1. IN AN AIR MOTOR INCLUDING A ROTOR CARRYING VANES AND WALL MEANS ECCENTRICALLY DISPOSED RELATIVE TO SAID ROTOR DEFINING A CHAMBER BETWEEN SAID ROTOR, SAID MALL MEANS AND SAID VANES, WHICH CHAMBER IS ADAPTED TO ROTATE ON AN INTAKE AND AN EXHAUST CYCLE, THAT IMPROVEMENT WHICH COMPRISES AN EXHAUST PORT LEADING FROM SAID CHAMBER ON SAID EXHAUST CYCLE, SAID EXHAUST PORT BEING SO DESIGNED THAT THE CROSS-SECTIONAL AREA OF SAID PORT IS SUBSTANTIALLY PROPORTIONAL TO THE RATE OF CHANGE OF VOLUME OF SAID CHAMBER FOR ALL POSITIONS IN SAID EXHAUST CYCLE.