×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Torque and power quiz4

## Torque and power quiz

(OP)
I spend a lot of time in the automotive community and a lot of time on car forums and groups.
Ever since I started to learn the actual relationship of torque and power it drove me crazy how few others in the automotive world actually understood this basic formula. People capable of building motors that make 500 hp per liter and who still think that torque is low end power.
I have gotten into enough arguments with people to learn that most would rather argue relentlessly cause that's what theys daddy taught them than sit down and think about the simplicity of the formula long enough to understand the relationship of the two.
I have decided to try a different tact and make a little quiz that gets people thinking about this from a different angle and maybe hoping they will get the point that torque and power can't be compared, and that torque does not mean low end power.
I just started on this tonight and it's 3AM. I want to do more to improve it but I also would love some input from others on ways I could improve it.
I would like it to be as detailed and informative as possible while still being interesting and keeping the person engaged and interested.

Tell me what you think of what I have so far.
What could I do better?
What are other questions I could ask or ways I could put things to get people thinking about the relationship without getting too bogged down in the math to loose too many people?
On that note should I focus more on the math or stay more with the basic relationship and principles?

http://matrixgarage.com/content/think-you-understa...

### RE: Torque and power quiz

Why do you believe that torque and horsepower cannot be compared?

They are simply two sides of the same coin and how you view the result depends entirely on the objective you are trying to fulfil. As you have stated the relationship between the two units is fixed and easily defined.

If your only objective is to accelerate a car at a maximum rate and you have complete control of gearing then it is clear that horsepower wins.

If you have huge weight and accel doesn't matter torque will probably win.

Look at the 10 HP Steam Roller which weighs 40 Tonnes and compare this to a 750 HP F1 car that used to go at 20000 rpm and only develop 300Nm of torque which is 'better'?

If you want to win a GP then clearly the F1 car. If you want to roll the tarmac on a 4:1 hill then the F1 car would look a bit silly.

The arguments in the Automotive world always start with the often quoted statement ' that Horsepower Sells cars and torque wins races'.

The weakness of the discussion that result are that they never state any basic cases of interest and are normally framed by a complete lack of understanding of the basic Physics.

It should also be clear that Tractive Effort would also be helpful in comparing two vehicles as gearing is taken into account.

Surely it is better to start with explanations of force, work, energy etc, etc and ensure that the basic groundwork has been covered thoroughly.

### RE: Torque and power quiz

(OP)
They cannot be compared and they are definitely not two sides of the same coin. That insinuates the general misconception of what torque and power are.
200 lb ft of torque cannot be compared to 200 hp because they are completely different things. As far as moving a vehicle is concerned that 200 lb ft of torque means nothing until you determine the amount of work it can do over time.
The point of the quiz is trying to get people to understand the relationship of torque and power in regards to work. In the case of the steam roller I don't feel it's practical to compare a 10 hp motor to a 750 hp motor as that power isn't needed to do the job.
To illustrate a point like this I would have a tendency to compare something like the two motors in my horsepower graph.

The red line is a 1.6 liter 4AGE turbo.
The blue line is a detroit diesel.
In an ideal world I would like to find a smaller motor that had a little more low end power because in this example the detroit does have an advantage in the low end over the 4AGE but it still shows that the advantage is not directly connected to how much more torque the detroit makes.
http://matrixgarage.com/pictures/tq-power/torque-g...
Despite only having 30% as much average torque in the short term the 4AGE would do almost as well as the detroit at a job like hauling a semi truck as long as it had the proper gearing to take advantage of it's power curve.
The Detroit would be chosen not because it's torque is particularly important but because
1. it would suck needing to spin the 4AGE to 4000 RPM to get started.
2. The high RPMs would wear out the clutch and drivetrain much faster.
3. The 4AGE would probably need to be rebuilt every 20k to 50k miles when the Detroit would likely make it 200k to 500k miles.

In your example You could compare something of a similar capability but at another extreme. For example with the right gearing a 10 hp nitro RC motor would properl the steam roller just as well but all the above issues would make it a less ideal option. The 80 lb 1400 RPM diesel motor is not the choice directly because it makes more torque but because it will be quieter and last much longer.

I hate sayings like that and it's exactly that mentality that I am trying to get rid of. That saying is so flawed that it should be easily thrown by the waysaide yet people are so invested in this myth that torque wins races that in the automotive world torque means something completely different than it does in the world of science and physics.
You can sit down with someone and explain to them that a gearbox will multiply torque of a higher hp motor to apply more torque to the ground but they will never accept it even if you throw all the math and science available to them. They believe that automotive torque exists outside of math and physics and that is what I want to make people understand.
I want people to realize the stupid truck commericals that advertise "the most torque in it's class" are 100% sales propaganda to sell suckers on their car when it has nothing to do with how the vehicle will perform it's job.

I don't really want to get into the definitions before the test because that is what seems to get people defensive or disinterested in the first place.
I will probably do a brief intro along the lines of
Most people think they understand the relationship of torque and power but very few actually do. Do you?

I am open to suggestions on what the intro could be.

I would like to have more at the end though. I would like to show the math and or theory that supports the answer. Still haven't figured out how to do that in this program.
That way anyone who is actually interested in why they got a question wrong can have at least enough basic information to point them in the right direction and give them some understanding of why.

At the end I would also be tempted to post a link to something like this where someone has already taken the time to explain it better and in more detail than I would.
http://www.epi-eng.com/mechanical_engineering_basi...
http://www.epi-eng.com/mechanical_engineering_basi...

I know that this is something that doesn't bother most people and the few others it does would rather not get into it with people who would rather remain in ignorant bliss than learn something new but this really is a major pet peeve of mine and does drive me crazy enough that I would like to do anything I can to try to get people to think about these things a little bit differently.
I am not the best at trying to teach or explain things so any input to improve would be very welcome.

### RE: Torque and power quiz

2
Well I got 7 out of 7, do I get a star?

While I agree the general population is confused about the difference between torque and power, I have given up trying to spread my knowledge to the great unwashed internet masses. I used to try to correct some of the bigger errors promulgated on internet forums only to find that the ignorant prefer to remain ignorant and are hostile toward anyone trying to point out the errors of their ways. It's like trying to teach a pig to sing, it's a waste of time and irritates the pig. Fortunately this forum, for the most part is different, and we can engage in educated discourse.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: Torque and power quiz

(OP)
dgallup
Gold star for you

I agree with everything you have said.
Unfortunately those same people are perfectly happy to spread that information to others.
I have for the most part stopped trying to change their mind but instead provide as much math and facts as I can hoping that I may sway someone with two brain cells to rub together to not just accept the other sides view just because it's the belief of the majority.

### RE: Torque and power quiz

(OP)
Just added a few new questions and got more detailed answers at the end.
I wish I could get them to show up after the grading but you take what you can get I guess.

### RE: Torque and power quiz

Taking this quiz reminded me of elementary school multiple choice science tests where the questions were written by "educators" that did not now much about science. I don't mean any insult. You need to start by clearly defining terms. If words have different interpretations to different people there can be no communication.

Work is mechanical energy and often the words are interchangeable. Power is energy produced per unit time. My ex-wife was somewhat technical with a degree in biology, but never really grasped what a unit of energy was or how that was different from power. Without understanding the concept of energy and being able to quantify it, there really cannot be any further useful discussion. The pig analogy is very appropriate.

Presenting data as a function of percentage of red line rpm without knowing what that red line rpm actually is, is meaningless.

When speaking of torque, is that at the flywheel, after the gearbox, or at the wheels. Is it max torque (at what rpm)? I do not think your quiz will help anyone.

### RE: Torque and power quiz

The confusion often comes from the shape of the torque curve and the gearing that is conventionally applied.

In most automotive applications, a gasoline engine will often be geared so tall in the highway cruising gears, in the interest of economy, that at normal highway speed, it is running well below its torque peak. It will be necessary to downshift in order to get anywhere, but diesel engines are often geared right near their torque peak at normal highway speed.

VW gasoline versus diesel are like this; YES the gasoline engine version will outrun the diesel in a drag race, but the TDI will usually be more fuss-free in daily driving and not demanding of so many downshifts.

While the theoretical possibility might exist that you could use a F1 car engine to haul an 80,000 lb tractor-trailer with suitable gearing, in reality you don't want to do that.

### RE: Torque and power quiz

Hi Yoshi. Looks good so far. I will try to think of some constructive suggestions. In the meantime a couple of nit-picks.
- Power is a measure of "work rate".
- Work is Energy, so power is also a measure of "energy rate" i.e. there are two correct answers for Q2
- It may be instructive to tell the examinees that Q3 and Q6 are the same three engines.

My summary of the torque/power debate.
1. The peak torque value for an engine (without rpm info) is useless.
2. The peak power value for an engine is useful.
3. Sure - the "torque curve" tells you everything, but if you have the torque curve, you know the power curve. As you point out, a torque curve drawn with a dimensionless speed scale (0% - 100% rpm) is useless. The equivalent power curve tells you (almost) everything you need to know.

je suis charlie

### RE: Torque and power quiz

(OP)
Compositepro

The reason I am posting here is for input on how to improve it.

How would you suggest I change the wordage to look better and be more accurate?
My biggest issue is how do I bridge the gap and make these principles easily understandable by the common person who didn't go to school for physics or engineering. I want to use simple yet accurate and clear wording and explanations.
I had thought about putting something like power is a measurement of work over time. Would this sound better?

As for the percentage of RPM I agree with gruntguru and that was my point for doing it like this. In relation to torque that graph tells you nothing. People like to think that because their truck makes 1000 lb ft of torque that it will do magical things. They don't understand that this figure is meaningless until you add RPM. The whole point of the torque graph is to bust these people who will look at that graph and assume that the motor that makes the most torque will do the most work.
The question with the power graph does tell you a lot. Not as much as it would with units of RPM but you can clearly see which motor can do more work over time.

When I say X engine makes X torque I expect that it is pretty clear that is at the crank. Any time I am talking about a transmission or other point of measuring torque I feel I made it pretty clear. If you can point out a specific spot that stands out to you I can take a look at it.

BrianPeterse

Thanks for your input and those are the exact kind of points I am trying to make here.
You are right that various motors have pros and cons. Otherwise there would only be one type. I am trying to get people to understand that the reason for having a Powerstroke in your pickup is not that having 600 lb ft of torque is necessary to haul a heavy load and that the actual benefits are things like low average RPM, long life and less wear on the drivetrain.

gruntguru

Thanks, I think you summarized what I am trying to accomplish very well.

Do you think it would be better to clarify that power is work/time making that more technically accurate than saying it's just energy?
Or would it be better to give both a more accurate designation of work rate and energy rate and make them both acceptable? I would lean towards the former.

Im not sure if you saw it but I figured out how to add a detailed explanation of each of the questions after the quiz. If you haven't seen it yet I would love any input on that as well.
In there I pointed out that they are the same engines.
I went out of my way not to point this out in the quiz because in question 3 I just want people to see that fatty torque graph of the Detroit and assume that it must obviously be the most impressive motor.

### RE: Torque and power quiz

The scaling of the rpm is just plain confusing and it makes the quiz become more theoretical vs practical. I would provide the power and torque curves with real rpm for 2 engines, a low rpm diesel and a high rpm gas and then pose questions using that real data, such as which one would accelerate a 40,000lb semi truck faster? or why is the diesel more suited for use in a semi truck?

The simplest definitions/explanations of torque and hp to apply to vehicles I have come up with are as follows;

HP tells you how fast your engine can accelerate your car. The HP at the wheels is the HP at the engine minus the drivetrain losses. Put more average HP to the wheels and your car will accelerate faster.

Torque just tells you the twisting force your engine applies to the transmission input shaft. The torque is meaningless for telling you anything about how fast your car can accelerate without knowing the engine rpm. Once you start using both the torque and the rpm, you are really using the HP information, since HP is proportional to torque times rpm.

I believe that people are mostly stuck on the idea that the torque is higher below 5250rpm so it must be the more important number.

### RE: Torque and power quiz

I think the dimensionless rpm scaling removes the ability to calculate to compare and makes a person think about the concepts of power and torque. So - a good thing.

je suis charlie

### RE: Torque and power quiz

2.7 l 2015 EcoBoost Ford F150 - 325 hp @ 5,750 rpm, 375 lb-ft @ 3,000 rpm , 6 speed auto

3.0 l TurboDiesel Ram 1500, 240 hp @ 3,600 rpm and 420 lb-ft @ 2,000 rpm, "unflappable" 8 speed auto,
note 240 HP at 3600 rpm = 350 lb ft , so the diesel is making at most few % less torque than the eco boost around 3600 rpm.

==========

Compare all the acceleration tests, including DAVIS DAM "FRUSTRATION"** 50-70-mph passing acceleration with 1,000 pounds of payload

"The EcoDiesel's torque comes on quickly and makes it feel quicker than the Chevy around town. Part of that is due to its exclusive eight-speed automatic transmission, which we agreed was the best here by far. "It's unflappable," said Seabaugh. "It's never caught in the wrong gear. It makes the Ford's and especially the Chevy's six-speed transmissions feel like they're from an entirely different era."

### RE: Torque and power quiz

Interesting set of numbers. One thing is for sure - the Ecoboost engine has a LOT more power than the other two - somebody's power claims are wrong. Not that I am a fan of turbochaged SI engines in trucks. Sounds like a recipe for reliability problems to me.

je suis charlie

### RE: Torque and power quiz

Interesting that they did the towing comparison with 7,000 lb trailers but the Ford towing capacity is only 5,000 lb.

Last year I bought a Ram 1500 with the 5.7 l Hemi mainly because of the ZF 8 speed transmission. Ford & GM were way behind on transmissions although GM is introducing an 8 speed this year and Ford is working on a 10 speed.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: Torque and power quiz

(OP)
Some stats from the first few days.
Every time I make changes to the quiz it makes a new revision so this is probably about 30% of the total test takers but all previous revisions had similar numbers.
The earlier ones had a little lower percentage above 40% and 5% to 6% passed. I believe some people may be taking the test multiple times. Another possibility is that some of the revisions have made the test a little clearer to some.
One interesting thing is that the 40-60% range is always the lowest. Right now it is tied with the 80% to 100% range but that's the highest it's gotten.
The incomplete tests are included in the 0-20% graph

### RE: Torque and power quiz

(OP)
Oh and passing grade is 75% just because that was the default.

