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What about fuel types & gas mileage?

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  • eliaselias Posts: 1,941
    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_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    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.

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  • eliaselias Posts: 1,941
    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_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    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

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  • shiposhipo Posts: 9,152
    "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_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    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.

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  • shiposhipo Posts: 9,152
    Which car in your stable are we talking about here, a Honda or a Porsche? ;-)

    Best Regards,
    Shipo
  • Mr_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    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.

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  • shiposhipo Posts: 9,152
    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
  • explorerx4explorerx4 Central CTPosts: 9,966
    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.
  • shiposhipo Posts: 9,152
    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 shouldn’t 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
  • explorerx4explorerx4 Central CTPosts: 9,966
    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?
  • shiposhipo Posts: 9,152
    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
  • vettevervettever Posts: 20
    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_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    I ran two tankfuls of premium fuel in my lowly Scion and actually LOWERED my fuel mileage by 2 mpg.

    Go figure....

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  • shiposhipo Posts: 9,152
    A noble experiment! That having been said, the results were at least semi-predictable. ;-)

    Best Regards,
    Shipo
  • Mr_ShiftrightMr_Shiftright CaliforniaPosts: 45,930
    I don't get it actually. Maybe I drove faster, I dunno.

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  • smokey75smokey75 Posts: 434
    Does your car recommend premium? If not that can lower your mileage.
  • waiwai Posts: 327
    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.
  • shiposhipo Posts: 9,152
    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
  • waiwai Posts: 327
    Do you know that my Jetta 04 VR6 has ignition control electronics to advance the spark event for higher Octane fuel or not?
  • shiposhipo Posts: 9,152
    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
  • dtownfbdtownfb Posts: 2,915
    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.
  • jaydhjaydh Posts: 16
    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.
  • calisteelcalisteel Posts: 22
    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!
  • imidazol97imidazol97 Crossroads of America: I70 & I75Posts: 18,524
    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.
  • calisteelcalisteel Posts: 22
    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.
  • imidazol97imidazol97 Crossroads of America: I70 & I75Posts: 18,524
    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?
  • eliaselias Posts: 1,941
    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.
  • shiposhipo Posts: 9,152
    "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
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