A model which converts flame arrival times at a head gasket ionization probe, used in a spark-ignition engine, into flame contours has been developed. The head gasket was manufactured at MIT using printed circuit board techniques. It has eight electrodes symmetrically spaced around the circumference (top of cylinder liner) and it replaces the conventional head gasket. The model is based on engine flame propagation rate data taken from the literature. Data from optical studies of S.I. engine combustion or studies utilizing optical fiber or ionization probe diagnostics were analyzed in terms of the apparent flame speed and the entrainment speed (flame speed relative to the fluid ahead of the flame). This gives a scaling relationship between the flame speed and the mass fraction burned which is generic and independent of the chamber shape.Experiments were run to check the accuracy of the model's predictions at 18 different operating conditions, including variations of equivalence ratio, engine speed, and flowfield inside the combustion chamber. Measurements of flame arrival time at three intermediate points between the spark plug and the liner with ionization probes and optical fibers were used to check the model predictions. The overall agreement between the model and the experiments was good. Calculations were also done using a thermodynamic burn rate analysis which gave the mass fraction of the mixture burned at a given time. These were compared to mass fraction burned predictions based on the model-generated flame contours and good agreement was obtained.