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reglar vs hight test gas
2

reglar vs hight test gas

reglar vs hight test gas

(OP)
Is it cost effective to use regular gas on an engine that is rated for high test? Is there any documentation around?
Thank you.

RE: reglar vs hight test gas

Yes. Providing the ECU can correctly adjust for it. Our Lincoln is rated for premium and I have never used anything but 87 octane in it.  No problems in seven years and 154,000 miles!

Rod

RE: reglar vs hight test gas

It depends, my particular car used to get a touch better mileage on 89 than on 87, it was designed for 89. it ran fine either way but the slight mileage boost made the extra 10 cents a gallon come out in the wash.
id say try it for a while and see.
also watch your tire pressures and keep clean oil and clean air filters, they seem to matter the  most.

RE: reglar vs hight test gas

A dirty air filter affecting fuel economy in a modern car has been exposed as a myth.  Read about it here: http://www.eng-tips.com/viewthread.cfm?qid=214125&page=2

As for the oil filter, I don't see a correlation to fuel economy either excepting accelerated wearout of the engine leading to increased blowby, but that would be a long term process and difficult to quantify.
Tire pressures & driving style (especially cruising speed, in long distance driving) are the 2 biggies under the owner/driver's control.  Gasoline octane rating typically has little or no effect on fuel economy since most driving is at part load, but will have an effect on maximum power, cost of fuel of course, and possibly engine longevity depending how well the engine management handles lower octane (probably not a concern in <10 year old cars).

RE: reglar vs hight test gas

Agree with most of your post but I have been told first hand of an engine (WRX STi) that was designed for premium that was more expensive to run on regular. In fact that is what I'd expect, most high performance engines run rather rich at WOT, they are on the ragged edge of lean/fully advanced at part throttle, using the knock sensor.

For a start, many knock sensors don't work at full throttle/high speed, there is too much noise, so you are working off the spark table, not the knock sensor.



 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: reglar vs hight test gas

If the oil gets thicker with age, it could cost MPG. Sometimes old oil is thicker, sometimes it is thinner. The thinner oil might gain increased MPG at the cost of engine life.

Presuming the engine can compensate to avoid damage (many modern engines can compensate) the economy issue comes down to several variables mostly mentioned or hinted at above.

The factors are:-

Effect on timing due to knock sensor.

Throttle position and it's effect on a:f ratio.

Price difference for the different fuels.

Ambient temperature.

Energy content of different fuels (ethanol containing fuels have low energy)

This link has a lot of info on fuels.

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

 

Regards

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

RE: reglar vs hight test gas

I have been told first hand of an engine (WRX STi) that was designed for premium that was more expensive to run on regular

Add a 2005 Mini Cooper (naturally aspirated) to the list.  When I had mine, it would take a ~20% hit on fuel economy if I put in 87 octane fuel.  At the time $0.20/gal was 10% of fuel cost, so putting in premium was a net savings...  that's part of what drove me to buy a pickup truck as a replacement - the fuel costs per month were about the same (ford ranger at 23-27mpg vs mini at 27-31).
 

RE: reglar vs hight test gas

Well, I'll need some better numbers to convince me that, for the vast majority of cars in use on public roadways, using premium grade (here it's 91 oct. @ ~$4.20/gal.) over regular grade (again, here 87 oct. @ ~$3.90/gal.)  is going to be 'saving'.  The exceptions exist.  My crew chief's WRX may be one...I doubt if I'll ever know for sure as he uses prem plus an additive, just to make sure.  Fuel cost is not an issue with him.

Clean oil?  Next you will be telling me that a clean car gets better mileage than a dirty one....hmmmmm?  Could be, if it is an indication of the condition of the mech bits and pieces.

So far, IMO, all this discussion is academic.  I don't see any fewer SUV's going 80+.  I don't see an increase in the local high school kids 'walking' or (gag) riding a bicycle.  I don't see a rash of new motorcycles as I did in 1973/4.  In fact, it appears to be "situation normal" (for Socal, anyway).  Speaking of Socal, I hear that there is at least one "hypermiler" club in the San Fernando Valley, if your interested.  

Rod  

RE: reglar vs hight test gas

Agree with Rod, there may well be some exceptions but the rule is sound.

