reglar vs hight test gas 2

[zekeman]

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

 

[evelrod]

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

 

[carnage1]

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.

 

[Lou Scannon]

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).

 

[GregLocock]

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

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

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

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).

 

[evelrod]

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

 

[Lou Scannon]

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

 

[drwebb]

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 . . .

 

[JSteve2]

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?

 

[evelrod]

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

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

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

 

[GregLocock]

http://naca.larc.nasa.gov/search.js...ll&Ntx=mode%20matchall&N=0&Ns=HarvestDate%7c1

Should have some relevant info. The curve of bsfc vs ignition timing is presumably well known, as is the curve of ignition timing vs octane, for typical engines.



Cheers

Greg Locock

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

 

[evelrod]

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

 

[JSteve2]

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. 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.

 

[ivymike]

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.

 

[GregLocock]

Yes, ethanol blending is a nasty little con.

Cheers

Greg Locock

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

 

[JSteve2]

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...