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curious about engine breaking

curious about engine breaking

curious about engine breaking

(OP)
I don't know if anyone can answer this question.  But I was just curious if engine braking a car (manual transmission) has any negative effect on the motor, or gearbox?

RE: curious about engine breaking

I do know that it's cheaper to replace your brakes than it is your engine.

RE: curious about engine breaking

I agree with the second poster, but so long as you do it correctly the loads are well within the capabilities of the components. Unless I am on a slippery surface I brake in neutral.

Cheers

Greg Locock

RE: curious about engine breaking

I agree with both previous posts except that it is may be  cheaper to replace motor and gearbox than to repair damage caused by loss of brakes due to overheating. It depends on why you want to use the engine as a brake.

If you need to use the brakes to the extent that fade becomes an issue, also use engine braking, otherwise, Greg's advice is sound, although it is mostly more convenient to just brake in the gear you are in , or the one you will need after braking with the downshift occurring during the braking.

Regards
pat   pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: curious about engine breaking

Brake fluid is hydroscopic (I hope I spelled that right) meaning that it has a tendency to absorb moisture over time.  If you have ever had your brakes get really hot, like on a long mountain down grade, and had the moisture in the brake fluid boil, and all the brakes just go away, (I have, more than once) you will use all the engine you can, including turning on accessories to absorb power, and use the brakes as little as possible.

That is the way my old daddy taught me to drive, anyway.

Now, in an area where engine braking is enhanced through the use of devices that modify the valve timing, and convert the engine into a compressor during engine braking, on long haul diesel rigs, the engine wear factor due to engine braking is documented at about a 15% penalty on engine life.  So, while probably not so severe for your case, it is nonetheless real.

rmw

RE: curious about engine breaking

If I down shift by e-a-s-i-n-g out the clutch and dragging the engine up to speed, rather than matching revs a bit before simply enagaging the clutch, I suppose each down shift adds a tiny bit of extra clutch/flywheel wear.  Starting-from-rest techniques and Shifting like that, up or down, might be the difference between a clutch lasting 3,000 miles (young man with 427 Camaro) and 240,000 miles (my old Volvo 240).

I think my current car has PLENTY of brake capacity even for coming down Mt Washington, so the overheated brake defense would be like wearing elephant repellant.

A greater issue for me is the smooth-ness upsetting the chassis or my passengers.

RE: curious about engine breaking

Tmoose,

If you ever loose your brakes on a long (and usually curvy mountain road), due to overheating or whatever, post back, and I will recommend a good brand of elephant repellant.  Actually your laundry man will wash a real good brand of elephant repellant out of your britches.

rmw

RE: curious about engine breaking

No adverse affect whatsoever, assuming you use good shifting/clutching techniques, ie speed matching, no lugging. Gives good braking quality without excessive nose dive, very important for cornering.

RE: curious about engine breaking

Constant negative pressure in the cylinders with the throttle closed may suck some oil past the rings and valve guides, but that will only result in a puff of smoke when you accelerate again.

Jeff

RE: curious about engine breaking

This is a bit off subject, but does anyone know if a car stops using fuel when engaged in engine braking? I heard this a while back, and it makes sense to me, but started quite a discussion (arguement, really) on a roadtrip this weekend.

Thanks.

RE: curious about engine breaking

Depends on the car.

Many (most? virtually all?) EFI or electronically controlled carburetter cars do shut the fuel off, most of the time, when the engine is in overrun, ie throttle is closed but revs are higher than idle.

However, in order to prevent excessive contamination of the cat by drawn over oil, or maybe just to keep the cat hot, several engines seem to give a quick burst of fuel at around 1800 rpm on decel. (VW Jetta 1988, Nissan funny little sports car from 1995 I particularly remember). This can be quite annoying.



Cheers

Greg Locock

RE: curious about engine breaking

I guess I'll add my two cents worth---having raced drum braked cars in the 50's and been left "wanting" on more than one occasion---a one hour race ment that you did the last 45 minutes sans brakes!
My dad tought me to drive in cars that required engine braking if you wanted to consistantly make it down from Cloudcroft, NM to Alamagordo---steep down hill mountain road in the 50's (still steep).  Times have changed.  Brakes no longer "fade" like they once did.  In fact, my wifes car has better brakes than many of the vintage racers I compete against today.  I still race, but no longer use the engine for anything but acceleration.  Braking is solely by the quite capable brakes.  Many of my friends drive modern sports cars and have never learned to use the engine for deceleration.  So far as I can tell, there is just no need, under normal conditions, to use the engine for braking.

