What about fuel types & gas mileage?

elias · May 2006

i usually 'benchmark' my gas-powered cars as best i can with regard to 87 vs 91 octane. for our volvos on long trips, i could never cause any mpg difference whether i used 87 or 91. with my 05 GTO, even for loafing-highway-cruising, it gets 10% lower mpg if i use 87 instead of 91. my tests for this stuff involve >500 or >1000 miles drives from new england to raleigh or orlando...

Mr_Shiftright · May 2006

That's interesting. I can't think of any reason why your GTO should get worse fuel mileage just loafing down the highway on 87 octane. I'll have to think about why that could happen. Perhaps your engine is fairly high compression?

elias · May 2006

ya the GTO has the LS2/C6-corvette engine - high compression. even in very cold temps it seems like the 87 octane hurts mpg noticeably, but i don't have enough/well-controlled data to know that for sure. i really can't notice a power loss with 87, at least not on public roads !
for years i drove LG4/305-TPI/LT1/LS1 Z28s through new england winters without traction control - and during cold weather i'd use 87 octane in those as "poor mans traction control", just to reduce the torque off-the-line a bit. the LT1/LS1 Z28 engines pinged objectionably if i tried that in summer however.

shipo · May 2006

"That's interesting. I can't think of any reason why your GTO should get worse fuel mileage just loafing down the highway on 87 octane."

I can. Even when loafing along (in fact, especially when loafing along), fuel economy is dramatically improved by advancing the spark timing. The higher the octane the fuel is, the further the spark can be advanced without the risk of detonation. It sounds to me like the GTO engine does its level best to keep the engine on that most efficient "Ragged edge of disaster", meaning, just a knife edge shy of detonation.

Best Regards,
Shipo

Mr_Shiftright · May 2006

Yeah, true, but with the engine loafing at 2,000 rpm the spark isn't going to need to advance...there's really no demand on the engine whatsoever. It is hardly working. It should run great on really cruddy gas under those conditions seems to me. I can't see why the knock sensor would even be operating.

shipo · May 2006

"Need" isn't the issue, fuel economy is. Think about it, even my 1966 Plymouth Valiant with the 225 Slant Six was equipped with a vacuum spark advance for steady speed low power cruising. That system used zero detonation detection and as such had to be extremely conservative. With the advent of all of our modern engine controls there is absolutely no reason that the ignition timing cannot be advanced to the point of detonation (and then backed off a hair) even under the lowest power driving environments.

The simple truth is that the further the ignition timing is advanced (shy of detonation), the better the fuel economy an engine will return. Why? The primary reason is thermal efficiency, retard the timing, the peak combustion temperature (and peak cylinder pressure) drops, advance the timing and the peak combustion temperature (and pressure) rises. The more heat extracted from the fuel, the more work the engine is capable of performing for any given amount of fuel.

Best Regards,
Shipo

wai · May 2006

I have experimented my Jetta VR6 with 91 octane the previous month due to high gas price instead of 93 octane which I used from the new car till now (18,000 miles). I actually get better performance with 91 octane, quiet engine, nice pick up at freeway or during aggressive accelaration and mileage is more or less the same.
The result might be different if this is a Winter season.
My Jetta required min. 91 Octane. But its more time consuming in pumping the gas as most gas stations don't have 91 octane gas, I have to mix it up myself. I suggest you to use 91 octane to save some money.

rbirns1 · May 2006

Hess makes very good quality gasoline. I've been using Hess for many years in several different cars with no problems or disappointment at all. They are consistently cheaper than the other major brands because they own all the stations directly. They also have a well earned reputation for the cleanest stations anywhere. Leon Hess was legendary for that. He owned the New York Jets and moved them out of Shea Stadium because the bathrooms weren't up to his standards. It's true.

Anyway, go ahead and fill up at Hess. The only difference you'll notice is the extra weight in your wallet.

shipo · May 2006

What is true for one engine isn't necessarily true for another. While your VR6 may not be able to take advantage of the extra detonation resistance of the 93 (in the form of an advanced spark), other engines are designed and built to do just that. As such it is wise to evaluate what is the best fuel grade on a case by case basis.

Best Regards,
Shipo

wai · May 2006

What is the fuel door or manual stipulated Octane required?

elias · May 2006

owners manual of my 05 goat recommends 91+ octane...
i'll try to remember to read the exact text and will post more later if that's not accurate.
i know the inside of fuel door just says "premium fuel recommended".
does the owners manual of *any* USA car recommend gasoline octane *larger* than 91? !?

