Abstrart-Experiments have been conducted with a homogeneous charge spark ignition engine fitted with a transparent dual (or divided) chamber cylinder head. High-speed cine photography has been used to chart flame progress. The rate of burning and performance of the engine have been determined for a range of pre-chamber sizes and pre-chamher to main-chamber interconnecting orifice sizes. A comparative series of experiments has been made with the same engine equipped wilh a disc-shaped single chamber cylinder head. Combustion proved faster with the dual chamber geometry; the rare of combustion has been shown to increase in accord with the calculated maximum orifice flow velocity during compression and the associated degree of turbulence generated. The range of turbulence intensity generated by the flow through the orifice is likely to have been greater than that noted in previous single chamber studies. A levelling OK in burning speed at very high turbulence levels, previously observed in stirred bomb studies, was encountered.