To obtain a better understanding of the combustion process in a spark ignition engine with an unscavenged horizontal prechamber connected to a main chamber by a torch nozzle of various area sizes (An), the flame propagations in the pre- and main chambers are simultaneously visualized with high-speed photography under such operating conditions as the optimum spark advance for best torque (MBT), volumetric efficiency ηυ=80±2%, air-fuel ratio of A/F<bsime>15 and engine speed of N <bsime>1000 rpm. The torch flame patterns in the main chamber are found to correspond fairly well with those of heat release rates computed from the ensemble-averaged pressure diagrams. On the basis of those flame photographs, several combustion characteristics, namely, the apparent flame speed, timing of actual ignition, ejected timing of torch flame and timing of combustion finish and combustion area fraction are also examined for each torch nozzle with different areas and an open chamber. In addition, the correlations between the above combustion characteristics and the engine performances are discussed.