Abstract: In a spark ignition internal combustion engine, an accelerometer is mounted on an engine component, preferably the intake manifold, and senses vibration including individual ringing vibrations due to engine knock or detonation. A reference signal is derived from the accelerometer signal during the portion of the engine cycle prior to possible knock-induced vibrations, which signal indicates the background accelerations due to noise. The number of individual ringing vibrations exceeding the reference signal during a predetermined amount of engine crankshaft rotation is counted. When the number of such counts exceeds a predetermined number during the predetermined crankshaft rotation, engine spark timing is retarded. If the number of such counts is less than a predetermined number during the predetermined crankshaft rotation, engine spark is advanced.