An extension of a thermodynamic methodology of TDC determination in IC engines is presented. The effect of an error on the TDC position coupled with an error on the compression ratio is analyzed in the temperature-entropy diagram. When the TDC position and the compression ratio are well calibrated, compression and expansion strokes under motoring conditions are symmetrical with respect to the peak temperature in the (T,S) diagram. Moreover, in case of an error on the TDC position, a loop appears, which has no thermodynamic significance. In the same way, in case of a compression ratio error the (T,S) diagram leans. Hence, an easy methodology has been conceived to obtain the right position of TDC and eventually to correct the compression ratio. This methodology is applied on motoring measurements to assess its performance.