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Quattro 02-19-2009 03:53 PM

87 octane and retarded timing
I have a 2001.5 Passat wagon with the 1.8T engine. It calls for 91 octane or higher. For the past year or more, I have been using 87 octane, as the car seems to run just fine on it (no pinging). I rarely challenge the turbo (i.e. accelerate hard), and I haven't had any fuel-related issues.

However, I've been reading about how the computer will adjust timing to compensate for the easier-to-ignite low octane fuel, and how this can effect FE.

Since "premium" (I hate the fuel "grade"'s so misleading) costs on average 20 cents per gallon more than "regular", am I offsetting any savings at the pump for a loss in FE? What I mean is, am I coming out even, or am I doing worse by running lower octane than what is recommended?

Since gasoline is so cheap right now, I have no problem paying $2.20 for high octane...but when it was $4.50, for some reason I balked.


jesse.rizzo 02-19-2009 06:34 PM

It's difficult to say. The only way to know for sure is to keep track of your mileage through many tanks of each and try and draw a conclusion from that. Even that may not be enough, since there are so many variables that can also change (temperature, wind, air density, your driving habits, the health of your car).

However, my vote is that you aren't hurting your FE. Since all gasoline in the U.S. is now required to have 10% ethanol, and ethanol raises the octane of gas, the "regular" (I agree with you on the fuel grading being dumb) gas may be 91 octane anyway. Many stations now charge the same for "regular" and "premium" gas.

But as I said, the most definitive way, and even it isn't very definitive, to know is through testing.

Quattro 02-20-2009 12:59 PM

Yeah, that's what I figured. I'm going to try a few tanks of 91/92 and see if there is a noticeable change in FE and/or responsiveness. I do feel like the car is a bit sluggish, but it does get up and go when you ask it to. I'm sure it's ready for a tune-up as well.


some_other_dave 02-20-2009 08:53 PM

Some people around here have reported a noticeable difference in FE when going from lower to higher octane fuel. There were those for whom the MPG difference was larger than the price difference. There were also people who noticed absolutely no difference at all. Some of thoe were people with rather similar cars, even!

So you have to do your own testing to figure out how your car and your driving are affected by a different octane level.

BTW, I always thought it was amusing that when fuel was at $4+/gallon, people would balk at spending an extra $0.20-0.30 for super over regular, when they didn't have a problem with a $0.20 margin at $2/gallon.


Duffman 02-20-2009 09:29 PM

Turbo cars usually have lower compression ratios, the octane is needed because of the boost. I doubt you are hurting your F.E. since you are claiming to keep off the boost.

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