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. The intensity of the heat and leakage losses has been varied in a broad range, and correspond phase lag errors of pressure diagrams measured at different engine speeds have been calculated. The standard deviation of the phase lag error has then been determined for the particular combination of the heat and blow-by losses. Since the phase lag error has to be theoretically the same at all engine speeds, the minimization of the standard deviation with respect to the heat and blow-by losses has finally been used to determine the best correlation between pressure and volume. The method was applied to a four stroke 7.1 / Dl diesel engine. The correlation between the pressure and volume has been determined with the accuracy of ±0.025 °CA.