There are several problems with the historical design. The vane tips radially contact the cam ring elliptic surface and cause a significant friction as the rotor and vanes turn. This friction is both pressure dependent and speed squared dependent due to RPM-squared centripetal forces. The speed is limited to about 6-7,000 RPM and the pressure is limited to about 2,500 PSI. Another pressure-related problem is that the pressure forces into the axial rotor to stationary kidney endplate clearance and buckles the device ends thus increasing fluid blowby referred to as 'volumetric efficiency'. Typically, vane pumps and motors have two external ports but there are actually two separate sets of chambers which form two separate pumps and motors. The internal plumbing is y-connected to create only two external ports.