sealing poppet having a stem shaft 76 unitarily formed with and extending downstream from a head 78 along a valve axis 520 the head has a back underside surface 80 engaging an upstream end of a compression bias spring 82 e g a metallic coil the downstream end of the spring engages an upstream facing shoulder 84 of a bushing guide 86 the bushing guide 86 may be unitarily formed with or mounted relative to the housing and has a central bore 88 slidingly accommodating the stem for reciprocal movement between an open condition not shown and a closed condition of fig3 the spring 82 biases the element 72 upstream toward the closed condition in the closed condition an annular peripheral seating portion 90 of the head upstream surface seats against an annular seat 92 at a downstream end of a port 94 from the discharge plenum for capacity control unloading the compressor has a slide valve 100 having a valve element 102 the valve element 102 has a portion 104 along the mesh zone between rotors the exemplary valve element has a first portion at the discharge plenum and a second portion at the suction plenum the valve element is shiftable to control compressor capacity to provide unloading the exemplary valve is shifted via linear translation parallel to the rotor axes the opening and closing of the compression pockets at suction and discharge ports produce pressure pulsations as the pulsations propagate into the gas in the discharge plenum and downstream thereof they cause.