Mr_Shiftright · May 2006

It's my present understanding that using 91 octane in a car designed expressly for 87 octane is pretty much a waste of money, unless you want to factor in perhaps a slight benefit from additives. This is what the engineers seem to tell the consumer.

elias · May 2006

Mr. Shiftright, i recall that a few years ago some law/policy/thingy passed requiring the oil companies to put the same additives/cleaners in all their grades of gasoline. so i think that possible advantage re additives/cleaners is no longer present.
i agree that there is zero benefit to using anything more than 87 octane in a car designed for 87.
the whole discussion is less important now that the percentage difference between low-octane gas and higher-octane gas is so much less than it was when gasoline was $1/gallon. there was a 20% difference back then between regular & premium. now there's a 6% difference.

Mr_Shiftright · May 2006

that's a good point, never thought of that.

I guess what I was driving at is the common misconception that premium fuel is some kind of "doggie treat" for your car :P

shipo · May 2006

"It's my present understanding that using 91 octane in a car designed expressly for 87 octane is pretty much a waste of money..."

Absolutely, positively no argument from me. I thought we were discussing the GTO which per a recent post requires 91 octane or greater, not a car that carries a recommendation of 87.

With your comments in mind, I'm thinking that in theory at least, it is possible to design a normally aspirated engine specifically for 87 octane, and then design the ignition system to be able to nominally take advantage of a higher octane fuel. That having been said, in reality my bet is that advancing the timing on such a car far enough to take advantage of fuels beyond say 89 octane is probably not going to happen.

Best Regards,
Shipo

Mr_Shiftright · May 2006

I'll be sure, on my next long trip, to run a full tank of premium on the same highway at the same speeds and see what happens, if anything.

shipo · May 2006

Which car in your stable are we talking about here, a Honda or a Porsche? ;-)

Best Regards,
Shipo

Mr_Shiftright · May 2006

the little Scion xA commuter car. The Porsche V-8 runs very happily on 87 octane BTW. Funny, but the only cars I ever owned that would not run on 87 octane without horrible protests were Volvos.

shipo · May 2006

I'm not at all well versed in "Scionese", however, I have heard the stray report claiming that some Toyota engines are capable of advancing their ignition timing to a certain extent. Far enough to efficiently consume Premium? Probably not. Midgrade? Depending upon how the engine is built, that is at least theoretically possible.

Best Regards,
Shipo

explorerx4 · May 2006

a lot of modern engines and transmissions adapt to 'your driving style'.
my pzev focus has a manual transmission, no tach, and an upshift light(flashback to the 80's).
one time i filled up with 93 octane. got my worst mileage ever, 24.9 average for the tank, and the upshift light never came on. for the life of the car i have averaged 28.9 mpg.
my explorer seems to run quieter on high test, but the mileage in not necessarily better, except when using winter gas in a cold snap.

shipo · May 2006

As a general rule, the gasoline that a car was designed for (and what is specified in the manual) is the fuel that will allow any given engine to deliver both the best mileage and the best power.

In the case of the Focus, there is no surprise that your mileage went into the dumper, Premium fuel simply isn't volatile enough for that engine.

As for your Explorer, unless your engine is heavily carboned up on the inside of the combustion chambers, Premium gasoline shouldnt make it run smoother at all. In fact, due to the effectively cooler burn that results from high octane fuel and a low compression engine, all you are doing is accelerating the process of carboning up the combustion chambers. Once enough carbon builds up, then "Hot Spots" can occur, and when they do, then you really will need Premium just to keep the engine from running in a constant state of detonation.

Best Regards,
Shipo

explorerx4 · May 2006

i think you explained what happened with the focus and i am not going to put it to the test again. regular is just fine.
with the explorer v8/auto/towing pkg, it seems like the transmission got out of sync with the change in the operaion of the engine. back to regular gas, everything is as was before.
what do you think about switching brands of gas because they have different formulations?

shipo · May 2006

While I've never been particularly brand loyal as far as my gasoline is concerned, I have always tryed to stay with the name brand gasolines if for no other reason than they seem to have a higher turn over rate than the smaller Brand X stations. As modern gasoline ages (and unlike wine, aging gasoline ain't for the best), it becomes less stable (as I understand it, I've never researched this aspect of gasoline), and as such less efficient.

Regarding different formulations, logistically speaking I just don't see how one brand can actually sell different gasoline than the next due to refining and transportation issues as well as various efforts at standardization such as the "Top-Tier" gasoline program.

