The bypass is there to bypass boost when not under load, and then to release boost(BOV) when you close the throttle blades after a pull. The valve has a spring in it that naturally holds the valve closed. There is also a vacuum line connected from manifold vacuum to the top of the bypass/BOV. Anytime you are not under load, and the motor is making enough vacuum, the vacuum holds the bypass open, allowing all the airflow the blower is making a place to go. When you get on the gas enough, that engine vacuum drops to near zero, the vacuum on the port on the Bypass/BOV goes to near zero and the spring presses the valve shut allowing all the air from the blower to go to the carb. Once you get off the gas and close the throttle blades on the carb, engine vac comes back and quickly opens the Bypass/BOV to release all the charged air in the charge pipe so it doenst slam into the closed carb blades causing the blower to surge.
As it is, at idle alone, the blower is making about 1/2psi of boost in the carb hat(not in the motor) with the bypass open! With the bypass closed it would be making even more boost in the hat at idle. The thing with the bypass valve is on the highway, if I am turning 3000rpm with steady throttle, the engine is making plenty of vacuum to keep the bypass valve open, which is a good thing. No need for the blower to be rolling down the highway trying to force a ton of air into a carb that barely has the throttle blades cracked open... that would only create heat and wear on the blower.
What makes this really fun is the instant boost! I have to check my logs, but theoretically at 4000 RPM at WOT this blower is already making 3-4psi of boost. I say theoretically because I can never really be at 4000RPM at WOT because my converter flashes higher than that. But anyway, cruising down the highway at 3000rpms and I wack the gas and the motor jumps to 4500rpms, the blower is already pushing a ton of air and as soon as I go WOT, the bypass instantly closes and all that bypassed air goes directly to the carb. Manifold pressure goes from -7psi(~13" of vac) to 4psi of boost instantly!
