A gear compressor or supercharger for compressing compressible fluids such as air, having a pair of intermeshing helical lobed rotors. A plenum or cavity is provided at a rear end of the gear compressor, situated rearwardly of said rotors and below an respective axis of rotation of each of said rotors, which spans at least a distance between said respective longitudinal axis of rotation of each of said rotors. In a preferred embodiment the plenum is in direct fluid communication with high pressure fluid which is discharged from said high pressure discharge port. The above modification improves the efficiency of the compressor, particularly at high revolutions.