This paper investigates a technique of calculating the completeness of combustion on a cycle-by-cycle basis. The technique introduces the normalized pressure rise due to combustion parameter (Ψ) to describe the completeness of combustion. This parameter is based on the Rassweiler and Withrow method of calculating mass fraction burned and is derived from the pressure-crank angle record of the engine. Experimental data were obtained from a Rover K4 optical access engine and analyzed with a combustion analysis package. A computer simulation was then used to model the data on a cyclic basis, both with and without the completeness of combustion parameter. The inclusion of completeness of combustion improved the simulation's ability to model the experimental data both in a statistical sense (COV of IMEP) and on a cycle-by-cycle basis.