The DD stands for Dual Drive meaning it has both a small port for low boost and big port for high boost. This is the head unit and it will work on the old style unit's flange. It is a great way for those of you with the old style blow off valve to upgrade for a relatively small amount of money. This is what is said about the blow of unit.

"The Blitz Blow Off Valve is essential to all turbocharged vehicles. The purpose of the BLITZ Blow Off Valve DD is to relieve compressor surge without reducing compressor speed. It controls boost pressure that is built up by the turbo and vents it into the atmosphere when the car is not under load (in between shifts). This prevents pressure from backing up into the compressor housing, causing the turbo impeller blade to rapidly stop or even worse, to spin backwards (compressor surge or backspin). Doing so can be potentially destructive to the turbine wheel and shaft. Repeated stress over time can cause eminent damage to the turbo charger and eventually lead to turbo failure. A blow off valve is actuated by pressure changes only, ensuring quick valve responses and complete closures at idle. The BLITZ Blow Off Valve not only improves performance and response, but also increases the life of the turbocharger." Blitz Blow Off Valve Features * DD (Dual Drive) for maximum response (small port for low boost + larger port for high boost) * Includes mini SUS filter * Urethane gasket to ensure complete seal * Tension adjustable * Holds to 1.2 BAR (18psi) * Hard Spring for high boost applications Get it only from RHDJapan!