In-cylinder pressure time history is usually measured in order to calculate both indicated mean effective pressure, as a global engine performance index, and the rate of heat release, as a quantitative information about combustion progress. Generally such information is readily provided by acquisition systems in terms of heat release rate, which is computed by means of simple inverse models based on first law of thermodynamics. Nevertheless attention must be paid to any measurement uncertainty connected to pressure cycle relief which directly results into inaccurate heat release analysis. As far as i.m.e.p. evaluation is concerned, a wrong adjustment of T.D.C. reference position, leading to incorrect pressure-crank angle phasing, had been recognized as the major error source.Starting from such considerations and by observing that greater is the in-cylinder pressure rise more important the influence of T.D.C. setting error becomes, in order to evaluate such an influence, heat release analysis has been performed for a D.I. Diesel engine. Therefore, pressure cycles have been acquired and, after a smoothing process, have been shifted with respect to the assumed reference T.D.C. position in order to simulate an experimental phasing error. The results of the corresponding net and gross heat release rate curves have been then compared showing that substantial differences can be observed both in shape and in total heat released also for an apparently small T.D.C. setting error. In addition interesting results have been found when the net heat released (i.e. the really available heat for its conversion in mechanical energy) has been compared with the active indicated work: their difference value, identified as a term proportional to the goodness of the heat conversion, has been recognized to be very sensitive to a wrong T.D.C. adjustment, thus involving a wide uncertainty in interpreting engine performance.