The Mach 2 is configured as a 100% recirculating valve (i.e. bypass, plumb back) and add on kits are also available to convert to 100% atmosphere venting. Use the recirc model for direct factory replacement when you want the performance WITHOUT the noise! Add the conversion kit when you want some noise with your performance! Factory bypass valves have two main shortcomings – one is that they are typically designed to start opening above a certain pressure to protect the engine, and the other is they are not optimised for throttle response. Oh, and you can’t vent them to atmosphere without stalling your engine. The Mach 2 solves all of these issues. It can effectively hold unlimited boost (unlimited in that no turbo could ever produce enough pressure to damage it), and the TMS technology ensures maximum throttle response. Oh, and you CAN vent with the atmo conversion to atmosphere without stalling your engine.