Preignition in an
internal combustion engine. Occurs when the fuel-air mixture
spontaneously ignites before it should. Typically when the
piston is performing its compression stroke, but before it reaches Top Dead Center. Can be caused by low octane gas, or by carbon buildup on piston heads (which is a sign of other problems such as oil seepage).
Predominant controlling factor in the amount of boost that can be run in a turbocharged engine. The higher the boost, the more danger of knock and the higher rated octane gas required to keep the engine running smoothly. High levels of knock are bad, and can destroy pistons and connecting rods very quickly, leading to an engine rebuild.