Quote:
Originally Posted by Nate
...high octane gas is more potent and will give you the same horsepower 87 gives you while burning less of it....
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Not true. In fact, the only reason that 91+ would ever give you more power in a given car is because the engine advances the timing and/or increases the boost to take advantage of the more detonation-resistant fuel. It does this by listening to the knock sensor--if there's no knocking, it can advance the timing (or increase boost).
In some cases, in fact, higher-octane fuel has (marginally) less energy per volume than regular-grade fuel! But in all cases that I know of, any difference is small enough to be "in the noise".
The only thing that the "octane" number of a fuel tells you is its resistance to pre-ignition. Nothing more, nothing less. It doesn't have noticeably more energy, it doesn't burn significantly slower, it just doesn't light itself off as easily. I've been schooled by several petro engineers on that subject over the years...
Now,
if your engine is built for high-octane fuel, you will generally get better performance and economy by using high-octane fuel. Note that many modern engines are built to deal with regular-grade fuel, but also to take advantage of higher octane when it is available. That's one of the advantages of running a knock sensor. Some older cars can take advantage of higher-octane fuel by manually advancing the timing (to a point!), but it isn't universal by any means.
Some people have been able to get better FE out of running higher-octane fuel. Some have shown a big enough improvement that the extra cost is worth it. Some have not shown any improvement at all. The best way to tell on your own car is through carefully instrumented and rigorous testing.
-soD