It also made me pretty happy my fiance got 7/8 right hehe.
She is not mechanically minded, her eyes glaze over when I start talking cars or motors.
Goes to show that some basic math skills and a bit of a brain really should be all it takes.

### RE: Torque and power quiz

How many of the "regulars" on this forum would admit they got any questions wrong?

### RE: Torque and power quiz

I got question 2 wrong but I blame poor wording.

### RE: Torque and power quiz

I don't like the use of "work/time" or work over time for power. Power is an instantaneous quantity, not usually time-averaged. dWork/dTime as dTime->0. It minimally needs the word "rate" in there.

- Steve

### RE: Torque and power quiz

SomptingGuy, IMHO, work/time is a physics definition of power and should be used. After all "work", "force", "distance", "velocity", "time, "power", etc all have formal definitions and don't make any sense or don't have consistent meaning if not used correctly or if a correct usage is not accepted. dwork/dtime is also fine.

Yoshimitsu, do you really hear people say that torque IS power at the low end? or do they say that high torque MEANS more power at the low end? The latter makes sense with respect to how engines are built and tuned.

I think some confusion is inevitable due to the fact that the piston IC engine is a tuned system that does not even exist as a working system at zero rpm and that is usually used over a restricted range of rpm with its torque and power depending on tuning choices (cam timing, sizes of ports and valves and lengths of ports). Other engines of limited power, like steam and electric, have the more logical relationship between torque and power with torque being highest at zero rpm and steadily decreasing with rpm while power remains relatively constant. BTW, if we look at a road vehicle as an engine (with variable gearing), it more closely approximates the constant power, diminishing torque characteristic.

### RE: Torque and power quiz

i work for a company that develops vehicles for moving stuff and lots of it, we develop a tractive effort vs speed curve for each vehicle

most people will never understand the difference between power and torque because they dont take the time to try and learn it.

### RE: Torque and power quiz

(OP)
inline6
Spending a lot of time trying to communicate with car guys, tuners and the general car community I think that so much of it is the education that they get from that community clouding the facts.
The fundamental basics of torque and power are pretty simple. My fiancee has a better understanding of it than most car guys do because she hasn't been taught her whole life that it means things it doesn't She just looks at the formula and a brief description you can find anywhere on the internet and she gets the basics. What frustrates me is that these other theories are so deeply imbedded in the car community that it makes it almost impossible for someone to accept the basic formula and principle and therefore the only way to make their world work is to separate the engineering/physics meaning of torque and power and automotive meaning of torque and power.

Tying into that.
140Airpower

There is a huge range but there are many people who believe that torque is power at the low end.
For example I started a similar thread on one of the car forums I am heavily involved in.
One of the questions in that thread.

#### Quote (So would an engine that produces more torque exit a corner faster than a higher strung engine? all else being equal.)

It is obvious that the poster assumes that torque is low end power.
I had to explain that what really matters is where in the power curve the motor is when it exits the corner and that a torque increase at X RPM equals a power increase at X RPM.
You can tell that this poster is not terribly knowledgeable and accepts and understands that and that is fine.

Worse than that are the people who can actually make 300+HP/liter and therefore must know everything.
If you hang out on speed talk or similar forums you will see incredibly capable engine builders saying torque to mean low end power.

I realize this comes a lot from dyno tuning where people will look at peak power and peak torque as well as the OEMs listing those two datapoints for motors but it has lead to a belief in the automotive community that has imbedded it's self so deeply that even fairly educated people severely misuse the terms without even realizing it.
You will see this in car commercials. In fact I always laugh at the people who bust out the old saying power sells cars and torque wins races when I hear one of these pickup commercials that brag about the most torque in it's class. Who gives a damn? All that number is doing is selling cars.

You see it in magazines with big reputations like Road and Track, Car and driver etc.
They will write about how the torque output of the car as though it is low end power.
At least top gear jokes about torques as though it's something they may not totally understand but they still use torque to define low end power and power to define high end power.

### RE: Torque and power quiz

Dynamometers measure torque at X rpm, and HP is calculated from that. Its just that simple.

### RE: Torque and power quiz

The hardest thing to understand was the english. 8/8

you confuse power and work in one question, I do not understand why you won't use power as a word. work/time could be taken to mean work and/or time, for example.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Torque and power quiz

Had this discussion before around these parts...

I think the confusion between the terms "torque" and "power" in general (non technical) usage is that journos and other writers need to correlate things with basic human senses, which are instantaneous things. I would argue that we are led to think that (for an IC engine at least) torque can be felt, power can be heard. That's what I read anyway. It's why petrol-heads don't like powerful quiet cars (can't sense what they think is power), even though they love the torque (can actually still feel that).

These are the only two base engine numbers we are given by manufacturers, which is not the fault of the journos. They have tried using other numbers (in-gear acceleration times, standing-start acceleration times, top speed), but these are all application related, not base engine and always qualified and explained to the point of boredom.

I'm amused by the extra confusion added by electric motors that are current limited and virtually silent. The journos really haven't managed to accommodate them yet other than to note that they have a lot of low speed torque.

- Steve

### RE: Torque and power quiz

Maybe as engineers we should come up with some new quantities to throw around. How about dWork/dPosition? That kind of stirs in everything except time. How much kinetic energy can you add to your vehicle in a given distance?

- Steve

### RE: Torque and power quiz

"Dynamometers measure torque at X rpm, and HP is calculated from that. Its just that simple."

But it looks like the inertia dyno industry does something a little different. Land and Sea is a former Massachusetts dyno company (now in Concord NH) that makes a bunch of products including stuff to instrument old dynos of different types.
They say this -
http://www.land-and-sea.com/dynamometer/dynamomete...
inertia-only "dynos" can not directly measure torque or maintain a steady state load (without also being equipped with some form of absorber and load cell or a parasitic drag source (e.g. a large fan or pump with a known loss curve).

This guy says it is common for inertia dyno software to monitor roller rpm over time, and for standard software to think of the energy at each rpm (after the operator enters all the inertias in the system.)
http://www.dtec.net.au/Inertia%20Dyno%20Design%20G...
"Calculate the energy of rotation for each of these angular velocity’s (sic) using the flywheels moment of inertia and then the change (Delta) in energy of rotation between these 2 points."

the "energy of rotation" has sissy metric units of Joules.
- One conversion is 1 joule = 0.74 pound-force foot. It is technically "work," not torque, even though the ft x lb units are mathematically the same ( work is force exerted through some distance, like a 216 pound Timberlake climbing up 40 feet of stairs at 5 Neponsett St. (pound-force is not pound-mass. High tech stuff that thinks of lbs as mass needs a correction factor and is confusing as heck)
- Another conversion is 1000 joule = 0.000372506 horsepower X hour, or 0.536 horsepower X second. So all we have to do is divide the energy change by the time increment in seconds and we do get HP directly.

### RE: Torque and power quiz

(OP)
dicer
Like Tmoose said
Load based dynos such as a water brake like Land and Sea manufacture do measure torque and RPM but you are still measuring for power. Without both torque and RPM you have nothing. Once you have torque and RPM you have power.

In the automotive world most dynos are inertial dynos and they measure the rate of acceleration of the fixed mass.

GregLocock

You mean it was hard to understand the wording in the last question? 8/8?

In which question do I confuse power and work?

SomptingGuy
I do not think it's that complicated for the most part.
I am not trying to suggest using more complex terminology or concepts.
I just want to see small changes. Instead of the journalist saying you can really feel that torque in the corners of X car but the power that y car makes on the straights is much better for them to understand and express that x car makes better low end power and car y has stronger top end power.
The biggest problem for me is when people try to compare torque and power or use torque to mean low end power.

### RE: Torque and power quiz

No, it was hard to understand (or at least not properly written) all the way through.

i don't know which question it was, you look for the one where you've confused power and work, and that's the one

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Torque and power quiz

(OP)
Well that doesn't do me much good.
If you have any specific suggestions on how to improve it I would love to hear it.

At one point I did have the questions just stated as work but after several suggestions I changed it to work/time which most agree is equivalent to saying power without saying power.
The point of not using power is trying to remove the words that already have such deeply ingrained beliefs and use synonyms or technical descriptions to show the principles without the pre existing judgement. It's also trying to get people to see that the work/time is what is actually important when trying to figure out how much work a motor can do.
Unfortunately it even between engineers, physicists and the like it seems almost impossible to come to complete agreement on terminology.

Some say that work/time is not an accurate or acceptable term for power while many others and many sources give work/time as the textbook definition of power.
At this point I have given up on making everyone 100% happy but I am still open to suggestions that could make it more accurate and technically correct.

### RE: Torque and power quiz

- Energy/time is certainly the definition of power. dE/dt is instantaneous power whereas E/t is average power.
- SOME (not all) dynamometers measure torque and speed, some measure Volts and Amps, some measure force and velocity.
- Power is a universal number which is constant and can be measured all the way from the fuel tank. Fuel flow rate can be expressed as power, and all the energy can be tracked to its final destinations eg work to propel the car, heat from the exhaust, heat from the cooling system, unburned fuel in the exhaust etc etc. All vehicles need power to accelerate, but torque is optional (ever seen a rocket car?)

je suis charlie

### RE: Torque and power quiz

gruhtguru, torque is not optional if we are in a wheel-driven vehicle. Otherwise it is optional. Even force and displacement could be optional, but not for vehicles.

### RE: Torque and power quiz

Yoshimitsu, you comment on a very interesting quote: "...For example I started a similar thread on one of the car forums I am heavily involved in.
One of the questions in that thread.
Quote (So would an engine that produces more torque exit a corner faster than a higher strung engine? all else being equal.)
It is obvious that the poster assumes that torque is low end power...".

Yes, I have often heard this kind of talk. But, I never thought of it as implying that torque IS power. I always visualize the torque and power curves of engines with "low end grunt" compared to engines with "high end power" and understood there was a trade off of better acceleration out of corners vs higher top speed. The idea is you can relate these engine characteristics with those torque and power charts. I assumed the speakers knew the difference and that their comments made sense. Maybe your quiz shows differently.

### RE: Torque and power quiz

Tmoose, why do you say "sissy" metric system? Is it that it's a system for people who object to confusion, time wasting and error generation? If so, I agree. Real engineers should not flinch at "roughing it" with the good old avoirdupois, English, American jumble.

### RE: Torque and power quiz

(OP)
140Airpower
You are right in that on a dyno graph the torque will be higher in the lower RPM and I believe this is part of what leads to this misconception but yeah there are a lot of people out there tho think that torque is low end power and many do not even equate the two being tied together by any formula but see them as two separate entities creating many misconceptions.
V8s make torque but 4 cylinders make power.
torque is better in the corners but power is better in the straights.
A 250 hp GSXR motor won't accelerate a car well because it doesn't make enough torque and you need torque to accelerate heavier vehicles.
Diesels are better for big trucks and towing because they make more torque.

If I ever get rich I am going to drop a high strung small displacement motor in a box truck, play with some gear ratios and a few other tweaks and race it up a pass against something like a 7.3 IDI such as the one sitting in my 26' Uhaul. Sure it would have a little more trouble getting off the line, sure it would go through clutches faster, sure it would need rebuilds more often but I just want to do it to see the look on peoples faces when some little sportbike motor or small built econobox motor beats the massive diesel to the top. There are a lot of people who's worlds would come completely undone. Many would believe that somehow you rigged or fixed it but oh well.

### RE: Torque and power quiz

Just because the bike engine makes enough power doesn't mean it is the most suitable for the application.

### RE: Torque and power quiz

(OP)
No I'm not saying it is by any means but not for the reasons that most people believe.

### RE: Torque and power quiz

140Airpower "torque is not optional if we are in a wheel-driven vehicle. Otherwise it is optional. Even force and displacement could be optional, but not for vehicles."

True, but the question most people don't ask is "what is the ultimate purpose of that torque?" and the answer is "to generate linear force, tractive effort, whatever you want to call it".

A chassis dyno cannot measure torque, because a car does not apply torque to the road surface.

je suis charlie

### RE: Torque and power quiz

Hi 140Air.....POWER,

The Joules are wimpy, not the metric system.

1 Joule < 3 quarter pounders raised 1 foot in the air, or a 2700 lb corvair about 0.003 inches off the ground, .
Then again, Me getting up off the couch prior to heading to the fridge is about 350 Joules.

### RE: Torque and power quiz

Tmoose, you are right that joules are wimpy. No good for raising Corvairs. Quarter pounders are another matter. With those, I only have to add a jack to do it.

Gruntguru, Roger about the chassis dyno. I never thought of it.

Yoshimitsu, I guess enough questioning will eventually bring out the misconceptions.

### RE: Torque and power quiz

Pascal is the really wimpy metric unit.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: Torque and power quiz

There's nothing wimpy about 1Pa RMS (94dB). Difficult to get youself heard over that.

- Steve

### RE: Torque and power quiz

Only if the Pascals are not DC.

Standard atmospheric pressure is about 100 kPa.

### RE: Torque and power quiz

8/8 earlier today, but the wording is still a bit confusing to this retired structural guy. I'd go back into the quiz, except that it would skew your results regardless of how I might choose to answer the questions on a second run through it.

#### Quote (yoshimitsuspeed)

They cannot be compared and they are definitely not two sides of the same coin. That insinuates the general misconception of what torque and power are.
To me, any misconception is that they're unrelated; that an engine could have one torque curve and an unrelated power curve. Is that what you're trying to clear up? Or just that peak torque and peak HP are somewhat more loosely related?

FWIW, I prefer to work with the torque curve for mathematically estimating vehicle acceleration and ignore the power curve beyond any use it may have in defining the torque curve. If nothing else, it's easier to deal with rotational inertias and their rotational accelerations (which must necessarily be consistent with the straight line vehicle acceleration, absent wheelspin) that way.

Norm

### RE: Torque and power quiz

(OP)
That is what I was trying to get at norm. I think there are many people who don't realize that a 10% increase of torque at a given RPM means there will also be a 10% increase in power at that RPM.
What I mean by they can't be compared ties directly into that. People will say for their build torque is more important than power or use torque to mean low end power.
The hope is to get people to understand how they are connected and the relationship between them.

