The supercharger on the March ST has a differential by pass valve which compares its boost to the turbo and slowly opens when the turbo boost exceeds that of the supercharger, then switching off the supercharger allowing a smooth transition between the two. Its not perfect but it does work pretty damn well. I know - I own one! Just out of interest I just had my march ST dyno'ed at 171bhp@5500 rpm. Thats from a 930cc engine..