Abstract: The compression of each cylinder of a reciprocating, internal combustion engine, expressed in a relative fashion as compared with the compression of another of the cylinders of the engine, is determined by sensing the peaks and valleys in an engine parameter which fluctuates commensurately with fluctuations in engine operating condition while cranking, such as starter current or voltage or instantaneous, subcyclic speed. The difference (D) between each peak and the preceding valley is measured, and these are filtered by averaging them with eight-tenths of the maximum difference, and each is expressed as a percentage of the maximum filtered difference.