What can spark knock in an engine be caused by?

Spark knock, also known as pre-ignition or detonation, can occur when the air-fuel mixture in the combustion chamber ignites prematurely. This premature ignition results in a knocking or pinging sound and can lead to engine damage over time.

Using hotter spark plugs than specified is one of the potential causes of spark knock. Hotter spark plugs have a design that retains heat more effectively, which can increase the temperature of the combustion chamber. If the heat level exceeds what is suitable for the engine's design and operating conditions, it can lead to earlier ignition of the air-fuel mixture, thus causing spark knock.

In contrast, colder spark plugs, smaller plug gaps, or larger plug gaps are less likely to directly contribute to spark knock. Colder spark plugs are designed for high-performance or high-stress conditions and can help prevent knock by dissipating heat more efficiently. Smaller gaps can reduce the spark energy, potentially delaying ignition, and larger gaps could lead to misfire or incomplete combustion rather than spark knock.