A supercharging system for supplying pressurized combustion air to a reciprocating piston engine of an aircraft at high altitudes. The engine has an output shaft on which the propeller is supported, together with a small ducted centrifugal fan. The duct leads to the intake side of the engine. A diverter is movable to one position constraining pressurized air to enter the engine through the intake side, and a second position which diverts compressed air away from the intake side. Preferably, the diverter also opens the intake side to unpressurized atmospheric air in the first position, so that the engine is naturally aspirated. Natural aspiration is employed at low altitudes and supercharging is employed at altitudes wherein there is an oxygen deficiency. Preferably, the centrifugal fan is located between the propeller and the engine block, has blades staggered from those of the propeller so as to be directly exposed to incoming air. The centrifugal fan blades are preferably of different pitch than the propeller blades.