Designed for turbo cars with an Air Flow Meter (VAM/MAF), also works great with Speed Density (MAP) applications.
Made from billet aluminum, this atmospheric blow off valve dumps the boosted air to atmosphere creating the "Whoosh" sound when the throttle is lifted.
Unlike single piston or diaphragm diverter valves, this valve uses counter balanced springs and dual pistons to prevent the valve from leaking air into the intake system at idle or part throttle. This prevents the valve from causing the driveability issues associated with single piston and diaphragm type valves when used on vehicles equipped with an air meter (VAM or MAF). Designed to be clamped to a 1" ID hose connection which is available as an option and on all of our intercooler kits.

6mm ID hose fits barely but certainly needs clamped on to prevent it from popping off. 4mm ID hose is tight/stretches over and would not need a clamp. So measure the port you plan to connect it to on the vacuum tree or turbo outlet and measure ports you may also connect to the same hose (like a boost controller) to determine which size you'd rather use so you don't have to adapt to different sizes. 

Includes:
Blow off valve, Replacement preload spring, 1" Silicone Hose for attachment to welded on 1" bung or silicone tee connector available in related items below