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Air pollution problems in California required control of vehicle exhaust emissions. The early development of catalytic converters and direct flame afterburners led to relatively complex mechanisms with substantial added cost to the vehicle. An evaluation of the primary factors affecting exhaust emissions indicated the feasibility of control by engine modification alone. A series of engine modifications were subsequently developed which reduced the exhaust emissions to below the California standard of 275 ppm hydrocarbons and 1.5 percent carbon monoxide and are discussed in this paper.