A compressor comprising: a compression mechanism, the compression mechanism being configured and positioned to receive vapor from an intake passage and provide compressed vapor to a discharge passage ; a port positioned in the compression mechanism to bypass a portion of the vapor in the compression mechanism to the discharge passage ; a valve configured and positioned to control vapor flow through the port , the valve having a first position to permit vapor flow from the compression mechanism to the discharge passage and a second position to prevent vapor flow from the compression mechanism to the discharge passage ; the compressor having a first volume ratio in response to the valve being in the second position and a second volume ratio in response to the valve being in the first position, the first volume ratio being greater than the second volume ratio; the valve being positionable using the pressure of vapor entering the intake passage and the pressure of vapor discharged from discharge passage to operate the compressor at the first volume ratio or the second volume ratio, characterized in that the compression mechanism is a screw compressor with rotors, and the valve is positioned below the rotors and extends across the compressor substantially perpendicular to the flow of vapor, wherein the valve comprises a valve body snugly positioned in a bore to avoid unnecessary leakage, wherein the valve body comprises one or more gaskets or seals to prevent a leakage of fluids and has varying diameters including a larger diameter portion and a smaller diameter portion , so that when the valve body is positioned in the closed position, larger diameter portion of valve body 102 blocks or closes off port , and when the valve body is positioned in the open position, smaller diameter portion of valve body is positioned near port to permit flow of vapor from port around smaller diameter portion to discharge passage , and wherein the ends of the bore are sealed. a compression mechanism, the compression mechanism being configured and positioned to receive vapor from an intake passage and provide compressed vapor to a discharge passage ; a port positioned in the compression mechanism to bypass a portion of the vapor in the compression mechanism to the discharge passage ; a valve configured and positioned to control vapor flow through the port , the valve having a first position to permit vapor flow from the compression mechanism to the discharge passage and a second position to prevent vapor flow from the compression mechanism to the discharge passage ; the compressor having a first volume ratio in response to the valve being in the second position and a second volume ratio in response to the valve being in the first position, the first volume ratio being greater than the second volume ratio; the valve being positionable using the pressure of vapor entering the intake passage and the pressure of vapor discharged from discharge passage to operate the compressor at the first volume ratio or the second volume ratio, characterized in that the compression mechanism is a screw compressor with rotors, and the valve is positioned below the rotors and extends across the compressor substantially perpendicular to the flow of vapor, wherein the valve comprises a valve body snugly positioned in a bore to avoid unnecessary leakage, wherein the valve body comprises one or more gaskets or seals to prevent a leakage of fluids and has varying diameters including a larger diameter portion and a smaller diameter portion , so that when the valve body is positioned in the closed position, larger diameter portion of valve body 102 blocks or closes off port , and when the valve body is positioned in the open position, smaller diameter portion of valve body is positioned near port to permit flow of vapor from port around smaller diameter portion to discharge passage , and wherein the ends of the bore are sealed. <EOS>