Originally Posted by Mike@GTM

The bypass valve needs to be open at idle since the throttles are closed (high vacuum in the manifold) and the supercharger is trying to force more air into the engine. The bypass valve operates on a pressure differential between the charge pipe and the manifold. The higher the differential pressure, the more the valve will open. It is operating correctly. Also, if the belt were to fail, the supercharger would stop turning and therefore would no longer be pressurizing the charge pipe, the pressure differential between the intake manifold and the charge pipe would be less than the spring pressure in the bypass valve and the valve will stay closed.

Putting a stiffer spring in the bypass valve will cause compressor surge since the charger would be trying to move a small amount of air at very high pressure ratios. Bad idea.

The reason why a blow off valve on a turbo is closed at idle is simply due to the fact that at such low exhaust gas flow, the turbocharger is not spinning fast enough to even generate positive boost pressure at idle.