The Mach 2 Atmosphere Upgrade Kit allows a GFB Mach 2 valve to be converted from full recirculation to atmosphere venting.

The kit includes a trumpet outlet, a firmer spring, and a 33mm hose plug.

To correctly vent a Mach 2 valve to atmosphere, the piston must remain closed at idle and the recirculation hose must be blocked. The 5933 Atmosphere Conversion Kit supplies a 33mm hose plug to seal the recirc hose, along with a trumpet that replaces the standard hose outlet on the Mach 2. While the trumpet is not required for operation, it significantly increases the venting sound. The kit also includes the firmer spring (part #6115).

Mach 2 valves are supplied as standard with a softer spring (part #6116), which is suitable for full recirculation setups or engines with an idle vacuum of 18 inHg or less. If the piston remains fully closed at idle with the existing spring, no spring change is required.

However, if the piston remains partially open at idle even with maximum spring pre-load applied, the firmer spring included in this kit should be installed. This helps prevent drivability issues such as stalling or backfiring when converting the valve to atmosphere venting.