RE: reglar vs hight test gas

OK, so why _do_ OEMs recommend (or specify) >87 AKI for cars in the US, since it can only be defensive to competitive models calling out regular gas?

I had an 84 Capri (3.8L V6) with +100Kmi that consumed noticeable oil and would ping enthusiastically on hard acceleration on 87 AKI gas.  I filled it ca. half with 87 topped off with 93 to get about a 90 rating for the price of the 89 mid-grade and prevented the knock.  I was a student at the time with no $$ or time for a new motor . . .

RE: reglar vs hight test gas

I can imagine a car that has to run so non-optimally on regular that there is actual fuel economy degradation. My guess would be that those cars are few (very high compression and/or supercharging), and that the percentages would be small - much lower than the possible 20% listed. Does anybody have specific data that meets some scientific standard?

RE: reglar vs hight test gas

drwebb, I can imagine an 84 V6 Capri pinking under full throttle.  Especially one with 100k on the clock.  De coking and or retarding the ign timing a degree or two would have cured the problem.  A student with no $$$ would have been wise to avoid "hard acceleration"...Or would that have been asking too much? ;o)

In the 50's when an OEM called for premium, it meant something.  We had a Buick that just could not operate on regular, I think at the time reg. was like 89 oct.  My dad always used super prem. 100 oct. Chevron (white pump) fuel.  I think the cost was something like $.39/gal. !!!

On this same question, I had a 77 Kawasaki that I rode to Mexico City and back in Jan. 1980.  You can only get bad and worse grades of Pemex down there, Extra and Nova (Spanish No Va means won't go).  After a couple days, you just learn to deal with the knocking...you can't miss the knocking on a bike.  We were down there several weeks and, ultimately, had no problems except when we came back to the U.S.---Kept looking for a gas station...couldn't find one...Until someone recognized the fact that here all gas stations are not Green/White Pemex!  Dumb, dumb, dumb.

Rod

RE: reglar vs hight test gas

It is EFI and knock sensors that have made the difference. By running on the onset of knock, maximum power can be extracted from either fuel without damage. How much effect that has on economy varies from car to car with high boost or especially high compression cars getting the biggest difference.

It comes down to how much extra MPG you really get vs the difference in cost. It will normally be a close call, slightly favouring the lower grade fuel. Those selling the fuel tend to do the cost benefit sums when setting prices, and expect an extra premium for the extra power as well as the premium for the MPG gains.

There will be no MPG gains in low compression engines or in use with no high load component.

Regards

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

RE: reglar vs hight test gas

Go ahead, tell me your theory about my unscientific methodology.  Your guesses are often pretty entertaining.

 

RE: reglar vs hight test gas

Greg, I am in awe of the pre war engineers and scientists.  Just proves the adage that nothing is 'new'.  We refine and tune for better performance, but the underlying theories are a century old in some cases.

Pat, wouldn't you think that if one is shooting for max fuel mileage, there should be no high load component ?

Isaac, I think you methodology has as much merit as any.  I'm a novice when it comes to fuel mileage.  I read a lot and listen a lot, but I have never found the need to go crazy over fuel mileage.  For me, a paid off gas guzzler is ultimately much cheaper (often more fun, too) than investing >$30,000 in a new car that will 'save' me ~$30/month!

Rod

RE: reglar vs hight test gas

ivymike:
It took me a bit to realize that was directed at me. I wasn't guessing about your methodology (or accusing you of anything). You didn't tell us anything about your methodology, and it was presented more in the nature of anecdotal evidence rather than scientific evidence. For all we know, you got the regular gas at the top of a mountain or on days you were hauling your boat with your mini Cooper. :P Or maybe your number was based on two mixed tanks of gas, for example.

I find the 20% number very surprising, practically unbelievable. I just wanted to know if such a number has been verified anywhere, or if upon reflection you thought your number was controlled enough to be presented as scientific.

