Instead of a smooth flame front initiated by the spark plug, high cylinder pressures cause pockets of unburned fuel to explode spontaneously. This creates a massive, uncontrolled pressure spike that hits the piston like a hammer. Sustained knock can shatter ring lands, melt pistons, and destroy engine bearings.
I'm pulling three degrees of timing up top because the datalog shows knock on cylinder four under heavy boost.
If you hear a sound like marbles rattling in a tin can when accelerating up a hill, your engine is knocking.
Also heard as
- detonation
- pinging
- spark knock
- Knocking
Do not mix it up with
- Preignition: Preignition happens before the spark plug fires, while knock usually occurs after the spark has ignited the initial flame front.
Related terms
Preignition
The uncontrolled ignition of the air-fuel mixture in a cylinder before the spark plug fires.
Anti-knock index
The standard numerical rating of a fuel's resistance to premature ignition.
Ignition timing
The exact moment a spark plug fires relative to the position of the piston during the compression stroke.