Abstract: This invention provides a method of converting a four-stroke reciprocating piston engine into an efficient two-stroke engine breathing through the original overhead valves. This is achieved by providing a reciprocating positive displacement pump having respective pumping chambers arranged with their outlet adjacent the inlets of the engine and feeding a group of power cylinders. The pump is driven through a step-up drive from the engine and the timing is such that each pumping piston leads alternate ones of the power pistons in the group fed thereby to their respective Top Dead Center positions, the inlet valve to each power cylinder to be fed opens before Bottom Dead Center and closes before TDC, and the outlet valve from the fed power cylinder opens before BDC and closes before TDC.