Abstract: An improved diesel combustion chamber of the prechamber type wherein a dual opening passage connecting the main and precombustion chambers has a large area base section below the dual openings to the prechamber and a restricted venturi between the base section and the outlet to the main chamber. In a preferred embodiment, narrowed channels in the head and piston connect with the narrowed outlet of the prechamber passage to aid further penetration across the main chamber of gas forced out through the venturi by increased pressures and combustion in the prechamber and the wide passage base portion.