Knock

Knock (also called detonation, ping, or pre-ignition) is engine combustion that occurs outside of the normal flame front propagation from the spark plug. Instead of burning smoothly from the spark plug outward, knock occurs when the unburned air-fuel mixture (the end gas) ignites spontaneously due to high pressure and temperature, creating multiple flame fronts that collide. The result is a pinging or knocking sound and rapid combustion that creates extreme cylinder pressure spikes.

Sustained knock is destructive. The pressure spikes can damage piston ring lands, break ring lands off the piston, damage rod bearings, crack pistons, or in extreme cases break connecting rods. Modern engines have knock sensors that listen for the characteristic knock frequency and the ECU retards ignition timing when knock is detected, protecting the engine.

Causes of knock include low octane fuel (insufficient resistance to spontaneous combustion), excessive ignition timing advance, lean air-fuel mixture, excessive intake air temperature, high compression ratio, and turbo overboost. Tuning involves balancing all these factors to extract maximum power without crossing into knock conditions. The knock sensor is the primary defense; aggressive tunes verify knock-free operation through dyno logs.