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Originally Posted by webby
twisted frame told you why. Detonation - the compression is high on the SI.
what is detonation:
a condition, in which, after the spark plug fires, some of the unburned air-fuel mixture in the combustion chamber explodes spontaneously, set off only by the heat and pressure of air-fuel mixture that has already been ignited. Detonation, or "knock", greatly Increases the mechanical and thermal stresses on the engine.
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I hate to disagree but I may also be talking about a different condition. I had always referred to this condition as predetonation. Which is a condition where the air fuel mixture ignites only by the heat and pressure just prior to the spark plug firing. The knock is then caused by the exploding gasses slamming down on the piston that is still in upward travel. The sound is created at the area where the wrist pin connects to the piston.