Question---does engine braking decrease the life of trans or engines?  In the "old days" I did commonly replace "U" joints and axels rather often.  A good lifespan for an engine was 100k miles.  A clutch was good for half that and valve jobs often came as early as 20k miles!
Was the cause the common practice of using the engine as a brake?  Was it inferior materials or construction design/tecnique?  Can we really compare yesterdays automobiles with todays?  I think NOT.
To assume that most people driving today can master the art of heel/toe would, IMO, be a stretch.  Indeed, most drivers nowadays can't even DRIVE a standard shift in the U.S. and the minority that can wouldn't know a 'double clutch' from a wing nut!

Rod

RE: curious about engine breaking

Whichever side of this old argument you come down on, I think it's important that you learn _how_ to decelerate a car with just the engine, because every once in a while, for a variety of reasons, you will be driving a vehicle with no useful brakes.  

With luck and practice, it could happen to you more than once in a lifetime.

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

RE: curious about engine breaking

Evel,

My first car was a BMW 2002, the Getrag transmission was infamous for its poor synchromesh.  The gas and brake pedals were however perfectly set up for heel/toe.  So, I learned out of necessity to do both, either with or without use of the clutch pedal.

While the car I currently own is an automatic, on the rare occasions when I do drive a manual I still double-clutch my downshifts as a matter of habit.  Passengers often ask what I am doing.

RE: curious about engine breaking

Yeah, my double declutching's like that!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: curious about engine breaking

AS long as you do not exceed the ratings of the drive components, engine braking shouldn't be a problem.

I do a fair amount of mountain driving, to and from mountaintop transmitter sites.  On the way down the mountain I ride in a lower gear letting the engine do the braking.  I apply brakes only when necessary for traffic or unexpected road conditions.

The "Sky Islands" of the Desert South West are quite a tourist attraction during the heat of the Arizona Summer or when snow covers the mountains above warm and sunny Tucson.  When traveling the Mount Lemmon Highway, the smell of burning brake linings permiates the air.  Flat Landers haven't learned the old mountain driving trick, Go down the mountain in the same gear you needed to get up.

Some truth can be found in most of the other posts in this thread.  Remember, everytime you make a service application of your brakes, you are wearing off brake lining and drums/rotors.  Often, after a long downhill run, you'll need them at or before the bottom.  Save the linings, rely upon manifold Vacuum (or air compression with a Jacobson style engine brake in a Diesel) to slow the vehicle and a service application of the brakes to stop.

I learned to drive in a '52 Chevy 1/4-ton pick-up and graduated to an Austin Healey and Shelby Mustangs.

I remain,

The Ol' Soldering Gunslinger

RE: curious about engine breaking

The front side of Mt Lemon is a piece of cake.  Try going down the back side without brakes!

Rod

RE: curious about engine breaking

Anyone who does any serious off-roading or rallying brakes with the engine, for general road use I would not say it is necessary but still a good tool to have in your box.

RE: curious about engine breaking

"Anyone who does any serious off-roading or rallying brakes with the engine,"-ajack1

yep, i prefer the engine braking when i dont need weight transfer, to make the front bite up and the back swing around in the infamous swedish flick a little LFB (Left Foot Braking) usually will do.

I find it interesting that no one here mentioned the use of engine braking in performance driving.

RE: curious about engine breaking

evelrod - I still race, but no longer use the engine for anything but acceleration.  Braking is solely by the quite capable brakes.  

NickE - I find it interesting that no one here mentioned the use of engine braking in performance driving.

Actually, someone specifically mentioned NOT using engine braking in performance driving.  Judging by the video footage, I'd say he's got a bit of experience too...

RE: curious about engine breaking

Incidentally Nick, you'll get the same load transfer so long as the retarding force is present at the contact patch.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: curious about engine breaking

For the past few months, I have been teaching my 15 year old daughter to drive a manual transmission.  She has taken quite nicely to catching a lower gear going into a corner (and at least partially using engine braking)and accellerating through the turn...much to her mother's chagrine.  In a month or two, I plan to graduate her to something a little hotter than the DOHC 4-banger in the '02 Ford Focus...like the tri-power 390 in the '64 Galaxie Rag-Top...Pray for me

I remain,

The Ol' SGS

RE: curious about engine breaking

Quote (ivymike):

"evelrod - I still race, but no longer use the engine for anything but acceleration.  Braking is solely by the quite capable brakes.  