Torque curve definitely has it's advantages in some situations as long as you know what the torque represents.

### RE: Torque and power quiz

Using the power curve to determine a best guess best shift point is pretty easy. If mechanically safe and possible Rev beyond the power peak to the point the HP in the lower gear will be higher than the HP at the shift point. HP cuts out a bunch of "red tape" calkulating the effect of gear ratio torque multiplication.

Ed Iskenderian described there is frequently an advantage of revving 10-20% beyond the power peak in "Valve-Timing-for-Max-Output"

### RE: Torque and power quiz

Tmoose - agreed. Aside from matters involving ultimate top speed there's nothing inherently special about the rpm at which peak power is developed.

To maximize area under the power curve you'd ideally want to have half of that area coming from the high side. That power curves aren't likely to be symmetrical about the peak power point would also be behind the "10% - 20% beyond" shift point suggestion, since gear spacings are typically wider than double that. (Side note, or maybe it's your main point - this very matter of setting shift points at rpms above the peak power rpm vs at it occasionally comes up on "enthusiast forums", where a couple of people will "get it" and the rest won't.)

This is a slightly different and I think more specific question than yoshi's, where vastly different powertrains are being compared against each other.

Geared appropriately, running at max HP will provide maximum acceleration at any given speed (traction permitting and ignoring rotational inertias and other secondary effects). But the maximum acceleration with that same gearing will still occur at max torque.

Norm

### RE: Torque and power quiz

(OP)
Another one from this morning.

Here we go with the science lessons. So when we put a V8 in a pick up we're wasting time, so put in a proper transmission instead? Torque and HP are related, give me something with torque then and forget the HP. Our 9 second Evo had 600 HP and 500 ft/lb, bu the 8 second dragster V8 had 600 HP and 900 ft/lb. Need we say more? Our Type R Honda had 650 HP and 450 ft/lb, it was not as fast as the Evo.

#### Quote (Same person)

I'm not a scientist or math wiz, but I understand what I read. I have also tuned enough cars to know that torque is more important than HP. I've tuned all kinds of car producing the same HP, but the car with the higher torque is faster in every case even with similar gear ratios. Can you explain to me in physics why that is? Maybe it will help me to produce more 9 second Evos or even lower my times.

These are the communities I spend way too much time in.
People who can get 200 hp per liter out of a motor but could believe it's due to fairy dust and unicorn sparkles and it wouldn't matter.

I actually came up with an analogy the other day.
A great chef doesn't need to know biology or chemistry to make a world class meal. They don't need to know exactly how yeast and sugar interact or how two different components combine to create a new flavor. In fact most will really know very little about that. They may think that bread rises because of little fairy farts making air bubbles in the dough. It wouldn't make their meal any less good or their cooking any less effective.

### RE: Torque and power quiz

Sometimes it's best to just smile and nod.

Let me add something that I saw with my own eyes at a local drag strip on test-and-tune night.

A very stock-looking and stock-sounding Mazda 3 lined up beside an older VW that was quite obviously hot-rodded and turbocharged. The VW made all sorts of chirpy and whiny noises in the process of staging. The Mazda made none.

When the light turned green, the Mazda took off in a very unremarkable, nondescript, completely-bone-stock manner. The VW, which had been making all sorts of noise from the engine being held just short of the rev limiter during staging, bogged and fell on its face. The revs dropped below the power band, the engine fell off the cam, the oversized turbo went off boost, and it slowly crept away from the line. Then, after a good couple of seconds, the engine woke up, the turbo spooled, the engine instantly revved to the limiter, and the front tires briefly lit up until the car caught up. Then the driver had to change gear.

And the engine bogged, and the car went nowhere fast, for another couple of seconds, until again it managed to creep into the RPM range where the turbo and the cam actually worked. Same thing. Lit up the tires, bounced off the rev limiter, time for a gear change.

Meanwhile, the Mazda with its driver-friendly bone-stock wide torque curve, was gone.

Obviously the VW had a too-big turbo and a too-big camshaft in the interest of making who knows how many hundred horsepower at 50 rpm short of rev limiter ... but anything below that, the engine was off the cam and the turbo wasn't spooled, and a lawn tractor would outrun it.

Your Facebook member would make a whole lot more sense if he simply plugged in the word "mid-range" prior to every incidence of the word "torque".

### RE: Torque and power quiz

(OP)
Yes exactly. I told him that what he was really talking about was power curve. You could use torque curve as well but until you add RPM the torque curve doesn't mean anything and once you add RPM you are essentially talking about power anyway.
But yes that is exactly the point and I think a lot of the problem is that people get so sucked into the 2 datapoint frame of mind that they actually think that torque number means something it doesn't.
Where it gets really bad is

#### CODE -->

give me something with torque then and forget the HP
and

#### Quote:

I have also tuned enough cars to know that torque is more important than HP
it really shows that this view of getting too invested in staring at peak tq numbers and thinking that is what means something it doesn't.

Your example above is a good one and shows the importance of tq/power curve. On it's own knowing peak power isn't a whole lot more useful than knowing peak torque but at least it gives you some idea of how much work the motor is capable of doing. If the VW had a CVT transmission he probably would have smoked the Mazda. Or maybe if the driver just knew how to drive the car.
I have had the same thing happen. My friend and I were just messing around, he was in his V6 or V8 Toyota pickup. I was in my MR2 with my 1.6 liter 11:1 compression, 256 cams turbo 4A. With a perfect launch I can have full boost by the time the clutch is all the way out. With that perfect launch I would have smoked him but I don't drag race, I don't practice my launches and I'm not terribly comfortable or smooth in a situation like that so I stuff it, drop the clutch too quick and might be lucky to be at 3k RPM by the time the clutch is all the way out. By that point he was already 4 car lengths ahead of me. Had we raced to 120 mph or so I probably could have reeled him in.
My car could easily take his truck with the perfect launch but because of his power curve just about any driver could get in it and take off at a similar rate because you let out the clutch, stomp on the gas and it will go.

### RE: Torque and power quiz

I keep coming back to the fact that IC engines are tuned systems. All complaints about not having enough low-end torque (or power) are really about operating the engines off tune. This is especially the case with turbocharged systems where having the turbo operating in its proper range is so vital.
Most of these problems just need the proper gear ratio. The maximum torque that counts is the torque at the rear wheels which is always best at the max POWER rpm AND the proper gear.

### RE: Torque and power quiz

(OP)
The 4AGE came in the 16 valve NA 4AGE, supercharged 4AGZE and the later 4AGE 20 valves.
People are always praising the 4AGZE because of it's amazing low end and how superior it is to the high strung 20 valve or a turbo setup because the supercharged motor accelerates 40% harder than either 20 valve or turbo at 2500 RPM. I am constantly trying to point out that even the lowest horsepower NA 4AGE makes twice the power at 6000 RPM as the supercharged motor does at 2500 so who cares on a motor that spins to 7500 RPM stock how hard it accelerates at 2500 RPM?
There are still those who insist that makes it a superior motor.

### RE: Torque and power quiz

An engine that has a narrow powerband up high in the revs will need the driver to row the gears all the time.

A torque curve similar to a typical diesel, will be less fussy about what gear it's in.

It makes a difference.

Sure, a turbo Suzuki Hayabusa engine makes about the same power as the engine in a tractor-trailer rig. Sure, you could tow 80,000 lbs with it if you had enough gears in the transmission. That doesn't make it a good idea.

### RE: Torque and power quiz

Brian, what you say is true for transmissions that require rowing, a dying breed these days.

Paddle shifting is a lot simpler than working gearshift knobs. With more speeds you can keep the revs closer to the power peak. Even with a torquer engine, you would want to run as close to the power peak as possible. If you had a CVT, you'd run at the power peak all the time, never at the torque peak. Then again, with a CVT you can design the engine to have the torque peak as high as possible. Forget the low end.

### RE: Torque and power quiz

Okay ... An engine with a narrow power-band up high will demand constant and repeated downshifts from the 9-speed automatic transmission on every grade or will demand its CVT to send the revs way up on every uphill, thus leading to driver complaints. A diesel-like torque curve will usually get into whatever gear ratio it wants at cruise, and pretty much stay put.

In a normal road car with a petrol engine, in this day and age, normal revs on flat-ground highway cruise are well below both the power peak and the torque peak, in the interest of economy.

### RE: Torque and power quiz

Brian, what you say is true, but the discussion of misconceptions about torque and power only becomes interesting when talking about the response of an engine to demand. As you say a CVT will respond to maximum demand by allowing the engine to rev up, I say to max power, not max torque.

### RE: Torque and power quiz

VW TDI torque peak near 2000 rpm, that's also about what they run at on the motorway. Need to accelerate? No it's not at peak power, but it's at least at whatever power it makes at peak torque, so something happens. On the one that I had, at 2000 rpm it had about 180 lb.ft of torque, so about 69 horsepower, which is not bad for a 105 hp engine, and that's without downshifting. Sure, you could downshift to get more if needed, but it at least had a decent amount right there.

Comparable non-turbo petrol engines might be geared a little shorter (say 2500 rpm highway cruise) but peak torque on those is well beyond that, and peak power beyond that again. A 1.6 petrol might have the same or a bit more peak power (at 6000 rpm) and it may have perhaps 120 lb.ft (at perhaps 4500 rpm), and less than that at highway cruise. Let's say 100 lb.ft for argument's sake. That's about 50 horsepower without having to change ratio. Want any more than that, and you will have to wait. If manual, for the driver to change gear. If automatic, first for the dim-witted electronics to realize that the driver wants more, then for the transmission itself to do the cog-swapping (or clutch-swapping or cone-shifting, as the case may be), all of which take a certain amount of time.

It's not all about how fast it will go, but for a daily-driver vehicle, it's also about how fuss-free it will go about its business.

The 1.6 petrol would probably have to downshift two gears to get what the TDI has right there ...

Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time.

### RE: Torque and power quiz

You must be a rather civilized fellow. "Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time". Not admitting to anything, but "most people" does not include all people.

### RE: Torque and power quiz

No, it most certainly does not include all people ... but I'll maintain that it includes most people, based on how easily I can outrun traffic using a 12 hp motorcycle or a 70 hp car.

The 12 hp motorcycle does get wrung out all the time. That's the only way you can ride something like that!

### RE: Torque and power quiz

The penalty for driving around with lots of extra BMEP available for throttle response in "an emergency" is going to look pretty bad in the brochure's EPA fuel economy ratings.

I rented some small auto transed something or other for a few weeks in 2014. It's insistence on shifting into a high gear at part throttle caught me by surprise. At very light throttle around town it would make that poor little engine's individual power pulses noisily and maybe even tactily apparent. A smidge more throttle would quickly, but pretty smoothly go down a gear or 2, which I bet was necessary to create the sensation of even modest acceleration.

### RE: Torque and power quiz

The penalty for driving around with a 5000 rpm torque peak available will be greater than that of driving around with a 2000 rpm torque peak available.

Just because a turbo Suzuki Hayabusa has more power than a 14 litre Cummins diesel in a 80,000 lb tractor trailer doesn't mean it's a suitable device for the job, no matter what gearing you use ... Can it move the vehicle, "yes", is it a good idea, "no" ...

### RE: Torque and power quiz

Apparently, this quiz is a theoretical exercise to help understand torque and power. In a theoretical context, you could say a Hayabusa engine will out-accelerate a 14 litre Cummins diesel.

I think you'd need a completely different quiz if you wanted to help people understand why the Cummins is a better choice in the real world. Or why an engine with a broader power curve is nicer to drive in the real world. It could also be noted in some text afterwards.

### RE: Torque and power quiz

#### Quote (140Airpower)

You must be a rather civilized fellow. "Sure, the petrol would win a flat-out drag race, but that's not how most people drive their daily drivers most of the time". Not admitting to anything, but "most people" does not include all people.

Maybe Brian's description of average daily driving doesn't fit all the people all of the time, but almost certainly covers all of the people most of the time.

It hardly matters which of my cars I'm driving; on the street I'm rarely at wide-open throttle . . . let alone way up there where peak power is developed.

When I'm out on the track, there is simply no point in running the engine at peak power rpm with barely 50% throttle going through a corner, as doing so would then force a potentially upsetting shift while there is still a lot of tire grip being demanded laterally. Being at or around peak torque rpm at the apex works much better, if only to help ensure that I stay on the black stuff and don't dig up any of the track's green stuff. Maybe all of my "enthusiastic driving" is closer to this than to the wheelspinning start, three shifts and coast kind of driving . . .

Norm

### RE: Torque and power quiz

#### Quote (Lionel)

Apparently, this quiz is a theoretical exercise to help understand torque and power. In a theoretical context, you could say a Hayabusa engine will out-accelerate a 14 litre Cummins diesel.

I think you'd need a completely different quiz if you wanted to help people understand why the Cummins is a better choice in the real world. Or why an engine with a broader power curve is nicer to drive in the real world. It could also be noted in some text afterwards.
From earlier,

#### Quote (me)

Geared appropriately, running at max HP will provide maximum acceleration at any given speed (traction permitting and ignoring rotational inertias and other secondary effects). But the maximum acceleration with that same gearing will still occur at max torque.
so what needs to be done is to get people to recognize the difference.

The "torque is everything" crowd only sees max acceleration in some [fixed] gear occurring at peak torque rpm and rests their case. The power people see max acceleration at some given speed in terms of using the most that the engine can provide . . . and ditto.

Norm

### RE: Torque and power quiz

Norm,
"Geared appropriately, running at max HP will provide maximum acceleration at any given speed (traction permitting and ignoring rotational inertias and other secondary effects). But the maximum acceleration with that same gearing will still occur at max torque".

The resolution of this "duality" of the IC engine characteristic is infinitely variable gearing (by whatever means, including electric drive). Without that we have what we have always had, a small number of gear ratios and the task of trying to get the most flexibility and performance -necessitating sacrificing maximum engine efficiency.

### RE: Torque and power quiz

As a "fix", an infinitely variable drive (typically a CVT) is at least a reasonable option in some cases. Including most folks' daily driving.

