Abstract: In combination with an internal combustion engine and an air compressor, for use in an aircraft, the compressor providing compressed air to the engine's air inlet, there is provided a variable overboard bleed valve. The bleed valve is operated by a valve controller which controls the position of the bleed valve in response to changes in engine operating conditions. This permits maintaining optimum compressor operation over a wide range of engine operating conditions without exceeding the critical pressure of the compressor, so as to avoid surge. The controller can be preferably linked to the propeller governor and the throttle control for the aircraft, so as to move in response to movement of the governor and throttle control.