"Valve Clatter", "Pre-Ignition" and "Detonation" are terms that describe the fuel charge being ignited before the spark plug fires. It is harmful to the engine because the noise you hear is the engine trying to reverse it's rotation. The octane number is the measurement of the fuel to resist "detonation". Eventually, the PCM will adjust for lower octane to resist detonation by retarding the timing, resulting in lower horsepower. Btw, this is the way (illegal) race car traction control works. They program in the ramp up rate of the engine and if the computer reads too fast a ramp up rate (e.i., wheel spin), it retards the timing to lower the HP to the rear wheels. A very simple system but outlawed by Nascar and every other sanctioning body. Our Traction Control works on wheel speed indicators and brake solenoids.
But I digress, if you want the most power from your GT (and who doesn't), just buy premium fuel.[8)]
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