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Detonation is an explosion of the unburned fuel-air mixture in the combustion chamber that burns from the heat created from the ignited spark. It creates an engine knock which is very stressful for the engine, and it often happens spontaneously.   How well gasoline resists knocking determines whether normal combustion or abnormal combustion occurs. After a spark at the plug. the flame starts across the combustion chamber. However, before the flame reaches the far side, the last of the charge explodes. The two flame fronts meet. producing a very rapid, and high-pressure rise. The result is a high pitch metallic rapping noise called pinging. Detonation can ruin an engine. The heavy shocks on the pistons put a great load in bearings and other engine parts. Pistons may chip and break. Severe detonation may lead to uncontrolled preignition. This can burn holes through the tops of the pistons.