But in the context of this thread, it side-steps around the value of understanding torque and power. You're still stuck with the matter of what rpm do you want the engine to flash up to on any given demand for acceleration - do you want it to hit max power rpm for max acceleration or peak torque rpm for adequate and more economical acceleration (given that BSFC more or less mirrors the torque curve) . . . or somewhere in between? IOW, as a powertrain engineer/developer/aftermarket tuner you'd still have considerable interest in the topic of this thread even if your expected end users couldn't care less about it.

Norm

### RE: Torque and power quiz

The Prius is a good, and well documented, example of how you use a stepless CVT. It works out the demand power, accelerates the engine to that point on its ideal operating curve (minimum bsfc for a given power) and then lets the trans do its magic.

Scatter plots of rpm vs cruising speed (hence power) obtained by users overlay very nicely over the bsfc map.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Torque and power quiz

Norm, obviously, for max acceleration you go to the max power condition. You would operate the engine differently for max economy.

### RE: Torque and power quiz

The Prius makes good use of its chain CVT. First, chain and push belt CVTs are friction devices that are more efficient than traction type CVTs. And the electric motor of the Prius eliminates the need for a torque converter for starting, which is normally a large source of losses in the drivetrain. The electric motor also reduces the total drive ratio required from the CVT variator. It can be difficult to get the optimum amount of drive ratio required from a single stage chain or push belt CVT variator. Chain or push belt variators are also very sensitive to slippage, so they require very conservative torque margins. Using an electric motor to supplement torque is a good approach to regulate torque transfer thru the variator.

### RE: Torque and power quiz

Prius does not use a chain CVT ... it is a planetary gear set used as a torque splitter, and the CVT function is done through the electrical system. All Toyota and Ford hybrids are like this, and the Volt is sorta like that.

The chain CVTs are in the non-hybrid Toyotas (and many others).

The planetary gear set arrangement means the Prius arrangement doesn't have to wait for the CVT to change ratio. If the driver wants instant torque, it can be delivered through the electric portion of the drivetrain. It only works because it is a hybrid.

The production chain/push-belt CVTs that I am aware of, still use a torque converter for starting off from a stop.

### RE: Torque and power quiz

Quick amplification - the Prius transmission is essentially a differential, or mixer. One input comes from the engine and the other input is from a motor gen. The notional output shaft which is direct coupled to the wheels also has a motor gen on it.

Since a differential is a torque balancing device, appropriate changes in the torques applied by the 3 torque generators/absorbers can be used to hold any one shaft at zero speed, or any other speed, and the torque balance can also be moved around.

It is a very flexible solution, and was first suggested and built in the 70s.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Torque and power quiz

I found this thread, after getting pretty disgusted at exactly what the OP said..
All the conjecture, urban myths, and just plain old misinformation either online, or down at the local pub is quite amazing!

I'm not schooled in mechanical engineering, but had a pretty good teacher.. 'Dad'. Dad was a mechanical engineer, and an ex-Marine officer.
45 years ago, I burst home from school, with 'new found' information! I just learned my buddies fathers 'dyno'd' their race cars and had 'x' amount of power.. I was sure to impress the old man!
After the lecture of the proper pronunciation of 'dynamometer'.. and a heated impromptu algebra lesson, I walked away with newfound information; HP= Torque X RPM/ 5252. What 3rd grader needs this kind of information??? Shoot, I was struggling with long division!

Anyhow, my career path took me away from the hows and whys of engine principles, and well, here I am!

I also am a Cummins nut these days, and had some pretty good bench racing sessions both recently, and in my youth with my old big block Dodges.

BTW, I took your test, and got 7/8. The one I missed, was the first question that had 3 different engines, and you wanted to know which one would reach redline first. I chose the one with 700+ torque.. the correct answer was 'not enough info'.
Perhaps I mis read the question?
I am going to list a series of statements here, ones that I believe to be true. Please correct me If I am wrong!
1. 2 things are easily measured coming out the end of the crankshaft: Torque, and RPM.
2. HP is 'calculated'.. it 'quantifies' what X amount of torque can do at 'Y' rpm.
For the life of me, I cannot understand why some will state " Torque launches the vehicle, but the 'horsepower thing' kicks in and pulls the car across the finish line". Since when is HP a 'thing'??
3. Isn't crossing the finish line and still accelerating just an extension of torque? (More torque is being applied to the rear axle- final drive- than what the car is trying to hold back? ie; a 500 ft. lb cummins crossing the line in 'direct drive' is putting 500 lbs to the ring gear, say 3.54 ratio... which multiplies it to 1770 at the end of axle. Our 32 inch tall tires then 'reduces' torque by 50% (16 inch radius) for a grand total of ~885 lbs. to the ground.
So, as long as the truck needs less than this to over come all drag.. it should still accelerate? (least until the engine starts dropping off the torque chart.. which is getting close via the rpm chart.

4. Perhaps the HP and torque debate was long ago conceived.. back in the caveman days of hot rodding. We didn't have much in transmissions. All were either 2,3,4 speeds, and pretty wide ratio. The big 3 had pretty much the same configuration. Same with rear gear ratios. It was easy peasy to see the difference in performance of a 340 Plymouth cuda, as opposed to it's Hemi brother..(stock form)
The 340 cars COULD compete, but needed very high numerically rear gears, (which increased the torque to ground).. but the engine needed to be built to turn 8,000 rpm or more, to get MPH. At the time, I never connected the change of gears actually increased the amount of torque to the ground.. but took away from a 'stocker' engine. We built the engine to handle more rpm.. better breathing, lighter components, stronger valve springs.. And our trap speeds actually increased.
Nowadays, we can custom build just about any ratio needed to 'fit' a particular power band, for a particular type of running.
I did some tractor testing for JD engine works..(work study program) I was just the goof that drove the mules around the track all day long.. but in the late 70's and early 80's, they did what was quite common industry wise: Increase engine rpm. They got claim of 'new improved' powerful tractors.. and sold a pile of them in the process. All they did was change up the governor springs, and perhaps a little modification to the fuel plate, and of course adjusted the final drive ratio. viola'.. 15-20% increase in HP.. Torque really didn't change though.. They sold a LOT of new green paint for just a few dollars parts change.
5. People were, and still are suckers for the HP 'thing'.. It's sexy, It commands attention.
6. on any engine graph, HP continues to build even if torque is pancake flat..(such as my cummins with near flat torque from 1600-2600 rpm) because of the 'math' in increased rpm.. And will even continue upwards even when torque decidedly starts dropping off.. again due to the math, But HP will soon follow (drop) as the torque no longer sustains the 'formula'.

I did a little 'test' on my towing rig just the other day. I was told my cummins could 'top a hill' easier at 2600 rpm, rather than at 1600.
This is in direct drive, and the test started at the bottom of the upcoming hill.
What I found was: Yes. 'easier' on me... I didn't have to shift! easier on my engine? ummmm, no. As I hit the bottom of this test hill, I watched my tach drop from 2600 rpm in 100 rpm increments per second..
Next run my tach dropped from 1600 rpm in 50 rpm increments per second. ( I was going slower, so less wind resistance)
Where's the Horsepower 'thing' here?? I should've had almost double the HP to the wheels to fling me over the top. The engine struggled just as bad.. The one thing that 2600 rpm gave me was 'time'. I told myself I'd shift when I hit 1400.. I just cleared the hill as I reached the magic number. Whereas the 1600 start point I only made it halfway up before I was forced to shift.

Guys, I am not trying to re invent the wheel here, just a 'fmi' friendly update to my noggin! Let me know if I am in the right path of thinking, or need to adjust! I am all ears!
I won't be alive for too many more years, I'd sure hate to have to wait until I meet up with dear ol Dad and have to explain myself again how I never 'got it right'!

### RE: Torque and power quiz

dang.. I messed up. The final drive ratio in tire size isn't 50% reduction! should be 33%??

### RE: Torque and power quiz

Note the speed you were doing at 1600 rpm. Now select a gear that gives the same speed at 2600 rpm. Hit the hill at 2600 rpm in that gear and the speed doesn't drop at all. Same task, more hp, more performance.

The two tasks you were comparing in the same gear are totally different tasks. Going up a hill at twice the road speed is storing gravitational potential energy at twice the rate so requires twice the power (neglecting wind resistance.)

je suis charlie

### RE: Torque and power quiz

On the JD tractor - if they increased the final drive ratio and ran the engines at a higher rpm then they did get more HP and the tractors would do more work per hour on the field. The change in final drive ratio DID put more torque on the axles and more force to the rear wheels. It wasn't just a number game to fool consumers. Well, the only game might have been trading engine life for HP.

You don't really put "torque to the ground". You have a force at the wheel to road interface.

As for your truck - as already pointed out you need to change the drive ratio to get 2600rpm at the same speed then test again. You proved it works because downshifting lets you start accelerating again.

I'm not sure you read many of the previous comments because you seem to have missed how a gear ratio change to allow a higher rpm will affect HP which affects the work the engine can do, even if the torque curve is flat or slightly falling off. You seem to think that people who follow HP because it increases with rpm are suckers since the torque remains the same or drops off.

### RE: Torque and power quiz

I Should've pointed out that in the Dodge test, 5th gear was used. (direct) in both instances. The point I was trying to prove to the guy who insisted that HP gets us over the hill 'easier'.. *without shifting*

both you guys missed the point here.
"Overall vehicle ratio"

Sure, I could've downshifted and put the engine in the 2600 rpm at the bottom of the hill. The ratio I was in was 1:1 (5th)
4th is 1.39:1 3rd is 2.09:1 Stabbing 3rd would instantly put me to a tad over 3200, 4th would've been about 2224.
This wasn't the point. My point was to show 'engine ONLY' scenarios. Not what happens to the torque AFTER it leaves the engine, and subsequently 'modified'.

My experiment here was to show how 'power' NOT torque fails at maintaining speed. Both trials showed clearly that the gear I was in, along with the load I was carrying, overwhelmed the output 'applied to the ground'.
By downshifting to 4th, the transmission.. *aka 'torque multiplier* now sends 1.39 times MORE torque to the axle than when it was in direct (5th)
In 5th, I was putting (theoretically) 500 ft. lbs out the rear of transmission. Which is now converted via ring and pinion by a factor of 3.54
(1770) to the end of the axle shaft. 'Felt' pressure to ground with 32 inch tires should be 1185 ft. lbs.
By downshifting, the same 500 ft. lbs, now is 695 out the rear of trans.. which is converted to 1648 ft, lbs *to the ground*

This only proves my point. Increasing torque to rear axle either helps us maintain speed going up a hill, or accelerating.
Same with the JD. They went from a final drive ratio of about 180:1 to just a tick over 200:1. Same forward speeds, same transmissions. We could now pull a larger plow, because of the 'torque multiplication' factor.

### RE: Torque and power quiz

I don't really understand your point. Are you trying to claim HP doesn't matter???

Saying the gear ratio caused an increase in axle torque is no different than just saying it was because of a HP increase.

Calculate the new HP at the axle due to your torque change and then compare it to the new engine hp. They'd better be the same. The nice thing about HP is that there is no need to do all those torque and force calculations. If you are driving the same speed and increase the available HP then you will be able to do more work plowing or climb a bigger hill.

### RE: Torque and power quiz

If you'd read my first post.. I clearly state I was here for my information purposes. either right or wrong.
So, here we are:
I've been told that HP is all that is necessary to maintain speed when going up a hill.
I used the perfect vehicle to play with.. My Dodge cummins. (very flat torque curve between 1600 and 2600)
I proved that when going up hill, both engines degrade pretty much the same rate even though the faster engine was putting almost 2x the horsepower down.. The faster speed degraded quicker due to the higher drag on the pickup..

Simply shifting gears no doubt saves the day here. My 'point' is made from NOT shifting. I'm talking 'engine only'..

the 1600 rpm engine with 500 ft. lbs of torque hit the hill with 152 HP (about 50 MPH)
the 2600 rpm engine with 500 ft. lbs of torque hit the hill with 247 HP (about 80 MPH)

the faster engine lugged down much quicker (at the start) but I figured the drag on the truck was much higher as well.
Since I was told that HP maintains speed, why isn't the much higher HP helping out in this case? Is it because the torque applied to the rear tires is the same in both cases?
To me, 'maintaining speed' is just that: not losing rpm. Both failed in 5th gear at pretty much the same rate (after throwing out the drag thing) the vehicle was overloaded for the gear it was in.
I'm here to learn. NOT prove anyone wrong. I am here to see If I am on the right track. These are my 'findings', and what I have 'heard' on the streets in my recent lifetime.
Here it is, my jugular exposed. LOL. Someone explain it to me!

### RE: Torque and power quiz

Rancherman, in my life I have only had one car that was an automatic. I love rowing through the gears and feeling the grunt from low rpms pull through right on up through the range. I really appreciate great low-end torque. But, when you say "...shifting gears no doubt saves the day here. My 'point' is made from NOT shifting. I'm talking 'engine only'..." realize that with a 9-speed auto you have no idea of what gear you are in or maybe not even that there are distinct gears. The flexibility and torque/power range of the engine is a non-issue. This is more and more the state and the future of engine operation. The engine you need for a Powerglide or a 5-speed manual is no longer needed. That is partly why a very small turbo engine can haul SUVs around. A lot of it is in the transmission.

### RE: Torque and power quiz

Rancherman. Neglecting air and road resistance, maintaining speed on a given hill requires a particular value of torque at the wheels. If the same torque is available at double the road speed, the vehicle will also be able to maintain double the road speed on the hill. However doubling the roadspeed with the same rear wheel torque requires double the power. It is available power that ultimately determines the highest speed the vehicle can maintain up the hill.

Power = mgv
Where power = power consumed to gain altitude (increase potential energy), m = mass, g = gravitational acceleration constant, v = vertical component of vehicle velocity (similar to rate-of-climb for aircraft)

je suis charlie

### RE: Torque and power quiz

"I've been told that HP is all that is necessary to maintain speed when going up a hill."

To test what you're saying you want to prove, speeding up approaching the hill is not maintaining speed.

When your vehicle struggles to climb a hill in high gear:
You proved that increasing hp and speed at the same rate doesn't help to climb a hill.
You failed to prove that more HP doesn't help you climb a hill (not changing speed).

