Knock retard

Timing the ECU automatically pulls when its knock sensors detect detonation.

Knock retard (KR) is the amount of spark advance the ECU subtracts in real time when its knock sensors hear detonation. It is the engine's self-protection mechanism, and it is your single most important feedback signal as a tuner. A few degrees of transient KR on a hard pull may be noise or a road bump; sustained, repeatable KR at the same load/RPM cells means your spark table is too aggressive there and must be pulled back. In VCM Scanner, log the Knock Retard PID (and per-cylinder KR where available) on every pull and review it before touching anything else. Remember the commanded-versus-delivered principle: the spark table shows what you commanded, but KR shows what the engine actually accepted. Never tune out knock by simply disabling the sensors or raising thresholds; fix the underlying timing, fueling, or fuel-quality cause.