So with a procharger, there actually isn't a blowoff valve option. A blowoff pops when you produce too much boost like with a turbo, a bypass valve pops when you suddenly reduce the throttle to keep pressure from damaging the super charger. The kit by default comes with a recirculating type bypass valve that sends extra boost back into the air filter. There is also an open type that will give that satisfying pspspspsps sound. There is no performance difference and I will probably be swapping mine out later on. I went with the closed valve initially thinking the whole system would be quieter and it could be a sleeper, but that's not turning out to be the case