### RE: Torque and power quiz

(OP)
I had someone pose a question on torque and power vs acceleration that managed to confuse me a bit. Maybe you guys can help me with this.
I have gotten so used to thinking in terms of maximizing acceleration through gearing that power and power curve should really be all that matters. Then they ask me something along the lines of

#### Quote (In a given gear shouldn't the acceleration curve match the torque curve?)

After thinking about this too much I believe the answer is yes but then I find it very confusing. This would mean that a motor that makes 100 lb ft at 1000 RPM and 50 lb ft at 2000 RPM would accelerate at half the rate at 2000 RPM as it would at 1000 RPM.
If you look purely at force applied to the mass this would make sense because at 2000 RPM the motor would be applying half the force to the road but the motor is applying half the torque at twice the rate so it should be able to do the same amount of work/time as it was at 1000 RPM.
Now if you shifted to a gear that halved your RPM it makes sense that your rate of acceleration should maintain the same because you are doubling torque at the same time so everything mostly makes sense. I'm just confused about why the rate of acceleration at 2000 would be less when the motor is doing the same amount of work/time.

### RE: Torque and power quiz

"I'm just confused about why the rate of acceleration at 2000 would be less when the motor is doing the same amount of work/time"

Because, in the same gear the car is going twice as fast, so friction loses and wind resistance are much greater. Power is work/time. It is also force x distance. At twice the speed you cover twice as much distance per unit time. On a flat and level track you can plot the power required to drive the car as a function of speed. This curve will approximately be a squared function due to air resistance. So just going twice the speed will require four times the power. Of course, just to get the car to start rolling requires power so this overpowers the square relationship at low speeds.

### RE: Torque and power quiz

#### Quote:

Because, in the same gear the car is going twice as fast, so friction loses and wind resistance are much greater. Power is work/time. It is also force x distance. At twice the speed you cover twice as much distance per unit time. On a flat and level track you can plot the power required to drive the car as a function of speed. This curve will approximately be a squared function due to air resistance. So just going twice the speed will require four times the power.
Nope. At double the speed, the force is quadrupled. But since you're now pushing that force at double the original speed, force x velocity is then 4 x 2 = 8 times the power requirement.

Norm

### RE: Torque and power quiz

(OP)
I am talking about a situation with friction removed. Just talking about basic principle of mass and acceleration here.

Without friction would a motor accelerating a mass with 100 lb ft at 100 RPM accelerate the mass at twice the rate as the motor with the same gearing at 50 lb ft at 2000 RPM?

If so then how come when they do the same amount of work/time?

If I'm not mistaken work is a force/distance. Power is work/time.

A crankshaft spinning one rotation at 50 lb ft should to 50 ft lbs of work. A motor making 100 lb ft will do twice the work/distance but if it is spinning half the speed they will do the same amount of work per unit time.

### RE: Torque and power quiz

#### Quote:

I had someone pose a question on torque and power vs acceleration that managed to confuse me a bit. Maybe you guys can help me with this.
I have gotten so used to thinking in terms of maximizing acceleration through gearing that power and power curve should really be all that matters. Then they ask me something along the lines of

Quote (In a given gear shouldn't the acceleration curve match the torque curve?)

This is part of what I was alluding to back on 15 March. Maximum acceleration at any given speed needs you to be geared to put the engine at peak power rpm at that speed. Once that gearing is fixed and you're looking at accelerations at different speeds, acceleration using that gearing must at least generally follow the torque curve.

It might help if the numbers used for illustration were not so closely related to one another (i.e. 2:1 for both torque and rpm), which actually implies that your motor is truly a constant power device. Say your 100 ft-lb @ 1000 rpm (~19 HP) motor makes peak power at 2000 rpm where 75 ft-lb is available (28.6 HP). You gear it for maximum acceleration at, say, 40 mph, so that's your 2000 rpm speed where you've got 75 ft-lbs times whatever gearing. At the peak torque point of 1000 rpm, you'll be doing 20 mph with 100 ft-lb available, times the same gearing . . . Ignoring aero and what-not, that sure sounds like a 33% improvement in acceleration to me.

Re-gearing the whole business to maximize acceleration at 20 mph only moves the max acceleration in gear point down to 10 mph, where it will be 33% better than the acceleration at 20. The new acceleration at 20 will be better than the old acceleration at 20 (as in the first arrangement), but that's not the question.

Norm

### RE: Torque and power quiz

(OP)
NormPeterson that's a good way of explaining essentially the same conclusion I came to but I still don't feel like that accounts for all energy potential.

Acceleration really comes down to mass and force. The torque is the force so this makes sense but why does the motor need more power/energy at a higher RPM to apply the same amount of force?

I have been trying to think of an analogy or a different way to put it to try to clarify. I think I might have it.

If we hook your 19/28.6 motor up to a generator. To make it easier I'll convert that to 14/21 KW motor.
At 1000 RPM it will make 14 KW of power and at 2000 RPM it will make 21 KW of power.
Now if you took that power and applied it to an electric motor 21 KW of power would give you more energy for acceleration than 14 KW of power.
What am I missing to think that in the direct drive application there is lost energy when at 1000 RPM the motor is able to do more work than at 2000 RPM?

### RE: Torque and power quiz

#### Quote (yoshimitsuspeed)

I'm just confused about why the rate of acceleration at 2000 would be less when the motor is doing the same amount of work/time.

Because it only has half the time to do the work.

If you divide the time it takes to do the work by a factor of 2, then you must also divide the amount of work done by a factor of 2 ... if you want to keep the power constant, of course:

power = work1 / time1 = work2 / time2

if time2 = time1 / 2, then work2 = work1 / 2.

Assuming you travel the same distance (i.e. you make the same number of revolutions), at 2000 rpm you will loose half the torque over that same distance (50 lb.ft instead of 100 lb.ft) but you will do it in only half the time. That is the whole concept of power: How much work can I do in a given time frame.

Here's an analogy:

You have two workers. You need them to move a pile of bricks from one spot to another. You are paying them by the hour, so in it is important that this is done within 1 hour.

The first one is a big guy, so he takes the whole pile and moves it slowly to the new location, taking the full hour to do it;

The second one is not as strong, so he only takes one brick at a time, but runs to the new location. Making all the trips necessary to move the entire pile takes him an hour.

As a boss, you don't care how the work was done: The whole pile is moved (total force carried across a certain distance, a.k.a. work) in the same amount of time. If you didn't look at them work, you can't even tell which one took the whole pile in on trip and which one ran with one brick at a time. The same work was done in the same period of time. We say these two workers are equivalent.

But if you gave only ½ hour to these guys, the guy who run with one brick at a time will move only half the pile (like with your engine example) and the other guy would move the entire pile of brick half way. They would still be considered equivalent power-wise, because they had half the time, they did half the work. Give them another ½ hour and they will finish the job, even if they switch around (The big guy would lift half the payload and could now travel the full distance twice as fast; And the other guy would still moved all the bricks one by one at the same speed, but would only have to travel half the distance).

### RE: Torque and power quiz

Power doesn't define the force; it's kind of the other way around where a force over some distance per unit time (velocity) defines the power. So at a lower velocity than the velocity associated with peak power, the force can be greater than the force associated with the peak power velocity even as the amount of power available is less. Basically, the velocity term decreases faster than the power does as you drop from peak power rpm to peak torque rpm. At half the velocity (20 vs 40) you still have roughly 2/3 of the power (14 vs 21), so the force must be higher. F = Ma, where M is held constant . . .

Norm

### RE: Torque and power quiz

Yoshimitsu, You said "...I'm just confused about why the rate of acceleration at 2000 would be less when the motor is doing the same amount of work/time...".

Don't forget that we are accelerating by applying thrust and power to the ground, ...and the ground is retreating at twice the speed at 2000rpm as at 1000rpm. Power is energy per unit time and energy is force x distance therefore power is force x velocity and for a constant power, when velocity doubles, force is halved. This is a simple fact that doesn't even account for friction and aero drag, etc.

### RE: Torque and power quiz

There are always people who will talk about how a vehicle without a lot of torque output will be slow, without any reference to horsepower; or, people will compare two vehicles of the same power output where one has more torque output, and assume that the one with more torque will be faster. Neither of these is true, as a lot of you know. I thought it would be fun to play with some numbers, to prove outright that horsepower is what matters, and that crank torque numbers are all but useless for comparing vehicles.

First, let's do a basic proof, using relations; none of the following are actual equalities, but I am going to use the equal sign to symbolize that they are proportional. In other words, I've dropped out constants to make the equations simpler and more general.

1.) Horsepower (HP) = Torque (TQ) * Engine Speed (RPM)
2.) Thrust (THR) = Torque (TQ) * Gear Ratio (GR)
3.) Speed = Engine Speed (RPM) / Gear Ratio (GR)

In this example, gear ratio is assumed to be a number describing the overall reduction from crank speed. Thrust is the force applied at the pavement, which allows you to find acceleration when given the weight of the vehicle.

Now, rearrange equation 1 to come up with the following:

4.) TQ = HP/RPM

Then substitute equation 4 into equation 2, for torque:

5.) THR = (HP*GR)/RPM

This can also be written as:

5a.) THR = HP * (GR/RPM)

We can rearrange the speed equation (eq. 3) above, to the form:

6.) 1/SPD = GR/RPM

And, through substituting this into equation 5a, we achieve the following:

THR = HP * (1/SPD) = HP/SPD

So, the result is that thrust is proportional only to horsepower and vehicle speed.

In a real-life situation, you don't have infinite gear ratios, and instead (assuming no gearchanges in the middle of our comparison) you have a constant in that place. The result actually remains the same, since then speed is only proportional to RPM, and you can drop it right into the bottom of the modified torque equation (eq. 4).

What does this prove? Well, this proves that at ANY given speed, the acceleration of a vehicle can be directly related to the horsepower production at that point, and the weight of the vehicle (assuming no wheelspin, of course).

So, if you have two identical vehicles traveling side by side, and one has an engine making 75 ft-lbs of torque at 14,000 rpm and the other has a sweet diesel making a cool 500 ft-lbs of torque at 2100 rpm, they'll accelerate at exactly the same rate. They're making the same power, and traveling at the same speed, so they have the same thrust.

On a side note, assuming they both have the same BSFC (which is a bit of a stretch, but it could be reasonably close), then they will both burn fuel at the same rate.

### RE: Torque and power quiz

(OP)
Thanks guys I can see how that all works.
I guess my thought with seeing it in terms of energy such as using a generator to power an electric motor with higher wattage is kind of a roundabout way of achieving that more ideal gear ratio so to speak.
That makes sense.

### RE: Torque and power quiz

#### Quote (snoopnoon)

There are always people who will talk about how a vehicle without a lot of torque output will be slow, without any reference to horsepower; or, people will compare two vehicles of the same power output where one has more torque output, and assume that the one with more torque will be faster. Neither of these is true

The sentence in bold will be true in most cases. We might also reference to a previous quote in this thread:

Here we go with the science lessons. So when we put a V8 in a pick up we're wasting time, so put in a proper transmission instead? Torque and HP are related, give me something with torque then and forget the HP. Our 9 second Evo had 600 HP and 500 ft/lb, bu the 8 second dragster V8 had 600 HP and 900 ft/lb. Need we say more? Our Type R Honda had 650 HP and 450 ft/lb, it was not as fast as the Evo.

It is obvious that he's talking from experience, we cannot deny the fact that between 2 engines with the same peak power, the one with the highest peak torque will offer better performance. But it is not because of the peak torque per say.

People usually say: «Only the power counts, not torque». What they should say is:

«Only the average power in the selected rpm range counts, not the average torque in the selected rpm range.»

If the torque is less at a lower rpm, the power will also be lower and that will lower your average power available at the wheel since the peak power values are the same between the two compared engines.

Of course, if we have a lot more gear ratios in the car with the lower peak torque, such that we are using a smaller rpm range (closer to peak power), it is possible to maintain the same average power as the one from the engine with the highest peak torque.

### RE: Torque and power quiz

#### Quote (snoopnoon)

There are always people who will talk about how a vehicle without a lot of torque output will be slow, without any reference to horsepower; or, people will compare two vehicles of the same power output where one has more torque output, and assume that the one with more torque will be faster. Neither of these is true, as a lot of you know. I thought it would be fun to play with some numbers, to prove outright that horsepower is what matters, and that crank torque numbers are all but useless for comparing vehicles.

I hope that when your derivation reaches its conclusion that your HP variable is being taken from whatever RPM is consistent with both SPD and GR. Not max HP as might be inferred.

When you throw consideration of rotational accelerations of things like tires and flywheels into the mix, any approach working with HP should suddenly become a lot less attractive.

Norm

### RE: Torque and power quiz

#### Quote (NormPeterson)

It might help if the numbers used for illustration were not so closely related to one another (i.e. 2:1 for both torque and rpm), which actually implies that your motor is truly a constant power device. Say your 100 ft-lb @ 1000 rpm (~19 HP) motor makes peak power at 2000 rpm where 75 ft-lb is available (28.6 HP). You gear it for maximum acceleration at, say, 40 mph, so that's your 2000 rpm speed where you've got 75 ft-lbs times whatever gearing. At the peak torque point of 1000 rpm, you'll be doing 20 mph with 100 ft-lb available, times the same gearing . . . Ignoring aero and what-not, that sure sounds like a 33% improvement in acceleration to me.

Re-gearing the whole business to maximize acceleration at 20 mph only moves the max acceleration in gear point down to 10 mph, where it will be 33% better than the acceleration at 20. The new acceleration at 20 will be better than the old acceleration at 20 (as in the first arrangement), but that's not the question.

This is where the «torque» people get lost. You have to compare the torque output at the same vehicle speed, otherwise it is a useless comparison: At a higher speed, the traction force will always be smaller for a given power output.

When you «re-gear the whole business», of course the torque is still 33% at 1000 rpm than at 2000 rpm; It will always be the case no matter what gear ratio you use. The important thing is the torque output when the vehicle's speed is the same in both gears.

With your example, if the gear ratio was, say, 3:1 in the first case then the torque output was 300 lb.ft @ 333 rpm (20 mph) and 225 lb.ft @ 667 rpm (40 mph). If you switch to a gear ratio of 6:1, then you get 600 lb.ft @ 167 rpm (10 mph) and 450 lb.ft @ 333 rpm (20 mph). The important comparison is that at 20 mph, you get 450 lb.ft in low gear vs 300 lb.ft in high gear. Ignoring aero and what-not, that sure sounds like a 50% improvement in acceleration to me. Nobody cares about the fact that you get 600 lb.ft @ 10 mph, because you can get 900 lb.ft at the same speed with a 12:1 ratio (Which is still a 50% improvement). Only in first gear will the maximum torque of the engine will matter.

