In order to improve the volumetric efficiency of a crankcase-supercharged 4-stroke cycle engine in the high speed range, rotary disc valves were used at both the inlet and the outlet sides of the crankcase. The effects of the size of the opening and the phasing of the disc valves on volumetric efficiency were investigated and compared with a system using reed valves.It was found that a volumetric efficiency higher than 120 % could be obtained over a speed of 3000-6000 rpm, which was not possible by using the reed valve system.SUPERCHARGING SYSTEMS utilizing Roots type blowers and/or exhaust turbochargers have been used effectively for increasing the brake mean effective pressure of 4-stroke cycle engines. The authors built a prototype cranckcase-supercharged 4-stroke cycle engine, which used the underside of the piston as a supercharging pump, and confirmed the effectiveness of the concept by motoring and firing tests. Some of the results are reported in previous papers. (1)(2)* The previous system used reed valves at both the inlet and outlet of the crankcase and was found to be incapable of producing a high level of supercharging in the high engine speed range. The authors also reported that, in a Schnürle scavenged 2-stroke cycle engine using a rotary disc valve at the inlet port of the crankcase, it was possible to improve the delivery ratio and the torque characteristics over a wide engine speed range by changing the size of the opening (cut-angle) and the phasing (timing) of the rotary disc valve with engine speed. (3) Therefore, it was considered that, by replacing the reed valves in the crankcase-supercharged 4-stroke cycle engine with rotary disc valves, volumetric efficiency improvement in the high engine speed range might be achieved. Based on this concept, a crankcase-supercharged 4-stroke cycle engine with rotary disc valves at both the inlet and outlet ports of the crankcase was built, and its volumetric efficiency characteristics were investigated for different valve designs.