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It Depends. . . (archive)

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Posted by Chris in Atlanta on March 02, 2001 at 10:02:26:

In Reply to: I thought the specs were a minimum value? posted by John D. on March 02, 2001 at 08:53:29:

On where the car is optimized, octane-wise. If the car was optimized for 96 octane fuel, and you were running 93 - you could benefit. Because the timing would advance itself to take advantage of the higher octane. Note that higher octane fuel helps not at all unless your timing is advanced to compensate for it.

But then the car would be retarding its timing for 99.9% of all drivers. Possible, but unlikely. More likely the fastest timing (allowed by the stock chip) will optimize at 91-92 octane (M engines - otherwise figure a few points below the fuel recommended in the owners manual), and higher octane will not benefit you.

Note (again) that the retard function will certainly let you run lower octanes, but if the top octane anticipated is relatively low - as seems likely - you would not benefit.

I tried a bottle of high-quality octane boost at an autocross (quite illegal in stock class (blush)) a year ago. I could feel no difference from the 93 octane I usually run.

Now, if you have a turbo or supercharger, or an aftermarket chip that is optimized for higher octane (possible but unlikely), higher octane could help you. But in a stock car - no.

I guess the question is at one level does higher octane become less and less a factor.

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