For the same output rpm, the output torque will always be greater in a gear ratio where the power is the greatest. That is the whole concept of power.

#### Quote (NormPeterson)

When you throw consideration of rotational accelerations of things like tires and flywheels into the mix, any approach working with HP should suddenly become a lot less attractive.

I never saw a transmission design that offered such an increase in inertia, that torque gain was overtaken. Even for the example, for a typical car, the effective mass of the car for a 3:1, 6:1 & 12:1 gear ratios would be 1.06, 1.13 & 1.40 times the mass of the car. So a 6.6% increase between 6:1 & 3:1 and 23.8% increase between 12:1 & 6:1. The 50% improvement in torque still makes a noticeable difference.

### RE: Torque and power quiz

#### Quote:

This is where the «torque» people get lost. You have to compare the torque output at the same vehicle speed, otherwise it is a useless comparison: At a higher speed, the traction force will always be smaller for a given power output.

<sigh> . . . I understand that, and said as much.

#### Quote (Me on 15 March)

Geared appropriately, running at max HP will provide maximum acceleration at any given speed <snip>

<double sigh> . . . and *power* people always manage to lose sight of what happens to acceleration when the engine cannot be operating at its peak power rpm; it's as if acceleration at any other rpm does not matter even if running at those rpms cannot be avoided. My point all along has been that (1) sometimes the reality of your road speed does not permit you to be at peak power rpm, (2) the acceleration at such other road speeds has nothing to do with either peak HP or its RPM, and (3) the maximum acceleration in any given gear occurs when the engine is operating (essentially) at its peak torque point.

#### Quote (Me on 15 March)

But the maximum acceleration with that same gearing will still occur at max torque.
Sometimes it's nice to know what happens under conditions other than peak power operation. Enlightening, even, when you realize that acceleration curves droop toward the tops of each gear because of more than just the increases in the speed and drag.

I was hoping you'd look at your own derivation of rotational accelerations instead of blindly using somebody else's rules of thumb and all of their [unlisted] assumptions. You might have found that it's simpler to work with torque, and from there seen acceleration as a function of torque in a different light.

Have you taken yoshi's little quiz yet?

Norm

### RE: Torque and power quiz

#### Quote (JackAction)

. . . . we cannot deny the fact that between 2 engines with the same peak power, the one with the highest peak torque will offer better performance.

Oh yes we can.

#### Quote (snoopnoon)

So, if you have two identical vehicles traveling side by side, and one has an engine making 75 ft-lbs of torque at 14,000 rpm and the other has a sweet diesel making a cool 500 ft-lbs of torque at 2100 rpm, they'll accelerate at exactly the same rate. They're making the same power, and traveling at the same speed, so they have the same thrust.

From snoopnoon's example it is clear that peak torque has absolutely nothing to do with it. The diesel has 6.6 times the torque but doesn't accelerate any faster.

The fact is (neglecting rotating inertias) if both engines have the same power, the one with the wider "power band" will accelerate better through the gears. That is to say "the engine with the highest average power across the band dictated by the gearbox ratio spacing". Generally speaking, the torque peak will not even be used when shifting at the optimum points - the revs will not drop that low.

je suis charlie

### RE: Torque and power quiz

Funny, now I'm getting blasted from two different comments that are exactly the opposite of one another:

#### Quote (NormPeterson)

*power* people always manage to lose sight of what happens to acceleration when the engine cannot be operating at its peak power rpm; it's as if acceleration at any other rpm does not matter even if running at those rpms cannot be avoided.

#### Quote (gruntguru)

Generally speaking, the torque peak will not even be used when shifting at the optimum points - the revs will not drop that low.

Can't we avoid those rpms or will the revs not drop that low? It doesn't really matter because I'll repeat the only important thing I've said that covers it all (I have enlighten the keywords):

#### Quote (JackAction)

«Only the average power in the selected rpm range counts, not the average torque in the selected rpm range.»

Norm, I like the way you started talking about the subject in your first posts and then you began introducing comments about saying that the torque value is somehow more useful than the power, and it's not.

Here's why: If I tell you that I have a car driving at 10 m/s with an engine that produces 500 N.m, it is impossible for you to tell me the tractive force of that car; not even a close estimate. If I tell you that I have a car driving at 10 m/s with an engine that produces 100 kW, I can estimate that the tractive force is 10 000 N. Simple as that. It doesn't even matter if your car is powered by a gas engine or electric motor - or a nuclear reactor for that matter. Anything else doesn't matter (as long as you have traction to support that force): weight (and other rotational inertia effect), drag coefficient & frontal area, rolling resistance, inclination of the road, if it's a FWD, RWD or AWD, or even what song plays on the radio: They have no direct effect whatsoever on the total tractive force you will produce at the wheels (i.e. except for some losses inherent to the different components' design).

If you use the torque, you will need to know the engine rpm (and you will now know the engine power) or the complete geometry of the car where - from the tire diameter - you will find the wheel rpm and - from the gear ratio - you will find the engine rpm and from that you'll get the power of the engine. Very complicated; with no addition of useful information.

This is why simple equations like these ones and this one or even a more complicated simulator like this one only needs the power-to-weight ratio to give an amazingly close estimate of performance. All the resistance forces and inefficiencies are combined in one big constant.

What I wanted to point out is that a statement like «You get a 33% improvement when the car is in the same gear and the engine is at different rpm» - although true - is a misleading information that doesn't take into account the vehicle speed difference and it only fuels the «torque» peoples' theory that torque is more relevant than power. Only power matters, there is no «if», «but» or «it depends». Whatever calculations you can do with torque, I can do it in less steps, and with less information, with power.

### RE: Torque and power quiz

Would the people fixated on horsepower numbers and nothing else, and think that everything can be taken care of with gear ratios, please explain to the class why a 9 litre 6 cylinder diesel engine is appropriate for propelling a box truck, and a far smaller and lighter Suzuki Hayabusa engine that makes the same (ish) power is not appropriate ... think of all the extra cargo you could carry if you were not lugging that huge engine around which weighs the better part of a ton on its own.

Perhaps the same people could explain why the reverse is also true ... bearing in mind that the diesel engine weighs twice what the whole bike does.

I've already tried, in this very thread, but obviously something must not be sinking in.

### RE: Torque and power quiz

Maybe the class doesn't realise that prime movers aren't perfect power-on-demand devices and have some serious intrinsic design/application trade-offs. It's amazing that most of them actually work and more amazing that we've managed to engineer contraptions that allow useful work to be extracted from them.

A super-efficient 2-stroke ship diesel would be ideal for a motorcycle. If efficiency was more important than driving pleasure. And if the bike was already big to start with.

A bike engine would be ideal for a heavy goods truck. If the tiny additional cargo it allowed offset the drop in efficiency. And the short life-span of the engine when operating at either full or zero load all day.

Steve

### RE: Torque and power quiz

Brian, your point about inconvenient gearing is well taken, but the example introduces even more compelling reasons a Hayabusa can't be used for trucking. You acknowledged this in an earlier post.

When I learned that a then typical long-haul diesel truck engine produced less horsepower than my 327 I wondered what would happen if they used 327s in heavy trucks. I didn't think the gearing for the higher rpms was an issue. I assumed the 327 would overheat on hills and would wear out in a small fraction of the truck engine's service life. I didn't even think about the fuel economy.
The Hayabusa could be geared for the job -as a stunt, but might catch fire or melt on a long grade and if it didn't it might be junk by the top of the hill. Otherwise, it could be a good idea.

Car makers are approaching the challenge of substituting tiny boosted engines for the typical large engines used for heavier vehicles and for meeting the performance of larger engines. It's not just gearing, 9 speed autos can handle it, it takes the water jacketing, cooling system and bottom end strength and wear characteristics of the larger engines being replaced.

### RE: Torque and power quiz

It remains to be seen whether those tiny boosted engines will live as long as a slightly larger, though unboosted engine. It sure doesn't seem like they use any less fuel in the real world, as opposed to the fictional, low-speed, light-load, gentle world of the official EPA Federal Test Procedure.

### RE: Torque and power quiz

Brian, good point there. I guess we'll see. Also we'll see if transmissions get more gears or if they have already gone too far.

### RE: Torque and power quiz

#### Quote (BrianPetersen)

Would the people fixated on horsepower numbers and nothing else, and think that everything can be taken care of with gear ratios, please explain to the class why a 9 litre 6 cylinder diesel engine is appropriate for propelling a box truck, and a far smaller and lighter Suzuki Hayabusa engine that makes the same (ish) power is not appropriate

Very simple, and it is all related to what has already been said: «average power» over the «selected rpm range».

The useful rpm range for a large truck needs to perform throughout the whole range. Even with a wide power band, they can have like 20 gear ratios; Imagine how many they would need if they had a narrower power band. You miss one shift and you lose your power: Not very practical. So they need high power in the lower rpm range. They are so heavy that they need to perform right from the start (v=0).

The motorcycles can get away with little power at low speed as they are very light and have very low drag resistance to begin with. So you can use an even smaller engine (even less weight and drag resistance) with an equivalent power band in a higher rpm range. You could also use the large truck engine and get the extra power at the starting line, but the extra weight of the engine will probably handicap you throughout the rest of the race.

Here's another way to look at it: Take your 1000-14000 rpm motorcycle engine and install a 5:1 gear reducer between the crankshaft and the clutch. Put the engine on a dyno and measure at the gear reducer. You will have the exact same values for the power curve, except that it will be in the 200-2800 rpm range. The torque values, though, will all be multiplied by 5. Now you can compare it with your diesel engine that have a similar rpm range. And the maximum torque will probably be lower in the case of the motorcycle engine because of the way it is tuned.

People shouldn't fix on the maximum values, but rather the average values over the rpm range that they will use. But if you only have the maximum torque and maximum power of an engine, the maximum power will give you an idea of the power curve over the whole rpm range; The maximum torque doesn't do that. Between an engine that produces maximums of 300 lb.ft and 350 hp and another one that produces 400 lb.ft and 250 hp - if geared appropriately in both cases - the first one will most likely give you better performance. But if, like posted earlier in an example, you have the same maximum power, but have a higher maximum torque in one case, the one with the higher lower end torque has also higher lower end power and that increases the average power over the entire rpm range.

### RE: Torque and power quiz

I had a new Chrysler 200 with the newfangled 9-speed as a rental this past Sunday through Tuesday. I never found any circumstance in which it actually uses 9th gear.

For how this transmission has been criticised in the press, I found it to be quite good. (I'm sure FiatChrysler has been working on the calibration.) Smidge reluctant to downshift, but most automatics are like that. My van (also a FiatChrysler product) has the ole 6-speed 62TE, and I'd gladly take the 9-speed in its place, even if it only functions as an 8-speed. The closely-spaced higher gears on the 9-speed are a lot better ... the 62TE has a huge gap between 5th and 6th.

### RE: Torque and power quiz

#### Quote (Jack)

This is why simple equations like these ones and this one or even a more complicated simulator like this one only needs the power-to-weight ratio to give an amazingly close estimate of performance. All the resistance forces and inefficiencies are combined in one big constant.
I am aware of the first two, which are essentially curve-fits to a fairly large amount of actual dragstrip results. Good approximations, sure. But they won't give you the acceleration data at any speed at all, so they're pretty much irrelevant to any torque vs HP discussion.

I didn't need to go very far into your third link to find Ft. Ft can just as easily be determined from (crudely) [torque] x [gearing] ÷ [drive tire radius], all of which would be knowns. From a power-centered approach, you still need to know gearing and tire radius, simply to find the point on the HP curve that your engine is operating at in that instant - as is suggested in your later post with "«average power» over the «selected rpm range»". Convince me that working with power in order to determine an acceleration curve between 35 mph and 55 mph and staying in a single gear would be easier.

Forget "effective mass" for rotational effects; do it properly as [torque] = [I] x [alpha], which will remain independent of rpm per se. Isn't it easier to deduct a parasitic torque that's a function only of the torque curve's variability than to deduct a parasitic power that's a function of both HP curve variability and the RPM at that point?

#### Quote:

saying that the torque value is somehow more useful than the power, and it's not.
Have you ever tried to construct a general math model of vehicle acceleration from basic vehicle data and first principles yourself? Mine even considers deceleration due to drag during the time it takes to execute each upshift. But hey, I've been at this one off and on since I was in college . . . and I'm now retired.

#### Quote:

Funny, now I'm getting blasted from two different comments that are exactly the opposite of one another:

Quote (NormPeterson)
*power* people always manage to lose sight of what happens to acceleration when the engine cannot be operating at its peak power rpm; it's as if acceleration at any other rpm does not matter even if running at those rpms cannot be avoided.

Quote (gruntguru)
Generally speaking, the torque peak will not even be used when shifting at the optimum points - the revs will not drop that low.

Our comments are entirely consistent. If your shifting from at or slightly beyond the power peak is such that you land above peak torque rpm, it's because the gear spacing was close enough to permit this. This is not always the case. In fact one of my cars develops peak torque at 4500 rpm and peak HP at 6000. Fuel cut is said to be 6250 or so. But the step from 2nd gear to 3rd is from 2.00:1 to 1.38:1, so a 2-3 upshift drops the revs to no more than 4312 (closer to 4100 would be safer). The first to second step is wider still . . . as is the 4th to 5th.

Norm

### RE: Torque and power quiz

One thing I was thinking, but haven't mentioned, is that for high speed racing, top end power can absolutely dominate. For high speed ovals this is obvious, but in road racing it's top end power that allows a car to pass on the straights. Trying to pass with grunt coming out of the turns can be defeated by a good opponent. So depending on the track, you may not think low-end torque is very important.

### RE: Torque and power quiz

#### Quote (NormPerterson)

Have you ever tried to construct a general math model of vehicle acceleration from basic vehicle data and first principles yourself? Mine even considers deceleration due to drag during the time it takes to execute each upshift. But hey, I've been at this one off and on since I was in college . . . and I'm now retired.

So I'm gonna stop right here as I'm not gonna go into a «I'm smarter than you are» discussion.