I'm sure this wasn't much help but there you have it. :-/

Best Regards,
Shipo

vettever · June 2006

If you drive a high performance car like a GTO you should not have bought it for the great gas mileage or the ability to run it on the cheapest gas available! I have a Corvette ZO6, the mileage is pretty good for the way I drive it (13 city/26 highway). I wouldn't dream of putting anything but hightest gas in this booger!!! I had a 68' Goat back in the day and it needed the "good stuff" so as not to knock. Fill er' up n' kick er' in the [non-permissible content removed]!!!
Vettever

Mr_Shiftright · June 2006

I ran two tankfuls of premium fuel in my lowly Scion and actually LOWERED my fuel mileage by 2 mpg.

Go figure....

shipo · June 2006

A noble experiment! That having been said, the results were at least semi-predictable. ;-)

Best Regards,
Shipo

Mr_Shiftright · June 2006

I don't get it actually. Maybe I drove faster, I dunno.

smokey75 · June 2006

Does your car recommend premium? If not that can lower your mileage.

wai · June 2006

Some cars' manual or fuel door sticker stated "require a certain MINIMUM Octane" Does it mean you should use fuel with that Octane or higher, but not lower. For example my VR6 requires min. 91 Octane, I can use 91, 93, or even 94 with no decrease in mileage or performance. In my case, I try all three, not much difference with mileage or performance.

shipo · June 2006

Some cars' manual or fuel door sticker stated "require a certain MINIMUM Octane" Does it mean you should use fuel with that Octane or higher, but not lower.

The only correct answer here is, "It depends."

If any given engine is optimized for 91 octane but has ignition control electronics that allow it to retard the spark event for more volatile fuels (lower octane) and/or advance the spark event for less volatile fuels (higher octane), then the "Minimum Octane" in the manual might could well be 87. That having been said, that same engine will lose both performance and economy when anything less than 91 is being used.

The flip side of course is when any given engine is optimized for any given octane rating and then a higher octane fuel is used. In this scenario, performance and fuel economy will suffer if the engine isn't able to advance the spark event sufficiently to start the fuel burn early enough. If, however, the engine can advance the timing, economy and power may actually improve slightly, of course, not as much as an engine optimized to burn said higher octane fuel.

Best Regards,
Shipo

wai · June 2006

Do you know that my Jetta 04 VR6 has ignition control electronics to advance the spark event for higher Octane fuel or not?

shipo · June 2006

Do I know? No.

I had a VR6 in my 1995 Passat and on that engine I don't remember it being at all sensitive to fuel above 91 or 92 octane.

Sorry I can't be more help than that.

Best Regards,
Shipo

dtownfb · February 2007

I'm not sure if this is the right forum but hopefully someone can educate me on gasoline providers.

I live in an area where the gas stations are dominated by the local convenience stores, Rutter's, Sheetz, Tom's, etc. Reading a few forums, the discussion of top tiers gasoline providers popped up. It dawned on me that none of these providers are in my area (South Central PA) or are they? Is there a way to find out who provides the gas for these stations? Obviously Rutter's and Sheetz don't own refineries and must buy from someone.

jaydh · February 2007

These are most likely UNBRANDED providers who load up their trucks at the local refinery and sell to local convenience store chains, among others, in your area. It's basically the same gasoline that you would find at the branded stations.

calisteel · July 2007

I read a lot of car forums for many different reason. But there seems be a common post in just about all the forums, GAS MILEAGE.

Many people get upset or can not understand why the GAS mileage of same vehicles differs so much. It is quite simple and it is not what most people talk about. Yes how you drive has a lot to do with it but the one main reason that mileage differs on two like vehicles is ALTITUDE.

ALTITUDE will determine how lean or rich your car will run. Higher altitude will provide better gas mileage if your car is tuned properly for the higher altitude.

Here is a simple explanation of what I mean.

With altitude, air density decreases, so a computer controlled fuel injection system will automatically decrease the fuel in the mixture to match the air density. Thus all the electronic sensors needed to keep the motor running correctly. You will find that horsepower will decrease, but mileage will actually increase.

This is why airplanes fly at the highest altitude they are capable of. When flying a piston engine aircraft, you manually lean the fuel mixture after reaching cruise altitude, you leave the fuel mixture at full-rich while climbing.

I hope this help explain why gas mileage is so different from so many people. The higher the altitude the better mileage. So people in Denver get better mileage then people in Death Valley California.

If you need a better explanation your local mechanic can also explain this to you.

Cheers!

imidazol97 · July 2007

But the computer adjusts the amount of fuel to match the air and the load on the motor. The amount of unburned oxygen in the exhaust is measured by the O2 sensor and the car will have to do the same amount of work to move the car 1 mile so the same amount of fuel is going to be used.

calisteel · July 2007

Not true. At a higher altitude your fuel mixture will be less as well as your power output. You move the same 1 mile at the same rate but through less dense air, so less power is needed thus lessening the fuel needs as well.