NickE - I find it interesting that no one here mentioned the use of engine braking in performance driving.

Actually, someone specifically mentioned NOT using engine braking in performance driving.  Judging by the video footage, I'd say he's got a bit of experience too...

Oopss... I'll just go back to lurking.

Greg- That may be true, it just doesnt feel the same, that may be the way my brakes are biased (pad cpd, disk size). I tend to see much more dive with (?foot?)braking than with engine braking.

Thanks for pointing out the errors...
nick

RE: curious about engine breaking

Nick, you see more "dive" with the brakes possibly because of geometry of the suspension bits and the fact that the brakes are MUCH more effecient at decelerating the vehicle.

SGS, steer clear of the high hp street machine for your kid.  BTDT and it aint pretty in the long run.  Instead, why not pick up a little used Miata and trick it out with the Spec Miata kit and take her to an SCCA approved professional school?  Cost in the long run will be MUCH less, trust me on that one!!!  Keep the high performance and racing on the track and leave the freeways to the rest of us dorks. (Being a competant and experienced race car pilot don't mean doodly on the street, but racing on the street is sure death)!!!!!!!!

Rod

RE: curious about engine breaking

Nick, Evelrod is pointing you in the right direction. By using engine braking you are using the anti squat geometry of the suspension for that axle, whereas when you use the wheel brakes you are using the anti (or pro) -dive characteristic of both axles. antisquat and antidive are similar in concept, but different geometrical constructions are required because the brake torque is resolved locally, whereas the drive torque is resolved at the diff in an IRS or IFS.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: curious about engine breaking

Thanks, and of course with every answer new questions are raised.

My car is a Subie, WRX (and like every other young american rally fan it's blue too....). Does the fact that its AWD with viscous diff's in both the rear and center affect this?  

RE: curious about engine breaking

NickE - the AWD does, but the viscous diffs don't (as long as all of the wheels are rotating at the same speed).  

Your engine is providing some braking at both axles, so you're invoking axle-height-based amounts of anti-dive up front and anti-lift out back.  Your outboard brakes work with ground-level-based amounts of A-D and A-L (which are greater than the axle-height-based effects, hence braking with the brakes gives you less pitch than does an equal deceleration using only the powertrain).


All - in slow-moving and stop-and-go traffic I use engine braking to as great an extent as is practical, though that's as much a function of all of my cars having manual transmissions as anything else.  All else equal, more engine braking is available, brakes don't get glazed, throwout bearings last longer, and I'm more relaxed for not having to keep a tap-dance going on all the pedals (save for having to occasionally alert some moron behind me who watches for brake lights but not closing speed).

It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine.  With all the braking out back,

As far as the usefulness of engine braking in performance driving goes, there is some limited applicability at least in autocross with front-engine rear-drive cars, which normally understeer.  You're planting the front tires for better grip and unloading the rears for less grip (directly) while simultaneously applying longitudinal load to the rears (indirectly leaving still less available grip for lateral purposes).  The overall effect is for the fronts to turn in better and the rear to drift a little wide for greater yaw response.  In my case with 5.7 liters of 10.2:1 engine and ~10 mph/1000 rpm gearing this effect can vary from "clearly noticeable" up to "more than I need or even want".

Norm

RE: curious about engine breaking

Quote:

It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine.  With all the braking out back,
should be edited/completed to read


It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine.  With a disproportionate amount of the total braking out back, a spin is entirely possible if you try to use the additional engine braking effects of lower gears.  Let's just say that a car with a rear end that starts making threats about leading the parade for a change has a way of getting your attention.  I can thank one of my folks' 6 cylinder mid-1950's cars for that little bit of (fortunately) no-cost education.

Norm

RE: curious about engine breaking

Engine braking is _way_ more dangerous in a FWD vehicle.

The slightest drag on the front wheels applies a retarding force, _ahead_of_ the center of gravity.  

Result?  Lift, and the car turns around, really fast.

