http://www.vividracing.com/blog/anno...off-valve-kit/

One important facet on turbo charged vehicles is the factory blow off valve also known as the recirculation valve. Because the Nissan GT-R features a twin turbo setup, there is 1 blow off valve on each intercooler pipe. With stock recirc valves, they relieve the pressure from the intercooler pipe back into the intake pipe which then goes into the turbo causing turbulence. This slows down the turbo spool up between shifts. However running an aftermarket blow off valve at a 100% atmosphere can cause MAF issues because there is no air being metered back. For modified cars running higher boost pressure, factory recirc valves can also leak boost from the OEM design at higher RPMs.

Agency Power’s blow off valves have been proven successful on many other vehicle applications with each holding up to 30psi of boost. With its adjustability **** on top to dial in your spring tension, the CNC machined 6061 aluminum component can be setup to run either 100% atmosphere or 50/50 for best performance. With the 50/50 setup, the blow off valves will still dump air back into the intake, but most will vent to atmosphere giving you that swoosh sound and keeping the turbos spooled between shifts. The anodized body with laser engraved Agency Power name includes a bolt on flange to mount the blow off valve on the stock or aftermarket intercooler piping. These kits are sold as a pair and come in black, blue, red, or silver. They fit all 2009 and up Nissan GT-Rs.