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Efficiency of a supercharger mainly depends on the clearances between the rotors and the housing, but it has hitherto proved impossible to reduce the clearances without a strong possibility of seizure. A two lobe Roots type supercharger has been developed which overcomes this problem by means of a special resin coating on the rotors. As a result, an increase of 18% in volumetric efficiency of the supercharger is achieved. Engine torque is increased by 30%. Drive power loss resulting from the operation of a supercharger at partial load when boost was not required caused fuel consumption to increase by about 10%. An electro magnetic clutch and an air by-pass valve controlled by the onboard computer were used in the new supercharger to eliminate this fuel economy loss. A passenger car equipped with the new supercharged 2-liter engine demonstrated about the same acceleration performance as a 3-liter class car without sacrificing fuel economy.