Although knowledge of detonation and preignition may be "old hat" to the old timers in aviation, lots of people in our industry are still somewhat confused over the difference between the two, and what causes either of them.
There is a limit to the amount of compression and the degree of temperature rise that can be tolerated within an engine cylinder and still permit normal combustion. When this limit is exceeded, detonation can take place. Piston engines are vulnerable to detonation at high power output because combustion temperature and pressure are, of course, higher than they are at low or medium powers. Leaning the mixture at high power can cause it.
Unless detonation is heavy, there is no cockpit evidence of its presence. Light to medium detonation may not cause noticeable roughness, observable cylinder head or oil temperature increase, or loss of power. However, when an engine has experienced detonation we see evidence of it at tear down as indicated by dished piston heads, collapsed valve heads, broken ring lands, or eroded portions of valves, pistons and cylinder heads. Severe detonation can cause a rough running engine and high cylinder head temperature.
Preignition, as the name implies, means that combustion takes place within the cylinder before the timed spark jumps across the spark plug terminals. This condition can often be traced to excessive combustion deposits or other deposits (such as lead) which cause local hot spots. Detonation often leads to preignition. However, preignition may also be caused by high power operation at excessively leaned mixtures. Preignition is usually indicated in the cockpit by engine roughness, backfiring, and by a sudden increase in cylinder head temperature. It may also be caused by a cracked valve or piston, or a broken spark plug insulator which creates a hot point and serves as a glow spot. Specifically, preignition is a condition similar to early timing of the spark. Preignition is a serious condition in the combustion chamber and will cause burnt pistons and tuliped intake valves.
The best temporary in-flight methods for correcting preignition and detonation are to reduce the cylinder temperature by retarding the throttle, enriching the mixture, opening cowl flaps if available, or a combination of all of these.