controlling the pistons referring to fig4 the compressor unloading system of the present invention may also be applied at the discharge end of the multi rotor compressor 140 described in co pending u s patent application ser no 08 550 253 entitled multi rotor compressor by shaw a male rotor 142 is axially aligned with and in communication with female rotors 144 and 146 male rotor 142 is driven by a motor in this example male rotor 142 has eight lobes 148 155 with a 150 u00b0 wrap female rotor 144 has six lobes 156 161 with a 200 u00b0 and female rotor 146 has six lobes 162 167 with a 200 u00b0 wrap accordingly the compression phase of the axial sweep with respect to male rotor 142 occupies 150 u00b0 of rotation with the timing between the closed discharge ports 174 176 and the closed suction ports 178 180 occupying the remaining 30 u00b0 of rotation duplicate processes are occurring simultaneously on the top and bottom of the male rotor unloader pistons 182 and 184 are positioned to stop at the discharge end face of female rotor 144 and unloader pistons 186 and 188 are positioned to stop at the discharge end face of female rotor 146 when the pistons are off the discharge end face vapor is pushed back to the induction side of the compressor instead of being compressed and then pushed out the corresponding discharge port pistons 182 and 184 are positioned on the discharge end face of female rotor 144 relative to the degree of interlobe volume reduction that has taken place before initial exposure to the unloader piston breakthrough area for.