Detonation

Uncontrolled secondary auto-ignition of the end gas after the spark, producing a sharp knock.

Detonation is abnormal combustion where, after the plug fires, the remaining unburned end gas spontaneously auto-ignites and collides with the normal flame front. The colliding pressure waves create the sharp metallic knock you hear and that knock sensors detect. It spikes cylinder pressure and temperature and, if sustained, hammers pistons, ring lands, rod bearings, and head gaskets. Causes include too much spark advance, a lean mixture, low-octane fuel, excessive intake-air temperature, and high boost. HP Tuners does not show detonation as a value directly; you infer it from the Knock Retard PID in VCM Scanner and from audio. Because it is fundamentally tied to fueling, validate your air-fuel ratio with a wideband before adding timing or boost, never trust commanded AFR alone, and treat any repeatable knock retard as detonation until proven otherwise. A safe tune builds in margin so normal real-world variation never crosses into it.