Abstract: While the crankshaft of an internal combustion engine is rotating, an encoder device produces a reference signal pulse and a series of encoder signal pulses during each crankshaft revolution and a microwave signal of a substantially constant amplitude and frequency is coupled through a microwave probe into the combustion chamber of a selected reference cylinder of the engine. The microwave signal waveform reflected from the reference cylinder combustion chamber while the reference cylinder piston approaches, passes through and moves away from the top dead center position during the compression and power strokes is processed to determine the encoder signal pulse count at which the line of symmetry thereof, the piston top dead center position, occurs. Initial ignition timing angle is then set relative to the encoder signal pulse count corresponding to the piston top dead center position.