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In two-stroke engines, the exhaust port timing has a great effect on engine performance characteristics. If the exhaust port timing can be varied in response to variations in the engine speed, an extensive improvement of performance can be realized. The Yamaha Power Valve System (YPVS) increases engine output with a valve which operates in the exhaust passage; this valve controls exhaust port timing in response to engine speed, despite the extremely high thermal load encountered in the exhaust passage. This literature deals with the construction and operation principles of YPVS and its effect on the increase of engine output. Also discussed is a series of tests and measurements made on the delivery ratio and trapping efficiency in order to elucidate why engine output is increased.