Abstract: In a combustion timing control system of the type wherein peak combustion temperature is sensed and combustion initiation timing is adjusted in closed loop to maintain peak combustion pressure at a desired constant crankshaft rotational angle, the combustion pressure signal is processed in apparatus having differentiating elements with a first time constant, smoothing integrating elements with a second time constant and further elements effective to vary both first and second time constants inversely with crankshaft rotational speed, wherein signal to noise ratio is optimized over the entire range of said speed.