The bypass valves on Roots or Screw type superchargers are different than bypass valves on turbos. Roots bypass valves are used to increase fuel economy in normal driving (not under boost) by allowing the air to enter the engine without going through the supercharger, thereby eliminating alot of the power loss due to compressing the air. Since Roots type superchargers are generally installed after the TB, they don't have to worry about compressor surge.
DemonSVT, please correct me if I'm wrong, but I'm hoping to clarify what you're saying a little. The wastegate controls the boost a turbo makes by limiting the amount of exhaust that goes to the turbo, thereby limiting how quickly it can spin, thus limiting the boost it can produce. It is installed in the exhaust stream before the turbo. The wastegate also protects the turbo from spinning too quickly and destroying itself by limiting the amount of exhaust gases routed to the turbo.
The blowoff valve is a safety valve that is installed in the intake stream after the turbo compressor to protect your engine in the case that the wastegate malfunctions, or someone bumps your boost controller and accidentally sets your boost to 80psi instead of 8psi or something. It is a backup device for protecting your engine.
I know we're discussing conventional turbo's here, but a wastegate is not required if you use a variable nozzle turbine turbo, since it can control the boost by itself.
Bob
