Experimentally determined and theoretically calculated concentrations of unburned fuel in the exhaust products of a combustion system amenable to theoretical calculations have been compared. Propane-air mixtures were spark-ignited in the center of spherical combustion bombs of various sizes. By this means, the relative importance of the quench phenomenon (a surface effect) has been evaluated. The results show that simple application of quench theory is not consistent with the experimental results. Use of a modified quench theory, based on the results from the bomb experiments, for theoretical calculations of unburned hydrocarbon in engine exhaust indicates that the exhaust unburned hydrocarbon concentration from a large-displacement, low-compression ratio engine should be much lower than that of a small-displacement, high-compression ratio engine.