Detonation (Engine Knock)

Normally the spark plug lights the mixture and a flame front burns smoothly across the combustion chamber. In detonation, part of the remaining mixture explodes on its own from heat and pressure. The colliding pressure waves cause a metallic pinging or rattling sound, especially under load at low revs.

Detonation is related to, but not the same as, pre-ignition, where a hot spot lights the mixture before the spark. Both can hole pistons, break rings and damage head gaskets.

• Common causes: fuel with too low an octane rating, too much ignition advance, a lean mixture, overheating, or carbon build-up raising compression.
• Use the fuel grade your manual specifies, keep the cooling system healthy and don't lug the engine in a high gear.