put the cheapest gas in your car and if you hear the engine pinging go to the next level and so on.
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The only time higher octane is of any benefit is if the car pings on the recommended grade, the timing is advanced, compression is higher than stock, or the car has forced induction. Pinging occurs when the spark occurs too early in the cycle. If the fuel/air charge is not compressed enough (i.e. harder to light) the charge explodes rather than burning at a controlled rate. The two means of eliminating this condition are to a) use higher octane fuel which is more resistant to ignition and burns slower and cooler (roughly 1 degree per point of octane) or b) retarding the ignition timing. This is exactly what happens when the knock sensor detects pinging.
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