An engine model is developed which incorporates the physical effects of deposit. Engine octane requirements are calculated from the model’s unburned gas temperature-pressure tracks using a recent empirical expression for the autoignition delay of hydrocarbon-air mixtures. The computed octane requirement for the clean model engine is in reasonably good agreement with experimental values, as is the octane requirement increase (ORI) due to the increase in compression ratio caused by the assumed deposit. The computed ORI due to the thermal effects caused by the deposit is too low by a factor of approximately 3, however. Possible explanations for this unresolved discrepancy are discussed.