The present study investigated the effects of fluctuation in pressure converted from the piezo sensor output, TDC position and clearance volume on the polytropic index in order to develop an improved method for obtaining pressure diagrams of an engine. The effect of gas leakage from the cylinder was also investigated. The influence of changes in pressure and volume were involved primarily in the sum of the indexes on the compression and expansion sides. Changes in TDC and gas leakage influenced the remainder of the indexes on both sides. These results were confirmed experimentally.