It happens before you can apply the brakes.

I figure it's the real reason why old Saabs had overrunning clutches.

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

RE: curious about engine breaking

I have no intentions of letting my daughter drive the galaxie on a regular basis, however one of these days she'll get behind the wheel of a car with more horsepower than the Focus she's been learning to drive in.  I want to take her somewhere where she can discover that all cars are not created equal.

Gee, I wish I knew someone at the Ford proving ground so I could get her some time on the skid pad.

I, on the other hand, learned on the road Ice of a Nebraska Winter.  Talk about the school of hard knocks.

--SGS

RE: curious about engine breaking

Many winters ago, as instructed by my DriverEd teacher, I was burning donuts on a freshly plowed shopping center parking lot.  Think 'skid pad with cushions'.

Someone in an adjacent residence must have been awake, at two a.m.  Along comes a cop car, so I slide to a stop and roll my window down.

"Son, what are you doing?"

"I'm practicing skidding, Sir."

Cop turns to his partner and says,

"NOW, I've heard everything."

 ... and drives off.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

RE: curious about engine breaking

I had almost the same experience - except when I got pulled over, they tried to pin some prior act of vandalism on me.  Apparently weeks earlier someone had run over the sprinkler system at the high school while spinning donuts on the lawn, and they figured "hey, this guy likes donuts, maybe it was him!"  

RE: curious about engine breaking

I would strongly recommend attempting to teach a relatively inexperienced driver to downshift as entering a corner. Even expereinced, skilled racers can get into trouble doing that.
Use the brakes to decelerate, and the engine to accelerate. They were designed for those functions. Brakes make poor engines, and vice versa.

RE: curious about engine breaking

To enlighten some of you on the effects of engine braking, let me give you some of my thoughts on this.
I have replaced the flywheel on my Integra with an 8.8 lb. flywheel. The factory flywheel is an 18 lb. boat anchor. The reason for this was to make the car more roadcoarse friendly and get more acceleration out of the engine. The whole idea was to lighten the rotational mass of the rotating assy of the motor. It made a tremendous differece.
Now, to get to the point of this thread. Upon taking the car to Gingerman raceway in Michigan, the benefits of the flywheel were very obvious. I was able to use more engine braking while downshifting, (even though I heel toe while blipping the throttle), and it helped me slow the car much quicker due to the lessened rotational mass. The engine was able to slow down quicker which in turn slowed the car faster under engine braking. The acceleration of the car was incredible. The engine really came to life on acceleration. I've seen this happen on an NSX I replaced the clutch in also. I replaced the factory flywheel in it with a 12 lb. It really came to life.
  I'd personaly rather buy brakes than a motor, but I enjoy driving too much to do that! Plus, I can rebuild the motor if need be. At the present, the engine has 40,000 miles on it.....HARD miles, with no issues. I say drive like you want to. Enjoy life while you can.
Have a great weekend guys,
Barney

Barney Demonbreun
Technician for Gary Force Acura, and owner of;
Ashland Motorsports
1304 Big Marrowbone Rd.
Ashland City, Tn. 37015

RE: curious about engine breaking

"go down the hill in the same gear they went up it"-
they do...

"D"

I learned on the junk, but haven't had anything vrey hot yet...
But the 3.8 V6 Corvar-thing may qualify...
Looks like it will be about 2200 pounds all up!

cheers
Jay

Jay Maechtlen
http://home.covad.net/~jmaechtlen/

RE: curious about engine breaking

First:RMW,  that word is "hygroscopic"

Second,  if it were cheaper to use the wheelbrakes than the engine brake, heavy trucks wouldn't all be equipped with "jake" brakes as they are in the U.S.
The Jacobs and similar brake devices enable an engine to develop as much braking horsepower as when accelerating,  and this can be up to 600 HP or so.

Particularly with trucks in the heaviest classes,  it is almoost impossible to fit them wtih wheelbrakes that will enable them to stop as quickly as the average car,  and the fade resistance is not so great either.

Probably 99%+ of the trucks we build today have "jakes."

RE: curious about engine breaking

Ever calculated the effective Horsepower of your brakes? It IS enlightening. Make it easy on yourself, use .5 for the pad Cf.

