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Originally Posted by DuSpinnst
The PCM when it gets a message from the knock sensor it adjusts the spark timing. Octane is actually a knock resistant measurement. When you use higher Octane your engine is less likely to knock. On old cars that don't have knock sensors need to be switched to premium fuel later on to prevent them from knocking. Now a days the knock sensor adjusts the octane level, but only in the direction down. So any car that says it requires or recommends 91 octane will run on 87 but a SIGNIFICANT performance reduction. Running 91 in an car that requires 89 doesn't allow the engine to run faster though.
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Exactly.It is not like there is a magical octane sensor that adjusts the timing etc.This is why if your vehicle only requires 87 octane it is a waste to run premium.But if the manufactor suggest premium you should always use it.