If that is the one between the intake and the ALDA, you can bypass it if you'd like. It's the overload prevention mechanism. The boost pushes the diaphragm in the ALDA to give more fuel, if that valve leaks or blocks the boost you'll have a slow car. Most of the 617 turbos I've had that valve goes in the trash and I run a straight hose between the intake and ALDA. I'm not sure how it actually worked, but if there was too much load I think it would stop allowing more fuel somehow. They were a bit cautious about overboost etc on these originally.