The effects of ignition location on combustion duration, thermal efficiency, exhaust emissions, cyclic variability, and knock sensitivity were investigated in two premixed-charge spark-ignition engines with disk-shaped combstion chambers and having high levels of swirlu. Except for very lean high-swirl conditions, peripheral ignition produced a longer combustion duration than did central ignition, but the difference was less than expected from geometric considerations alone. Flame holding at the spark electrodes, observed in high-speed schlieren films, enhances mass-burning rates for off-center ignition locations. For a swirl number of 4 no significant differences with ignition location were measured in thermal efficiency, heat-transfer losses, exhaust temperature, cyclic variability, or exhaust emissions, but with peripheral ignition the engine had a greater knock sensitivity, lower knock-1imited peak power, and a misfire problem of unidentified origin. For a swirl number of 7 an increase in heat-transfer losses and a corresponding decrease in thermal efficiency were measured for peripheral ignition. Thus with intake-generated swirl, central ignition was found to be desirable in the two engines tested.