08. On the backside of the plenum is a blow-off valve (arrow). The blow-off valve is a spring-loaded valve that is connected to a line that is tapped into the backside of the butterfly in the throttle body (vacuum side of the engine). When you’re on the bike at full throttle/full boost and then you close the throttle body off and go from 8 pounds to nothing, it creates pressure, which creates a vacuum that sucks open the blow-off valve to relieve excess boost pressure left inside the manifold. This helps prevent compressor surge, prevents slowing the turbo down, helps the turbo last longer, and increases responsiveness.