RE: curious about engine breaking

Or make it easier still,  and assume a .9 G stop.
----------------------
And for Greenracer, who's "never heard of a motorcycle racer using engine braking"  - dare I suggest that your knowledge of motorcycle racing is limited to just the past few years?

Hints:  Stirling Moss,  Mike Hailwood,...

RE: curious about engine breaking

Hi Rob.
I have spoken to Mr. Moss, years ago at the Long Beach GP and since I was on course and on my motorcycle at the time I am sure we may have spoken about motorcycles.   However, to my knowledge, Mr. Moss never raced motorcycles.

Perhaps you ment John Surtees and Mike Hailwood.

As to the question of engine braking on a motorcycle---I still ride the 1948 Norton that belonged to the 4 time World Trials champion Bill Young---I may slow a LOT more than he would have, but engine braking is really dodgy.

Mike Hailwood did not start racing until he was 17 and by that time brakes on most all bikes were quite good.  He won his first championship in 1961 on a 250 Honda.  

Engine braking on race cars of old may have been, indeed, probably was a necessary evil.  You must finish to win and you cannot finish if you relied solely on the drum brakes of some/most of the older race cars, e.g., Jag XK 120 or, in my case, a 50 MG.  You just have no idea how thrilling it is to approach the same corner at the same high speed that you did just  one lap before and reach for the whoa pedal and find it as hard as a rock with no apparent effect on diminishing your velocity!  Early drivers often had large laundry bills.

One last comment---On many of the F-1 and Indy/Cart cars, G loadings greatly exceed 1G.  I saw a accelerometer reading of 4.0 on one Al Unser Jr. car after just a short jab at the brakes. Since I have no firsthand idea how many G's those cars pull in a corner, and my experience was in the early 80's,  I can only assume the magazines and TV commentators are fairly accurate.  I do know firsthand that standing at the apex of T-1 on the Long Beach GP circuit and watching an F-1 car slow from near 200 to a bit over 75 is indeed one of the most awsome sights I have ever witnessed.

Rod

RE: curious about engine breaking

Current truck brakes can't dissipate enough heat to handle long hills, heavy loads, and reasonable speeds all at the same time, I think. That's why they are required to descend a lot of steeper roads slower than cars are.
The engine and exhaust can dissipate quite a bit of energy, and do it for a sustained period. I suspect that it is cheaper overall to accept a bit higher engine wear and avoid the weight and other costs of a brake system that could really do a complete job. No reason, for example, that brakes couldn't use active cooling via liqud coolant and a air-liquiid heat exchanger.
The system could dissipate arbitrarily large amounts of heat and be completely fadeproof.
Of course, it would cost money, complexity, weight, etc.
I'll bet that forcing large volumes of air through each brake assembly could increase capacity greatly. Still costs something- what's the tradeoff? Of course, disk brakes probably cost more than drums, but they are on nearly(?) every car today...
Comments?

Jay Maechtlen
http://home.covad.net/~jmaechtlen/

RE: curious about engine breaking

Quote (JayMaechtlen):

"No reason, for example, that brakes couldn't use active cooling via liqud coolant and a air-liquiid heat exchanger."

This is done in certain forms of racing. WRC Tarmac cars for example. I've also heard rumors that its done on the Semi-Truck racing in Europe.

RE: curious about engine breaking

Evel -
Yes of course I meant John Surtees!   Got his book "Speed" sitting on my bookshelf!

and Jay:
Disc brakes are coming on heavy trucks.  They're being tested now.
Anything that adds weight, though, like the airflow or heat-exchanger ideas,  would be flatly rejected by many truck owners,  since weight  reduces payoload,  and payload is money.

RE: curious about engine breaking

Sorry

I can't resist it any longer.

If you are really curious about engine breaking, as you say in your heading, to experiment, you will need to use a very large hammer as engines tend to be very strong.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: curious about engine breaking

Engine braking has its place.. especially in mountain driving.. and using it effectively might save your bacon some day.  Anyone not using it is asking for trouble..    

The most extreme I've done, and where heavy use is mandatory  was the return trip back down Pikes Peak..   

It's steep enough to require second gear (manual tranmission) and with no throttle it was reving the engine around 4000rpm most of the way, and in some places by adding the compressor load by turning the AC made the difference in not having to continuously ride the brakes..

