Using n-heptane and iso-octane as gasoline surrogate fuels, the laminar flame speeds, flame thicknesses, reaction zone thicknesses, and flammability limits of fuel/air/residual gas mixtures are computationally studied over ranges of pressures, temperatures, and dilution levels representative of unthrottled HCCI operation. These calculations are further used to characterize the combustion regime for spark-initiated HCCI combustion. In a turbulent premixed combustion regime diagram, the local flow conditions at the time of spark are estimated based on the in-cylinder turbulence levels reported in the engine combustion literature. Although the calculations show that these highly-diluted mixtures would be flammable, the flame speed may be so low and the flame so thick that the in-cylinder turbulence level may be sufficiently high to destroy the laminar flame structure. In order to increase the flame strength so that the local flame structure will not be affected much by the in-cylinder turbulence, spark would need to occur at a later crank angle during compression or the fuel mixture can be stratified near the spark plug.