Detonation is when unburnt parts of the mixture reach such a high temperature that it ignites spontaneously at an early stage of the power stroke. In fact, the rate of pressure rise will occur too early in the power stroke for the piston to move downwards. Therefore, most of the chemical energy created will be wasted as heat. The cylinder walls, the piston’s head, the sparking plugs, and the exhaust valves are being hardly damaged when detonation occurs as this will cause a sudden knock. The conditions in which detonation is most likely in a piston engine are a high manifold pressure leading to a high cylinder head temperature and low RPM.
The risk of detonation can be reduced by:

• First of all, always using the correct grade of fuel designed for your aircraft’s engine. Using a too low octane rating will greatly increase the likelihood of detonation.
• Making sure the proper mixture is being supplied to the engine in any conditions. The greater the amount of fuel for a given amount of air, the greater the power that can be obtained without detonation.
• Avoid situations where the temperature and pressure of the mixture raise before it burns, such as applying carburetor heating at high power settings (takeoff/climb).

However, if you ever experience detonation during flight, the following corrective action should be carried:

• Reduce the manifold pressure (= retard the throttle)
• Fully enrich the mixture.