Instead of telling you that I'm smarter than you, I'll show how I smart I am (maybe it's less than you, but I'm still proud of what I accomplished): The simulator from the third link, I wrote it. The theory behind it, I developed it and showed how I did it, up to the last detail. For what I did not do, I cited my sources. It is all there for you to analyze. So trust me when I'm telling you that nowhere in that simulator, the engine torque nor the rpm was needed to determine the acceleration of a vehicle. I only used the power alone.

Instead of telling you that I'm smarter than you, I'll show you how humble I am. If you look at the About page, in the Thanks section, at the very end, you'll find the name Suresh Gopalan. He's one of the few who wrote me to tell me they found a mistake on my work. He's the only one who was actually right. If you find a mistake in my work, I will add your name to this list and I promise that my Thank You will be truly sincere.

All I care about is the knowledge of science and its sharing. All my argumentation is made towards that goal.

### RE: Torque and power quiz

#### Quote (BrianPeterson)

Would the people fixated on horsepower numbers and nothing else, and think that everything can be taken care of with gear ratios, please explain to the class why a 9 litre 6 cylinder diesel engine is appropriate for propelling a box truck, and a far smaller and lighter Suzuki Hayabusa engine that makes the same (ish) power is not appropriate ... think of all the extra cargo you could carry if you were not lugging that huge engine around which weighs the better part of a ton on its own.

Perhaps the same people could explain why the reverse is also true ... bearing in mind that the diesel engine weighs twice what the whole bike does.

I've already tried, in this very thread, but obviously something must not be sinking in.

The explanation is perfectly simple, as I already pointed out 11 days ago. The topic of this thread is the quiz and the quiz is a THEORETICAL exercise attempting to help people understand the relationship between HP and torque.

### RE: Torque and power quiz

#### Quote (Jack)

I developed it and showed how I did it, up to the last detail. For what I did not do, I cited my sources. It is all there for you to analyze. So trust me when I'm telling you that nowhere in that simulator, the engine torque nor the rpm was needed to determine the acceleration of a vehicle. I only used the power alone.

Not too surprisingly, I found that you are considering many of the same things that I have, and you even define top speed in specifically the same language that I do (when a = 0.000...). And a few that I haven't, at least not to date (aero lift, off the top of my head). Unfortunately, in terms of HTML or whatever language those pages were created using, I'm less than a newbie, so it's taking a while.

I do have a question - since you're using power, and since there are plots involving longitudinal acceleration, what are you using for power for determining acceleration at points that would require operating the engine at other than peak power rpm? There's an acceleration curve with a couple of wiggles in it that cannot be explained by anything I've found yet. Oddly enough, I see torque determined before HP, although it is not peak torque.

I think a factor for at least overall powertrain efficiency is in order. As an example, 300 HP engines identical to the one in my Mustang typically dyno something like 265 - 270 at the wheels, this most likely being measured using the direct (1:1) 4th gear, which even if you ignore all the gears in constant mesh not carrying power still leaves U-joint and hypoid axle gear effects absorbing torque/wasting power. About rotational inertia - flywheel, clutch, tire, and wheel weights and diameters are subject to considerable variation and aren't necessarily fitted to a vehicle with optimum acceleration in mind. Hence, it might be useful to keep them independent of total vehicle mass and (better still) separate from each other. If nothing else, it would allow comparisons involving changes to those components (is a 5 lb weight loss per wheel worth doing? how about that aluminum flywheel instead?).

More challenging, and of particular interest to the dragstrip guys, would be a good acceleration model for the first 60 feet. I don't have one, and it's not for not having tried. If I was smarter, maybe I might.

About top speed and the difference between theory and reality . . . if the car is not geared optimally to put the engine right at peak power rpm at that speed, you won't get there. There used to be a magazine that published drag at 100 mph in their data panels (I think it was "Car Life", which ceased publication decades ago). Some constant or other times the cube root of (HP over drag@100) would get you to a theoretical top speed. Whether the car was geared to actually accomplish that feat was of course a separate question, but it was still better than wild guesses and wishful thinking.

Norm

### RE: Torque and power quiz

First, you must understand the objective of the simulator, which is closely link to the subject of this thread: Identify the important characteristics of a vehicle to achieve a certain level of performance. What do you need to do the ¼-mile in 11 s, reach 150 mph or go from 0-60 mph in less 4 s? Before fine tuning gear ratio & power curves, before reducing the weight of your wheels, What are the basics? This is where it relates to the OP: You need power and traction. If you don't have enough of either of those thing, you are wasting your time studying any models you are making.

What the program does is giving you a realistic maximum performance level for a given vehicle. No matter what fine tuning you do, it will most likely never be better than what is shown on the graphs.

So it is assumed that the vehicle is at constant power all the time. Knowing that most people who will use this tool will use it for a vehicle with an ICE, I introduced some average inefficiencies (89% for typical drivetrain and 94% for direct drive) and I've assumed the average power that can be maintained is 95% of the maximum power given. More info in the Converting engine power to wheel power section.

The first part of the acceleration curve is limited by traction. The best drivers will maintain enough power to be at this limit in the first part of their acceleration (Of course we assume the drivetrain is designed to reach that limit as well). The «wiggles» are introduced by an assumption I made, that there will be some kind of transmission that is shifting, to make everything more realistic, as explained on the site:

#### Quote (HPWizard.com)

This site assumes an average mass factor of 1.07 for a direct drive. For a typical road vehicle, it assumes a 6-speed transmission shifting every 1/6th of the top speed with the following mass factors for each gear ratio (from first to last): 1.19, 1.12, 1.08, 1.07, 1.06 and 1.05. This will also work with most older vehicles with 3- or 4-speed transmission as well, as such vehicles are usually not set up to reach their potential top speed.

My goal was to quickly compare vehicles, with as little data as possible. If I want to evaluate how a 1964 Corvette, a 1982 mazda RX-7 and 2006 Audi A4 compare to one another, what do I need to know? I've reduced it to 4 variables: Power, weight, traction and vehicle layout, which are data easily available (except for traction; even though it is probably the most important one, performance-wise). I make assumptions for all others and you can input your own aerodynamic, rolling resistance and basic vehicle geometry if you wish. But even if I let the user input them, he will most likely guessed them just like I do, and if he has access to the real numbers, he's probably not looking for free apps on the web to do his calculations.

You can find a «usual» method to do the same calculations on one of my «competitor»' site, blackartdynamics.com. You have to input much more data ... and then the program will do assumptions anyway (For example, the frontal area is calculated based on the width X height, not sure how). The engine power curve is based on max torque and max rpm. Not even max power! You have to enter data to model a torque curve until you reach your max power. Very tedious and you need a lot of data. If your sources are magazine articles and such, good luck!

In the end, you will find that the results are mostly the same with both simulators. IMHO, close enough that, with all the assumptions made, you are probably well within the margins of error.

I guess my point is that there is no point to introduce a lot of precision in your input data, if that data is made of wild guesses anyway. It's just discouraging and that's when everyone sees math & physics as boring and useless.

#### Quote (NormPeterson)

More challenging, and of particular interest to the dragstrip guys, would be a good acceleration model for the first 60 feet. I don't have one, and it's not for not having tried. If I was smarter, maybe I might.

I feel your pain. But after discussing with some tire engineers, the drag tire has a very special compound which has a friction coefficient that varies a lot, especially at launch. I'm pretty sure some very hard to find data has to be known to have a good precision in that small period of time.

You probably did not notice, but on the different graphs on my simulator, the data appears when you point your cursor on the curves. If you point your cursor on the time curve for the ¼-mile at the 60-ft line, you will have the precise time.

Here's a random case study to show how you can use the simulator for reverse engineering:

Take this Corvette.

I entered the following data (you have to uncheck Simplified version):
• (transmission) Engine power: 887 hp
• Mass: 3300 lb (approx. reference)
• h/L: 0.18 (guessed)
• F/R: 53/47 (reference)
• Friction coeff.: 2.68
• Rolling resistance: 0.02 (guessed)
• CdA: 0.583 m² (approx. reference)
• ClA: 0 m² (guessed)
I adjusted the friction coeff. to get the 60-ft time and the power to get the ¼-mile time and speed. You'll note that the 1/8-mile time & speed match also.

The friction coefficient seems rather low for a drag slick; So the car/driver probably doesn't maximize its grip for the launch (transmission/suspension/driver?). If I convert to wheel power, I get 789 hp. If the car is known to produce more than that at peak power, the gear ratios or shift points are probably not well selected to fully extract the maximum power from the engine. And that is true power at the track, not «SAE net» standardize.

### RE: Torque and power quiz

Quick thoughts . . .

I guess it's out of scope for the purposes of your model, but suppose you wanted to see what happens in street settings where you might start out with the engine nowhere near its power peak. Something like acceleration from 30 - 70 starting in 3rd gear as might happen as you accelerate up to highway speed from an unusually slow ramp corner.

I would think that blackart would need to specifically use peak HP and its RPM just to define a second point on a torque curve. Between two points and a pair of slopes you ought to be able to get something usable enough for making comparisons.

Variously, I've played with tire grip and several other parameters for the 60' matter (there are many inputs). To me as a road course guy it's really just to get the sheet to reflect reality as opposed to any performance I'm particularly interested in achieving as a driver, but I would like to get it there eventually.

I did not notice anything pop up when I hovered the mouse over the curves because it comes up very slowly. I have no idea if the slowness is a problem at my end, your end, or with something in between.

FWIW, I recently ran my sheet a couple of times for the new Z/28 and got quarter mile numbers right around 12.3/116. Car and Driver got 12.7/116 and Motor Trend got 12.3/117.2 . . . some of the Camaro5 dragstrip guys want to see an 11.9x out of that car in stock form pretty badly, and I really don't see it happening except by freakish confluence.

Norm

### RE: Torque and power quiz

#### Quote (NormPeterson)

I guess it's out of scope for the purposes of your model, but suppose you wanted to see what happens in street settings where you might start out with the engine nowhere near its power peak. Something like acceleration from 30 - 70 starting in 3rd gear as might happen as you accelerate up to highway speed from an unusually slow ramp corner.

The closest thing the model will tell you is the lowest time you can possibly take to go from 30 to 70, assuming you can get near peak power. You take the time from 0-30 and subtract it from the time 0-70 and you get the 30-70 elapsed time.

I know the situation that you described can happen in real life, but I would think that if someone is looking for performance, he would downshift to get in the peak power rpm range. And that brings us back to the OP and the importance of power vs torque in acceleration.

### RE: Torque and power quiz

Story time:

I remember some years ago, a university lecturer of mine had this misconception in a machine dynamics class. He said to a couple hundred students that if you want your car to go faster you need to buy a car with more torque. He said it multiple times, and this annoyed me because I hate people in positions of authority handing down completely false dogma. I wrote up a really simple bit of maths during the lecture that showed that with power and vehicle mass (engine and vehicle) I could predict acceleration, but with torque and mass I couldn't. Showed it to him during the break. He looked at it for 10 seconds then waved it off and said he couldn't go through it (this is highschool level maths by the way) but he had graduate students that had analysed it and they came to the conclusion that torque was what was important.

Could never take him (or his grad students, apparently) seriously again.

In any case, I have a feeling you will lose most people with that quiz of yours because once the thinking gets tough, they will simply close the tabs and say 'meaningless academic tripe', and won't engage hard enough to realise what you are trying to teach them. You really need to get it through to them in one profound sentence. The quiz feels a bit patronizing to me and it will feel more so to someone struggling with answers and getting told he is wrong.

And if this is your reaction to the torque vs horsepower debate, you're gonna really have a fun time with all the other stuff floating around with the automotive hobby (which is rife with this stuff, because it is just that level of accessible vs complicated where everyone can form an opinion but no one can verify it), all the vehicle dynamics misunderstandings are massive and far harder to explain/prove. At some point you just gotta ask them what happens to an airplane on a treadmill and fade into the distance while they all quote their qualifications at eachother right after completely missing the point.

Good luck!

### RE: Torque and power quiz

HP is an arbitrary number 33,000 lb ft per minute, it was a number that was settled on as the definition of 1 horse power. Torque is the true quantity measured.
As far as testing an IC engine on any sort of dynamometer or prony brake, there are losses that gum up the accuracy.
Again HP is a calculated value from the measured torque.

### RE: Torque and power quiz

#### Quote (dicer)

HP is an arbitrary number 33,000 lb ft per minute, it was a number that was settled on as the definition of 1 horse power. Torque is the true quantity measured.
As far as testing an IC engine on any sort of dynamometer or prony brake, there are losses that gum up the accuracy.
Again HP is a calculated value from the measured torque.

HP might be an arbitrary number, but it is a unit. What this unit represents is power, and that is a very real thing.

Power is the result of the combined effect of torque AND rpm. That is what is important to understand. Torque by itself doesn't tell the whole picture, neither does the rpm. You need to know both, and that what power is useful for.

Imagine your statement rewritten this way (changes highlighted):

HP is an arbitrary number 33,000 lb ft per minute, it was a number that was settled on as the definition of 1 horse power. rpm is the true quantity measured.
As far as testing an IC engine on any sort of dynamometer or prony brake, there are losses that gum up the accuracy.
Again HP is a calculated value from the measured rpm.

It would still be true, but that doesn't mean that an engine that can reach 10 000 rpm will give better performance than one reaching 5 000 rpm.

### RE: Torque and power quiz

Land and Sea is a dyno manufacturer in Concord, NH. They offer a bunch of dynos and dyno upgrade options.

From Land and Sea's website -
" inertia-only "dynos" can not directly measure torque "

http://www.land-and-sea.com/dynamometer/dynamomete...

So, for dynos that track roller rpm vs time, and thus compare the change in kinetic energy, HP is the more direct measurement, and torque is "derived."

### RE: Torque and power quiz

Yes. The fact that measuring power output of a rotating type engine is most easily and accurately achieved by measuring torque and rpm, does not make torque a more fundamental quantity.

What engines do is convert heat energy into mechanical energy. The rate at which they perform this conversion is the power output and is the most fundamental quantity to be measured.

There are engines coming with no rotating parts whatsoever, converting linear motion of free pistons, directly into electricity via induction coils. What will be the fundamental output from these engines - Volts?, Amps? or power?

je suis charlie

### RE: Torque and power quiz

Combustion pressure times the area of a cylinder bore = some force.

