Quote:
Originally Posted by dwx
It's absolutely cylinder pressures which require higher octane...
Spark advance can cause detonation, but the reason you have to pull back timing is based on cylinder pressure, whether it's a NA or FI application.
87 octane fuel doesn't burn correctly with higher cylinder pressures and spark, it explodes, which causes detonation. To say you could tune a car for 87 octane and get the same amount of power as a car tuned on 93 is completely incorrect.
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I'm wondering how much direct injection changes this. Just a few years ago you would have never thought we could boost a 12.5 to 1 compression motor with 5-8 lbs. of boost on 93 octane, but yet here we are, doing it reliably. I would guess(just a guess) that a DI motor can use a lot less spark timing since the ability to control the injector timing and precise amount of fuel injected seems to help control detonation in a big way. I have been trying to find info on how spark timing relates to fuel injector timing on a DI motor in the context of crank degrees. Plus the advances in combustion chamber design that promotes fast, even burns. But I'm still looking for that info. Anyway, I was just curious, and thought I'd throw it out here. I guess I should have expected some of the replies I got....