The TRD Sc is a "Roots" style supercharger, while the URD is a centrifugal supercharger. A Roots type super charger is also known as a "Positive displacement" supercharger, as it pumps its displacement into the motor, i.e. a 65 cubic inch roots blower will pump 65 c.i. every revolution of the impellers. A centrifugal supercharger is like a tubo charger, but with a gear drive instead of an exhaust impeller drive.