RE: reglar vs hight test gas

I don't have the data anymore (it was a 2003 that I sold in 2005, not a 2005).  When I owned the mini, I had a reputation for being a bit of a nut about fuel economy.  It is my standard practice to record every input of fuel to each of my personal vehicles, along with the date and mileage.  I periodically transfer my numbers to a spreadsheet where I plot fuel consumption and mileage vs time.  Fuel consumption is plotted using a 3-tank moving average (dotted line) overlaid on the raw data points.  Strongest influence generally seems to be the season - weekly avg ambient temperatures from noaa overlay the fuel economy graph quite well if the scales are set appropriately (in the case of my pickup truck, its gradual circle around the drain has been a stronger influence lately).  I drive the same route to work every day, at the same times, in very similar traffic conditions.  For a long part of the time when I had the mini, there were only 3 stoplights on my 20min drive to work, so I basically cruised at 45mph-55mph the entire way, save 4 periods of hard acceleration.  I don't stop for fuel unless I'm worried that I won't make it home from work.  In the case of putting 87 fuel in the mini, which asked for 91 fuel, the difference was so striking as to convince me in a single tank that there was a clear difference and that would lose money using 87.  I repeated the experiment on several occasions, however, because of regular discussion online about how 87 octane fuel would work just as well as 91, or according to some, better.  Most recently (just before i got rid of the Mini) Greg L had convinced me to try again, with the argument that the difference in fuel economy couldn't be greater than the 10% (at the time) difference in fuel prices.  Turns out that it was... so I started shopping in earnest for a vehicle with a lower payment and 87-octane fuel.  Perhaps the station I filled at had "bad" 87 octane fuel - maybe they used more ethanol in the cheap stuff (would guess the opposite, though) - who knows.  

 

RE: reglar vs hight test gas

Well that's certainly better than an anecdote.

Ethanol is usually in at not more than 10%, and has 67% (roughly) the energy content, so that only explains 3.3% even if true, although anymore you can often find ethanol in any grade. 3.3% is significant, of course.

Now I'm curious - do you know the compression ratio of the mini? I assume it was not supercharged...

RE: reglar vs hight test gas

By my sums on energy content, 10% ethanol blend needs to be at least 3% cheaper, octane rating being the same.

Different octane rating, then it gets more complex due to many of the factors already covered as well as the different a:f and energy content.

I have a very unscientific test. Every time I refill, I reset the trip meter. I watch the mileage vs gauge. It is a pretty good indicator of whether I am doing better or worse than normal.

Occasionally I try private brand or independent fuel, sometimes ethanol blends, sometimes higher octane, but I guess about 95% of the time I use std unleaded which here I think is typically 89 to 91 depending on brand.

Regards

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

RE: reglar vs hight test gas

"Pat, wouldn't you think that if one is shooting for max fuel mileage, there should be no high load component ?"

For the best (lowest) BSFC, you want the engine operating very near its maximum load, and typically at its peak torque.  For any lower throttle setting, the engine will perform less efficiently (power produced by the engine is lowered, but all of the rotating friction losses are still there - think of an engine at idle speed, barely producing enough power to keep rotating, but still burning fuel: its efficiency is pretty much zero).  

Unfortunately, most of us like to drive cars that can accelerate hard enough to be at freeway speed at the end of an on-ramp, thus the peak power we want is quite a bit more than is required for "cruise".  Thus, we have systems that shut down part of an engine (one side of a v-8) or reduce pumping losses (Toyota VVT), or hybrid systems that use stored electrical power to provide acceleration, while the IC engine is sized for the cruise condition.

RE: reglar vs hight test gas

btrueblood
IMO the older engines had to spin up to become efficient, but my Z06 runs 1500 RPM at ~65 with 24MPG.
I run the 87 octane and have had no problems but I have not done a mileage check other that the computer trip mileage.

I don't know anything but the people that do.

RE: reglar vs hight test gas

My 2zzge engine is 11.5 to 1 compression ratio and I would not dare to put a lower octane in it on purpose.  One time I noticed the engine ran rough, had no acceleration and would not go into lift.  The station or I myself must have gotten a lower octane fuel. I drove it very gingerly for a while and refilled with 91 octane.  The engine still didn't run like it was suppose to so I disconnected the battery for 15 minutes to reset the ECU.  It ran like it was suppose to.

As for price, I love it.  The gap between low and high octane is a constant $.20/gallon.  As gas has gone up, the % in price has declined.

