To vent the Mach 2 to atmosphere, you need to ensure the piston stays closed at idle and the recirc hose is plugged. The 5930 Atmosphere Conversion Kit includes a 30mm hose plug to block off the recirc hose, a trumpet to replace the hose outlet on the Mach 2 (not necessary for functionality, but makes the sound louder!), and a firmer spring.The Mach 2 is supplied with a softer spring that's suitable for recirculation venting or cars with idle vacuum of 18inHg or less. If the piston stays closed with the existing spring in the Mach 2, there is no need to swap it out. However, if the piston hovers open at idle even with the spring pre-load maxed, the firmer spring in this kit should be installed to prevent stalling or backfiring.