In thicker or more dense air you need a more rich fuel mixture increasing the power output to move you 1 mile thus burning more fuel.

Even though the car is doing the same speed and distance the output of fuel needed is greater or lesser depending on the fuel mix based on air density. You can not get around it.

imidazol97 · July 2007

I thought the computer kept the mixture the same. The air valve in the throttle body lets more or less fuel mixture into the intake manifold to give more power.

What is the difference in air resistance at 900 feet altitude compared to 0 feet?

elias · July 2007

thanks calisteel. your point about altitude is well-taken.
also, it explains why I've been seeing such lousy mpg while driving in the Marianas Trench.

shipo · July 2007

"ALTITUDE will determine how lean or rich your car will run. Higher altitude will provide better gas mileage if your car is tuned properly for the higher altitude."

Ummm, no. Modern fuel injected cars measure the weight of the air coming into the engine and provide the exact same amount of fuel per pound of air at all altitudes and all temperatures and all humidities. Fuel economy as measured by the conventional Brake Specific Fuel Consumption (BSFC) measurement won't change one iota regardless of any of the above "Density Altitude" factors.

"This is why airplanes fly at the highest altitude they are capable of. When flying a piston engine aircraft, you manually lean the fuel mixture after reaching cruise altitude, you leave the fuel mixture at full-rich while climbing."

Ummm, no again. GA aircraft can extend their range at altitude simply because air resistance against the airframe is lower at altitude. Said another way, I take out say a Skylane and set it up as lean as possible to cruise at 100 knots in level flight at sea-level, and then measure the hourly fuel flow. I then take that plane up to say 17,000 feet and lean out to the exact same hourly fuel flow setting, that plane will have a true airspeed of 137 knots. That, and only that, is why aircraft get better economy at altitude.

FWIW, I've seen anecdotal evidence that suggests that cars will see some gains in economy at altitude (but no where near as dramatic as with aircraft due to the rolling friction of the wheels), especially turbo-charged cars.

Best Regards,
Shipo

Mr_Shiftright · July 2007

I would think you'd need to also advance engine timing at altitude to maintain the same fuel mileage. But perhaps the re-calibration of the fuel mixture negates that necessity?

shipo · July 2007

General aviation engines have fixed timing and zero ability to adjust it one bit from where the factory set it and where the FAA certified it.

That said, stoichiometric is stoichiometric is stoichiometric. Given the proper air and fuel ratio, the mixture will burn exactly the same regardless of whether the aircraft is at a density altitude of sea-level or 17,000 feet. ;-)

Best Regards,
Shipo

Mr_Shiftright · July 2007

I think that pre-computer, automobiles required a bump in timing (timing advance) to compensate for the carburator's inability to adjust proper fuel mixture at altitude.

shipo · July 2007

Hmmm, thinking back to the 1970s (when I turned a wrench to make a living), I don't remember tweaking the timing to help cars deal with altitude, that said, I do remember rejetting any number of carburetors.

Best Regards,
Shipo

Mr_Shiftright · July 2007

Well the bump in timing was great for cars that were going up 5,000 feet for a few days--you wouldn't want to re-jet just for that.

shipo · July 2007

Ahhh, got it. If one of the cars I worked on in San Diego was only heading up to the 5,000 foot mark for a few days, I wouldn't bother touching the car, however, the bump in timing doesn't sound like such a bad idea. The only cars I rejetted were the ones that were heading up into the Rockies or the Sierras to stay (or at least to stay for a considerable length of time).

Best Regards,
Shipo

Mr_Shiftright · July 2007

Oh advancing the timing definitely works--dramatic improvement in fact.

dchen2003 · July 2007

I heard some people say some of the gas is not 100% gasoline but with 10% alcohol. Because alcohol has lower energy density that cause lower MPG. Does any one know What brand of gas station use this kind of blend fuel? And what gas station has the best fuel based on your experience.

Thank You

shipo · July 2007

Typically the areas that mandate oxygenated fuels (i.e. most metropolitan areas) also mandate E10 (gasoline with 10% ethanol). If you live in such an area you're out of luck as 100% gasoline cannot be had for love nor money. ;-)

Best Regards,
Shipo

imidazol97 · July 2007

Is alcohol being used in lots of areas now? I thought after he Katrina mess the gov advocated using as much ethanol as possible.

What about fuel types & gas mileage?

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