Detonation or spark knock is the sound and related effects produced in spark-ignited internal combustion engines by instantaneous ignition and combustion (autoignition) of the gaseous fuel-air mixture ahead of the advancing flame front.

After spark ignition, a flame travels outward from the spark plug and, under normal combustion, will progressively burn the entire fuel-air charge. The burned gas liberates heat and expands, leading to increased pressure and temperature in the unburned gas ahead of the flame front. The unburned gas may be raised above its autoignition temperature. If the flame front velocity is too small the unburned gas spontaneously ignites and burns instantaneously. The instantaneous combustion results in a very intense pressure wave that produces the audible, high-frequency (pinging) sound known as spark knock.

Besides sound, spark knock can result in pitting, or erosion, of the combustion chamber, damage to spark plug electrodes, and possible structural damage to the engine. Spark knock also leads to loss of engine efficiency by inducing spark plug preignition, resulting in overly advanced spark timing. Spark knock also causes intense turbulence within the cylinder, aggravating heat loss from the burned gas to the colder cylinder and head surfaces and reducing efficiency.