A methodology for predicting the cyclically variable pressure time history based on correlations derived from optically observed flame fronts in the engine is presented. The methodology relies on relating the observed flame propagation to engine design and operating parameters like swirl, engine speed and chamber geometry etc. The correlations are developed using typically 25 cycles of engine data for each operating condition. The cyclic variability is incorporated in the model by analyzing the observed variability in the flame kernels corresponding to 1% burnt mass. The pressure time history is predicted using a spherical flame front model. The model is able to predict both the pressure history and the cyclic-variation reasonably well.