Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.


Detonation can be caused by:
Lean fuel mixture
Fuel octane too low
Improper ignition timing
Lugging
Carbon deposits
Excessive milling of heads or block, which will increase compression ratio.
Pre-ignition, as the term suggests, is the ignition of the fuel-air mixture before the regular ignition spark from the spark plug. If the regular spark occurs shortly after the pre-ignition, the colliding of the two flame fronts will cause a pinging noise. Preignition causes loss of engine power and can cause severe damage to pistons, rings and valves.

Detonation and pre-ignition are so closely related that it is en difficult to distinguish one from the other by sound. Each can lead to the other and either condition can cause extensive engine damage.

Pre-ignition can be caused by:

Carbon deposits that remain incandescent
Spark plugs too hot a heat range
Spark plugs not firmly seated against gasket
Detonation or the condition leading to it
Sharp edges in combustion chamber
Valves operating at higher than normal temperature because of excessive guide clearance or improper seal with valve seats.
Overheating
Ignition crossfiring. Induced voltage in spark plug wires that run parallel to each other for long distances