Engine-out HC emissions were measured in SI engine experiments in which the piston topland crevice size was changed systematically. For a warmed-up engine, the HC emissions were found to be modestly sensitive to the piston crevice size-a 10% change in size results in approximately a 2% change in HC emissions. This low sensitivity is explained in terms of a crevice HC diffusion/oxidation model in the expansion process. When the piston crevice is sufficienlly small, however, the model shows that the HC emissions have a one-to-one correspondence with the crevice size.