RE: reglar vs hight test gas

Hey- I'm not the only one who has noticed the fixed price gap means that 'high test' is a better value now.

Here's more anecdotal data- I have a Volvo V70 2.4T that 'recommends' high test (91 RON, if memory serves) but 'allows' regular.  I understand that advanced timing theoretically should give marginally quicker accel and maybe even smoother run, so I wanted to justify fancy gas.  On several trips to grandma's house I alternated fill-ups with 87 and 93 AKI and recorded mileage.  The difference compared to standard deviation convinced me that I couldn't tell a mileage credit for fancy gas.  Maybe now that premium is relatively cheaper I should try the experiment again using in-town service.

Is there a CAFE requirement that OEMs have to recommend the same fuel grade they qualify models with??  I'm still wondering what's in it for them to call out fancy gas unless they believe it reduces their warranty costs or customer complaints- but I doubt many average drivers would complain that their mileage or acceleration was noticeably different.

RE: reglar vs hight test gas

With every car I've owned, I did a study of regular vs. premium gasoline and mileage.  Generally, I got slightly better mileage from premium.  On a cost-per-mile basis they were always equal within reasonable doubt.

That said, I tend to go with performance.  In winter, the low-test fuel performed better.  In older cars with knocking or dieseling tendencies, premium fuel in the hot summer helped avoid those difficulties.

RE: reglar vs hight test gas

Should never have spoke of it.
Just bought fuel and for the first time in ages the spread was more than 10 cents. only 11 an 22 but still ...
  I'm pretty sure there are vehicles that are more economical to operate on premium ( I hope so, I've put considerable effort into making it so.)
  That said, I'm sure there are a thousand  times as many wasting money on unnecessary octane                                                              
   

RE: reglar vs hight test gas

"IMO the older engines had to spin up to become efficient, but my Z06 runs 1500 RPM at ~65 with 24MPG."

Ok, you probably have me there, my numbers are from memory for 4-cylinders...

But my point is (was) that running an engine at a higher load gives best efficiency.  Compare your numbers to my 4-cylinder 1.8L that runs ~2500 rpm at 65mph with 33-34 mpg.  I would bet it puts out something a bit closer to its maximum load (as a percentage of its capability) at that speed, relative to a V8, thus it gives better mileage.  Your motor would pretty much tear the shorts off mine in a drag race, but the little motor wins in efficiency, simply because it better matches the average driving power requirement.

RE: reglar vs hight test gas

If things keep going the way they have been, I will change to a turbo 4 Cly maybe a 3L twin turbo Eco tech engine.

I had a turbo T bird that I spent money on that my wife got 33 MPG and I got 16 MPG so driving a 4 Cly isn't the complete answer to good mileage. A side note; in 4 years I went through 3 turbos and 2 motors...also expensive..

  

I don't know anything but the people that do.

RE: reglar vs hight test gas

Hmmmmm.  A couple observations, nothing personal since I had a long relationship with a stock and modded Tcoup...
One...If your wife averaged 33mpg, she probably needs a new calculator or, a Hondayota something or other.
Two...Since you got 16mpg, I certainly can see why you went through three turbos and two engines. I approve but, you shoulda had me do your new engine. clown

My wife and I averaged 22/26 mpg from new until 100k miles where she holed a piston...crapping the turbo, etc.
New engine (Evelrod Racing build @ 295hp at the rear wheels on the JBA dyno)...we managed 22mpg most of the time and that engine went over 200,000 miles!  Sold it because we could not keep an AC compressor in it and it wouldn't pass smog last time. I liked that car!...  "1983-1995 RIP" Sure was a fast car. Used it in many, many SCCA schools at RIR and Willow Springs...fast on the dry lakes too, 155mph on the rev limiter...clocked!

Rod

RE: reglar vs hight test gas

I'm still enjoying the Ford 2.3T in my daily driver, now heavily modded (in a streetable way).
I agree with Rod's mileage observations, but IMO a stock 2.3T has plenty of safety margin when maintained in tip-top condition, and as he notes, when judiciously modded, is capable of vastly increased power without sacrificing longevity.

