Computational fluid dynamics (CFD) has been widely applied to screw compressors. CFD has allowed engineers to gain a better understanding of the complex flows that occur inside the screw compressor. In many cases, the details revealed are only possible with CFD compared to 1-D simulations. Both CFD and 1D can reduce development costs and as in this case, guide decisions on the choice of hardware to be tested. This paper presents the flow details of the suction process in a screw compressor and the effects on performance. The cases presented are a twin screw rotor set applied to two different suction port designs. One suction port matches the helix angle of the rotors and closes at the maximum suction volume. The other suction port stays open for 20 degrees of rotation past the maximum suction volume and does not match the rotor helix angle. The paper presents the effects of the geometry difference at various rotor rotational speeds.