Some force operating at the crankpin = a torque about the crankshaft's axis.

Seems pretty fundamental to me, though I guess you have to be thinking in terms of a free-body diagram. The analogous linear motor's property would probably have to be the force required to push the free pistons through the magnetic field.

Norm

### RE: Torque and power quiz

2
torque is fundamental but it still doesn't tell you how useful the "engine" is unless you also know rpm.

or in the terms of a linear frame of reference where force is fundamental you need linear velocity to know how useful the "engine" can be.

i like the comments about converting chemical/heat energy to mechanical energy being why power is more relevant. if only people looked at energy methods for thinking about things.

For example:

- Accelerating a car from rest to say 60mph requires none other than a change in kinetic energy, the time taken to achieve this governed by energy/time (i.e.POWER)

- Going up a hill with 200m change elevation at constant speed is simply change in potential energy, the time taken to achieve this i.e the SPEED at which you can do this is governed by energy/time (i.e.POWER)

### RE: Torque and power quiz

#### Quote (ionline6)

- Accelerating a car from rest to say 60mph requires none other than a change in kinetic energy, the time taken to achieve this governed by energy/time (i.e.POWER)

- Going up a hill with 200m change elevation at constant speed is simply change in potential energy, the time taken to achieve this i.e the SPEED at which you can do this is governed by energy/time (i.e.POWER)
Power may make for the simplest explanations in such cases, but it's not the only possible approach.

If, and let's say in the absence of discussions such as this one, you were to look at any number of dyno sheets with both torque and HP plotted, you'd probably conclude that an ICE is closer to being a constant torque device than a constant power machine. Much closer in most cases, I suspect.

Norm

### RE: Torque and power quiz

the fact is that peak hp is a better indicator of performance potential than peak engine torque. the truth is the average hp in the operating band is more accurate again, yet the average crank torque is next to meaningless as a measure of performance potential.

if one was to use a CVT transmission with infinite possible gears and wanted to generate the most tractive effort at every possible speed assuming sufficient traction then they would run it at the rpm of peak hp as opposed to the rpm of peak torque.....

### RE: Torque and power quiz

when you look at performance potential based on engine torque numbers you need rpm, whereas with power you do not as it comes out in the wash so to speak

### RE: Torque and power quiz

I'm glad that you included the word 'potential'. Because in virtually all daily driving, non-competitive track day sessions, and even for a large part of actual competition where there are corners to be negotiated you won't be operating at peak power very much of the time. Even conventional (non-CVT) automatic transmissions don't kick down such that the engine is driven to run all the way up there either.

Really, you should keep rpm in mind when using an estimate of actual acceleration based on power, if only in order to have some idea about how much of it you really have available for the situation at any given instant.

I think I mentioned something about CVTs a month or so ago . . .

Norm

### RE: Torque and power quiz

#### Quote (NormPerterson)

Really, you should keep rpm in mind when using an estimate of actual acceleration based on power, if only in order to have some idea about how much of it you really have available for the situation at any given instant.

I don't understand what you mean, could you give an example? Like I showed you in my simulator, I don't know anything about engine rpm (or transmission or wheel rpm for that matter) and can still give an estimate of acceleration. What if you have an engine that doesn't rotate, as previously mentioned by others? The fact is that once you know the velocity of the car, every other car components' velocity is a mere geometrical relationship for «matching» purposes.

In the end of it - to get the greatest acceleration - if you want to do the proper «matching» of the engine rpm to vehicle speed, do you need to match the maximum torque of the engine or the maximum power of the engine? The maximum power will always give the greatest traction force at the wheel.

And even if you don't want the greatest acceleration; Or better, you want zero acceleration and keep the speed constant, are you gonna match your engine rpm and load to obtain a defined torque or a defined power? The answer is again power. Yes, at the wheel you need a defined traction force and velocity, say 1200 lb @ 45.8 ft/s or 100 hp. But you could set your transmission in low gear, setting your engine at 6000 rpm and adjusting your throttle to get 87.5 lb.ft or you could be in high gear, setting your engine at 2000 rpm and adjusting your throttle to get 262.6 lb.ft. As long as you get 100 hp. That is why we say that torque or rpm doesn't matter, only the power, i.e. the combination of both torque and rpm.

### RE: Torque and power quiz

once the rpm get to peak power at 40mph they stay there

kicks down to 5500rpm which is very near peak hp

naturally the goal isnt always maximum performance in a road car so the system can be "tuned" to give certain results at part throttle

of course with conventional manual you try and maximse power band but sometimes the gearing is non ideal for the corner and you do as best you can.

### RE: Torque and power quiz

Interesting videos, inline, and very much what I'd expect.

The way I understand it, there is some sort of torque management going on in Nissan's CVTs. You'll note that the revs initially flash to about 3000, well below the peak torque point (4400), and remain there until the car has moved through about 15 mph. My guess is that the torque capacity of the drive "pulley" (available friction at the small diameter for large "gear reductions"? ) represents some sort of limitation. Nissan's CVT did have a few "teething pains" as I recall.

That at least Nissan's V6 CVT-equipped cars offer soft initial acceleration is a fairly common complaint on the Maxima forum that I belong to. But it's not at all surprising that those who can get past the slightly sluggish zero-to-20 are quite happy with the performance otherwise.

Norm

### RE: Torque and power quiz

The initial flash to 3000 rpm is due to the torque converter - these transmissions have a conventional torque converter. It locks shortly after starting off from a stop. Note the flashing orange light on initial start-up ... no doubt this is the traction control system limiting torque output.

Tests like this don't expose that style of CVT's weakness ... transient response when an abrupt ratio change is demanded. Yes, normal automatics can have that weakness, too, but it's dependent on programming rather than on how quickly the cones and belt can shift position. Yes, you can ask either transmission to hold a lower ratio ... thus throwing away the efficiency benefit.

### RE: Torque and power quiz

Examples are easy, Jack

Street driving - and your satisfaction with your car in that environment - is more about response than it is about drive-it-like-you-stole-it absolute maximum performance. Quite a lot of near-wide-open-throttle and WOT driving falls not just within the regions identified as "unavailable power", but also to the left of the intersection with the line representing availability of a still lower gear. In nonsteady speed driving averaging 40 to 45 mph I commonly drive around in 4th which is about 2000 rpm, which is neither lugging the engine nor forcing it to feel "too busy". Should I feel a need for more acceleration than 4th can provide, 3rd has enough to get the job done. I just don't encounter situations where grabbing 2nd for 10 or 12 mph would be necessary or make much sense.

There are turns on the road courses I have driven on where running in 2nd gear would just barely be possible. However, since cornering eats up a good amount of drive axle grip it is worse than pointless to use 2nd, especially given that an upshift to 3rd would be required barely past the apex for rpm reasons regardless of how much or how little of the "extra" acceleration potential you could actually use. Non-optimal gearing? Of course. But no single transmission gearset, axle ratio, and tire size combination can be optimum for all corners at all road courses. You work with what you've got, just like you do on the street, and it's almost never along the max power line.

Maybe it's because I am NOT oriented toward drag racing (or its illegal street cousin and the sort of bragging that goes with that) that it's always been easier for me to work with torque. Doesn't mean I don't understand it from a power point of view. The easiest way for me to illustrate this is that when I downshift I'm specifically seeking a larger mechanical advantage for the available torque rather than accessing a greater amount of power. Pretty much the same thing with a different mindset . . . notice that I didn't say anything about chasing a bigger torque number either.

Norm

### RE: Torque and power quiz

#### Quote (NormPerterson)

The easiest way for me to illustrate this is that when I downshift I'm specifically seeking a larger mechanical advantage for the available torque rather than accessing a greater amount of power.

But when you say this, combined with your previous statement «that an ICE is closer to being a constant torque device than a constant power machine» I would think that a downshift is more of a power increase than a torque increase.

When you downshift - assuming same throttle opening - the engine torque stays more or less the same and the engine rpm increases, such that the net effect is a power increase.

Of course, at the wheel, the rpm is the same and the gear ratio translates the power increase into a torque increase at the wheel, but still, wouldn't you agree that it is fundamentally an increase of power from both the engine and wheel points of view?

### RE: Torque and power quiz

Like I said above, I'm looking at a downshift as providing a more favorable mechanical advantage (it's certainly what you make happen inside the transmission, regardless of what's going on with the engine ahead of it).

Yes, I could think of it in terms of accessing more power, but that just isn't the way I visualize it.

As a structural guy, things like torque (aka "moment"), mechanical advantage(s), drive tire radius, vehicle mass, rotating masses, etc., are all easier to picture as pieces of the whole than power is as sort of a shortcut around needing to know some of them. It's certainly easier from a torque perspective to compare the results of changes made individually to, say, transmission or axle gearing or tire size, when you're actually working with those items as variables.

That does bring up a different point - why would people do these sorts of performance predictions or analyses? If it's for estimating maximum performances only, using power is appropriate. But if your intent is for help in deciding whether some given mechanical modification is going to give you what you hope for, or if the improvement it's predicted to give is going to be worth the time and expense, maybe the power approach isn't sufficiently detailed.

If I was looking to maximize acceleration around some specific speed or speeds, or to maximize top speed, of course I'd choose gearing that would have those speeds put the engine up around or right at peak power. If I was a drag racer and serious enough about it to keep tinkering, I'd probably gear my car to be running a little past peak power rpm as my car crossed the finish line, which would either require knowing the peak power rpm or involve a number of gear swaps. But once that gearing had been finalized, what I might want to know about acceleration at other speeds and/or in other gears would still be based on torque and all the rest of the variables.

Norm

### RE: Torque and power quiz

Scroll down for scatter plots illustrating the relationship between acceleration and power/wt and torque/wt for a number of actual vehicle tests. Shows a clear correlation of acceleration to power and none for installed torque.

je suis charlie

### RE: Torque and power quiz

If there was an EDIT function I would have included RPM that is the time factor that determines the HP from the measured torque.

### RE: Torque and power quiz

#### Quote (Tmoose)

So, for dynos that track roller rpm vs time, and thus compare the change in kinetic energy, HP is the more direct measurement, and torque is "derived."

I've seen that argument before for an inertia dyno, but the dyno very much uses the basic torque = inertia x delta speed / delta time formula for the torque around the roller shaft. But, it will typically show the HP curve first because the HP is simply the HP so it's the easiest curve to create. Heck, a meaningful HP curve can even be created without knowing engine rpm. But, for the torque to be meaningful it really needs to have engine rpm on the X axis. So, the dyno first has to take the engine rpm calculation from the ignition pickup and figure out the effective gear ratio between the engine shaft and the dyno roller shaft before calculating the engine torque vs rpm curve.

### RE: Torque and power quiz

#### Quote (gruntguru)

Scroll down for scatter plots illustrating the relationship between acceleration and power/wt and torque/wt for a number of actual vehicle tests. Shows a clear correlation of acceleration to power and none for installed torque.
I made a few stops along the way, and the derivation specifically starts out with torque, power not appearing until later. Hmmmm.

#### Quote:

SO WHAT IS POWER AND WHY IS IT USEFUL?

An Engine's power output is simply Flywheel Torque x Flywheel RPM. Essentially it is a figure which tells us how much potential the engine has for accelerating a car when it is provided with the correct gear ratios. We can use it to simplify the equation above:-

(((Power / Road Wheel RPM) x Driven Wheel Radius) - (Resistance Forces)) / Mass = Acceleration
He also uses the word 'simplify' . . . IOW, a convenience (without implying there's anything wrong with making life easier here).

#### Quote:

Now we can see that more Power = more acceleration; we don't need to know if this power is generated by having more flywheel torque, or a higher RPM, we just need to know it is higher.
For theoretical comparisons, sure, that is all you need (I've never argued otherwise). For design/redesign purposes with at least some of the hardware "fixed" and only knowing what that power is, probably not. Using power at 50% of peak power revs might be more useful, but it's not inherently better than using something like 85% or 90% of peak torque given that the values of such variables as gearing and tire size are readily available.

Norm

### RE: Torque and power quiz

#### Quote:

Heck, a meaningful HP curve can even be created without knowing engine rpm.

I'm afraid I'm not grasping what you'd be plotting HP against if not rpm.

Norm

### RE: Torque and power quiz

"Quote: Heck, a meaningful HP curve can even be created without knowing engine rpm.
I'm afraid I'm not grasping what you'd be plotting HP against if not rpm.
Norm "

Power can be plotted against vehicle speed, not knowing engine rpm or even if there is a rocket engine or any engine involved at all. It can be plotted against time or accumulated energy (1/2mv^2), etc.
This is especially relevant when we plot hp vs vehicle speed under acceleration.

I think engineers look at quantities as "fundamental" if they are easiest to measure. On a brake dyno you just look at the reading on the scale and multiply by the radius of the beam. But the scale measures force, not torque and it is represented as potential energy (compression of a spring or lifting of a balance weight, etc). Therefore force is a derived quantity.

### RE: Torque and power quiz

Just to elaborate on the brake dyno, the irony is that to load the engine you use a friction brake or water brake, etc because, unlike a static torque measurement you have to have the engine running. The process produces a lot of heat and this heat is proportional to the engine's speed and the retarding force which shows up as weight on the scale. This is exactly POWER. It takes power to measure torque on a dyno. So torque is derived from power. But, it is not convenient to measure the delta T and precise flow rate of the cooling water so engineers get the power figure by calculating backward from the measured torque.

The vehicular power figure for a car without an engine or engine not running would be for the brakes retarding a downhill coast or, like in "Furious 7", a car dropped from a plane converting only some of the potential energy into kinetic -which would be a perfectly efficient process except for air drag. The power figure would start at zero and build up to the point of terminal velocity and level out, slowly increasing with decent into thicker air.

### RE: Torque and power quiz

You could plot power vs % if you wanted. Assume the initial dyno rpm is the starting percent and the peak dyno rpm is 100% speed. Fairly simple to do and it would roughly gives you the power vs rpm once you equate 100% with the peak rpm of the engine.

### RE: Torque and power quiz

I really need to type better. If you plot HP vs % dyno rpm by using the peak dyno rpm as 100% then you get a HP curve which you can roughly match to the engine operating range. You can't do that with torque and get a meaningful curve.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!