They have a mandatory brake check stop after coming down the top third of the mountain.. The guy holds his hand about 2 feet from the brakes and if he feels heat at that distance, you are directed into the parking lot for a mandatory cool off..

Monarch Pass, Colorado is another good one.. about a 8 mile downhill grade at 9% slope most of the way.. It's straight enough to be doing 60mph in places and really spooky braking hard coming into a curve...

RE: curious about engine breaking

i think that there really is no more additional wear on the motor unless you do something drastic, such as 3rd to 1st, in reality you shouldn't even be needing to downshift for 1st except for from a stop. 2nd gear is far enough on the downshift unless you have a lamborghini which does like 90 in first!

RE: curious about engine breaking

Hi, I'm new here, but I thought I'd just add a bit of personal experience. Following on a little from the racing motorcycle mention earlier. I do a few track days on my Triumph over here in the UK, and as I've been learning my way quicker I've found engine braking to be very handy. I engine brake all the time when normal braking anyway. But when going into some corners it can be useful to roll off the throttle and weight up the front, giving steeper geometry, allowing a faster tip in. If you had to brake for the corner anyway this is obviously unnecessary.

Just a thought,

excellent forums by the way, some of you guys know more than I ever will!

Kurtis714

RE: curious about engine breaking

I would rather rely on my brakes to slow down my car as they have the antilock feature. If I were to rely on the engine for braking, I would be concerned about going into a skid.

RE: curious about engine breaking

EddyC--

Actually when I was living in the great white north (UP Michigan) and coming down a really steep hill in an ice storm engine braking was a far better idea (at least it worked in this case).

Here's what happened when I just pumped the brakes: Immediate lockup, not a chance of braking effectively. (this is with full on winter tires (blizzaks) and no abs)

but when I downshifted the fact that the motor was still spinning the wheels prevented them from going in to full lockup. At this point I had to use all three pedals to keep my speed under control to make the sharp turn at the bottom.

*(for anyone familar with the area this was Quincy Hill, site of the deepest copper shaft in the area (9200'))

Now ABS is good and has saved my car once or twice. However I can stop my car faster on dry pavement if the ABS is turned off, I think because I can feel the threshold better. This also applies to gravel and lightly packed or fresh snow surfaces, there is a wedge of loose material that can build up underneath a locked wheel. ABS rarely shortens stopping distance, but it does allow you to push full on the brakes and still steer.

RE: curious about engine breaking

Ref "additional wear":
I assume (ass/u/me?) that the additional engine revs induced by downshifting would create some additional wear.
Surely the total wear of a powertrain is a function of a number of things, including the sum of (torque*crankshaft rpm) and some number of other factors...
Without overrevving a motor to (near?) failure, you might not notice the additional wear- too many other variables, and modern motors last a long while anyway.
cheers
Jay

Jay Maechtlen
http://home.covad.net/~jmaechtlen/

RE: curious about engine breaking

"But I was just curious if engine braking a car (manual transmission).........."

"All of you guys that are against engine braking don't  work in Heavy Truck.........."

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

Geographically challenged Eskimo mom, sending her teenage son off to vacation in Hawaii - "Nanook ! You promise me you will be sure and wear that parka and those mukluks"


RE: curious about engine breaking

uh, coasting in neutral would give better fuel economy.

RE: curious about engine breaking

I don't see how it's true, but it's apparently true, I have read in places, and there are a few posts up there... that say that decelerating while in gear gives you the best possible fuel economy.

regarding engine braking, I use it for the most part on hills and freeways, I have to drive down a grade when coming home from work, and if I put it in neutral my car will easily pass 85 going down that hill, but put it into 5th and just lay off the gas and it does a perfect 70 mph, so I save gas and I save my brakes (and I save my kidneys too because I have a midly warped brake rotor I haven't gotten around to replacing)

I don't use it in regular city driving though.

Oh and regarding that post on downshifting into 1st, most manual tranny cars have a limiter built in that won't allow you to do this until you're under 15 or 10 miles an hour to prevent the engine from redlining, plus the collosal magnitude of the jerk it gives you is so bad you normally don't ever try it again anyway.

RE: curious about engine breaking

I have read in many places that people have been kidnapped by aliens.

I have never seen this downshift limiter on a manual transmission. How does it work.

