Abstract: A supercharging system for an internal combustion engine, consisting of a positive displacement air pump mechanically driven by the engine, and an electromagnetic clutch activated during high torque demand operating conditions. The system produces a high torque output in a high torque demand range from a relatively small displacement engine, with economy operation during normal engine operating conditions. A flow modulation control system provides a smooth transition from natural aspiration to supercharged operating conditions and includes a sensing of the pressure drop across the throttle plate and modulation of the supercharger air flow into the engine intake in accordance with the sensed pressure differential. The flow modulation also enables high efficiency supercharging with the engine operating under partial pressure supercharging as required in ascending a grade.