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Originally Posted by lynch mob
Really why don't you explain to the class how the PCM will know the difference between 87 octane and 91 or 93 octane?So what you are saying is the PCM will recognize a lower octane fuel and adjust the timing etc. based on the octane?I really would like to hear this answer.The only thing the PCM will do is retard timing if it hears spark knock.Which i doubt would happen if you run 87 instead of premium in this engine.
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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.