It would be real handy if they also put it on reverse

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: curious about engine breaking

I could give you a reasonably long list of manual gearboxes that do not have this feature, if it is anything more complex than baulky synchro.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: curious about engine breaking

I think it's a balky synchro.  Every car I've had with a manual trans felt as though it was locking me out of first at relatively high speeds, but some persistence would usually overcome that (on a bad day when i couldn't find third).  

RE: curious about engine breaking

Ingear 0% throttle give me teh best mileage in my car because the injector duty cycle also goes to 0%.

Greg-I think thats what hes talking about. That said I've never driven anything other than a synchro box and have never had any trouble w/ 2->1 shifts. Although double clutching isn't too hard.

RE: curious about engine breaking

Ingear 0% throttle give me teh best mileage in my car because the injector duty cycle also goes to 0%.
...but you throw away X additional horsepower in engine pumping, friction, etc. losses as you spin the engine at 3000rpm instead of 800rpm...

RE: curious about engine breaking

ivymike- right thats where the "braking" effect comes from. but truly I will see lower fuel usage per unit time if coasting in 5th vs coasting in neutal.

RE: curious about engine breaking

Trust me, guys...I come from a place and time when coasting down hills saved me mucho dinero.  I can guarantee you that, in the long haul, staying in gear ALL the time WILL NOT be as efficient, vis a vis fuel conservation, as coasting in neutral!  One trip in particilar from High Rolls, New Mexico to Las Cruces the 'Okie Overdrive' managed to give me >50 mpg in a 1949 Mercury V-8 !!!!! Gasoline was only $.25/gal but, then again, I only made $.50/hour mopping floors in a drug store, part time.

As to the 1st gear inhibitor...nawwwww...but the old Moss four speed in my '50 TD came  close.

Rod

RE: curious about engine breaking

As long as the ECU/PCM stops sending pulses to the injectors during overrun (but obviously not during conditions of idle), less fuel will be consumed than if the engine is allowed to rest in the 'idle' state while decelerating.  This is completely consistent with any experiences involving 50+ year old carburetor technology, too.

I've been hoping for a description of the 'low gear inhibitor' and perhaps some 'performance tips' as well.  I think one of mine is out of adjustment, and my shop manual doesn't seem to be of much help . . .

Norm

RE: curious about engine breaking

while decelerating.  ...sure, during a short-duration braking event, but the question was about coasting (presumably for a while), not rapid decel.  
 * If you're going down a hill, and plan on going up the next one, then coasting in neutral will let you store energy in the vehicle on the way down to use on the way up.  
 * If you're going to drive up to a light and stop, you'll get better fuel economy if you idle the engine and coast a long way to the light (gradually decel) instead of running under power until the last 50 feet and engine braking + pedal braking to stop (rapid decel)...


RE: curious about engine breaking

Right mike, however if I just leave it in gear and let the momentum decay (and I can balance it with distance by gear choice) until I just have to use the brakes slightly I'll use less fuel.

(Note this isnt how I drive, just an extremely stretched rubber band of an example. Kind of like a 30profile 22" spinner.)

RE: curious about engine breaking

I tend to think most guys out there racing sportbikes are using a combination of engine braking and standard braking to scrub off speed.  At least I do on my 600cc bike.  The comment made earlier about getting off the throttle for front dip-in works.  Although you shouldn't really be on the throttle until you're ready to crack it and commit to your cornering line.  For those interested, books by Keith Code and Nick Ienatsch will cover this for sportriding.  

As the 600's are geared lower than the literbikes, I'm generally dropping down a gear before entering the corner so that I can get the engine rev'ing higher in the sweet spot (usually 10K RPM or so).  A small throttle blip is used to allow for the rev's to match.  Basically you blip it high a learn to anticipate when to drop the clutch so the rev's match.  I'm not fast enough or confident enough to brake right up to the apex, so I'm usually off them a bit before that.  But if I'm moving in hot, I'm using both my engine (from the downshift), the front brakes, and the tires to scrub off speed.  From there it's just a quick countersteer, cracking the throttle, and then rolling it on through the turn...hopefully with a knee scraping pavement.  =)   

For the question on over-revving, sportbikes and racers are leaning towards the slipper clutches to limit revs and avoid damage to the gearboxes.  Is this what you were referring to?

I've never tried racing one of the vintage deals.  That might be a fun restoration project too.  Any recommended sites for info?

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