The disclosure describes apparatus for determining the relative compression of the chambers of an internal combustion engine by analyzing the voltage or current waveform of the starter system during cranking. The waveform is rapidly and repetitively sampled by an analog-to-digital converter which converts the amplitude waveform into corresponding digital signals. A processor selects one of the signals having either a maximum or minimum value to represent each of the chambers. The processor automatically determines the number of chambers in the engine, determines the relative compression of the chambers and displays the result to the operator of the system.