Detonation, also known as knock or pre-ignition, is an undesirable engine characteristic that refers to spontaneous combustion of the remaining fuel/air mixture in the chamber after normal spark-initiated combustion. It is a pinging sound that can sometimes be heard during acceleration and throttle tip-in. Detonation can cause serious internal damage to the engine, including melted spark plug electrodes, cracked piston rings, melted or cracked pistons, hammered rod bearings, and blown head gaskets.

Detonation happens when a heat source within the combustion environment ignites the fuel/air charge prior to initiation by the engines ignition system. It is caused by excess heat and pressure, leading to engine knock. Detonation can be caused by several factors, including over-advanced ignition timing, lean air/fuel mixture, carbon deposits, and overheating engine.

To avoid detonation, it is important to ensure that the engine is running properly and that the ignition timing and air/fuel mixture are set correctly. If the engine is experiencing detonation, there are a range of things that can be done to prevent it, including using higher octane fuel, retarding the ignition timing, and increasing the fuel flow.