RE: reglar vs hight test gas

I just read the last post from hemi and it dawned on me that that 83 T-bird was one of the exceptions to the 87 octane rule.  After it was modded (bigger turbo, ported head, hotter cam, bigger injectors, altered ECU, etc.), I could not retard the timing enough to keep it out of knock under full boost.  Even with the 93 oct available  in the early 90's, I needed water injection to keep out of knock on a hot day pulling the hill on I-15 out of San Diego. I used racing gas for much of the time...the cat was 'empty' and I was not too worried about the added cost of the race fuel...I think it was something like three bucks a gallon or therabouts.
I suppose I could have backed boost down to some 'reasonable' numbers...what fun would that have been.
I could spin the tires going into second gear (first was kinda doggy...low boost) and it would go plumb sideways into third. Coming out of T-9 at Willow, I could 'haze' the rear tires in 4th gear!!! I know. I'm too old for that...NOT.

Rod

RE: reglar vs hight test gas

I was naive (dumb) about turbos I killed the first one at a car wash when I accidentally hit it with the spray. It made a whimpering noise and started smoking. The dealer was close by and it was warrantied.
The next one just sucked in the impeller and contaminated everything.
And the last one; I was told to let the turbo idle down as there was no reserve oil to the bearing and should have listened, if I do it over again I would PAY attention its cheaper that way.

Cheers

I don't know anything but the people that do.

RE: reglar vs hight test gas

A water-cooled center section goes a long way to reduce coking after shutdown, in turbos that tend to run hot, e.g. those applied to spark-ignited engines.

RE: reglar vs hight test gas

hemi, only the 83 Bird had a non water cooled center bearing.  Water cooling helps but does not cure the coking problem. Cooling, along with quality lubricant changed at no greater than 3000 mile intervals and taking care to let the turbo spool down and cool for a few minutes will do the rest.

I doubt if I will ever buy another street driven car with a turbo unless there is some very large profit in it for me.

Rod

RE: reglar vs hight test gas

Rod, you've obviously had a bad first experience, but remember the '83 turbo Bird was engineered over 25 years ago.  By now the European & Japanese brands especially have really figured out turbos in spark ignited engines, but the North American brands aren't afraid to dabble with them anymore either.  Change the subject to diesels, and a non-turbo diesel is nowheresville these days.
In my case my DD has been the Ford 2.3T '85 vintage for the past 16 years and have never had a turbo failure.  I've torn down & reassembled the turbo a few times, but only the first time was for maintenance (compressor seal passing oil), the rest were mods in pursuit of higher performance, so I'm just now on the third set of bearings/seals at 200,000 miles.  Just to fill in the picture, I'm currently running 22psig boost (at 5000' elevation), and expect to have around 300 whp when tuning is complete.
Sorry for the thread hijack; we should start a new thread I guess if this turbo discussion has legs.

RE: reglar vs hight test gas

Hemi, you misunderstand my posts.  I have nothing against turbos, I loved that Bird, used the crap outa it for 12 years.  Only sold it because I just got too old for a street hot rod and my wife (whose car it was, technically) was complaining about the AC always on the fritz.  Incidentally, I used 18 psi on the street and 24 or so when I could get by with it. In 1988 (when I rebuilt the engine) the Bird was FAST!!!  There were no Mustangs that could even touch it. It also was a 'ticket' magnet ;O(  !!!

I have only lost one turbo due to ignorance, I guess. That was on my Corvair Spider in 1964.  The first turbo in the Bird was cratered due to the molten Al from the piston failure at 100k miles.  The turbo on my Dodge/Cummins, now at >200k miles and 17 years old, is doing just fine.

Rod

RE: reglar vs hight test gas

Understood.

Cheers!

RE: reglar vs hight test gas

Saturn ION owners with the 2.4L engine report that they achieve better fuel efficiency with 89 than 87, presumably because of the knock sensor. I don't recall whether the difference was significant enough to justify the most expensive fuel. Personally, if I had a car that recommended the higher fuel, I wouldn't mess around with the lower stuff. I'd be afraid of fouling the plugs and chamber with the extra carbon running rich, and I think one knock is one too many.

RE: reglar vs hight test gas

Knock is not a boolean, it is a continuum.  Each engine type has its own limits (frequency-intensity-duration).

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