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A statistical method is described which allows a very accurate determination of the correlation between pressure and volume. The basis of the method is the pressure measurement of an engine under motoring conditions and correcting the pressure diagram with respect to heat and blow-by losses. Computation of the polytropic exponent dependent on error of phase lag determines the correlation in the pressure volume diagram. Since the internal processes in an IC engine under motoring conditions differ significantly from the ones under firing conditions, some modifications in the heat and blow-by losses models are necessary in order to be successfully applied in the proposed method. A simple optimization approach has therefore been used to correct both models. Several pressure diagrams measured under engine motoring conditions and at the different engine speeds have been studied.