Valve is vacuum controlled based on engine RPM, above 3,000 RPM the valve opens to provide free flow of exhaust. If you are interested in more volume, the vacuum hose can be disconnected & plugged. Just forward of the engine compartment electrical box (fuse & relay) is the connection. Disconnect & plug the vacuum source. Valve is held open by spring, closed by vacuum (you'll notice that at start up it is open for a short period of time) presumably until the vacuum reservior is charged.