Detonation or spark knock is erratic combustion. Detonation happens when excessive heat and/or pressure causes the charge to ignite. This can cause multiple flame fronts in the combustion process instead of a single flame kernel. When these flames fronts collide, the violent reaction produces a sharp increase in cylinder pressure. This is often heard as pinging or knocking. The force is so violent it can damage head gaskets, pistons, rings, and even rod bearings.
This occurs in all engine but continued and prolonged detonation can cause serious damage.
Pre-Ignition is typically caused by a hot spot during the combustion process, before the spark plug fires the cylinder. It can be caused by excessively hot Carbon deposits, overheated spark plugs (too hot of a heat range) or hot sharp edges on a spark plug, piston or in the combustion chamber.
Too lean AF condition can promote hot spots.