Application of apparent heat release analysis appears to be very convenient in finding the optimum ignition timing (MBT) of reciprocating engines. The crank angle where apparently 50% of heat has been released was called the combustion phasing angle. The combustion phasing angle present at MBT ignition timing has been defined the optimum combustion phasing angle. The value of the optimum combustion phasing angle appears to be close between 7 to 8 deg after Top Dead Centre, independent of process conditions. This knowledge makes it easy to readjust ignition timing of engines when fuel quality or air to fuel ratio vary, or when design changes have been carried out.