You dont need one mate as BMW have installed a recirculating one as standard. All petrol and diesel cars from the factory have one but instead of the boyracer pssst the air released goes back into the inlet tract. The info above on how they work is very innaccurate. A dumpvalve or recirc valve actually does prolong turbocharger life. When you accelerate the turbo spools up forcing air up the inlet pipework, when you back off or change gear the turbo is still spinning so forcing this air up but its meeting a dead wall at the throttle butterfly. This air then 'backs up' effectivly trying to go backwards through the turbo known as compressor surge. This air tries to spin the compressor the wrong direction whilst the turbine side of the turbo is still trying to turn it the correct way. This over time wears the bearings and can cause the turbine shaft to snap. the dumpvalve or recirc allows the boost pressure to be released when you back off, this allows the turbo to spin freely with the boost pressure simply leaving the valve and causing no damage. The turbo itself isnt being slowed by the air trying to escape as this has been removed so actually spools up much more quicly reducing lag. The turbo can basically keep spinning and producing boost pressure whilst you change gear or back off without the air trying to slow it down and damage it. Boyracers fit loud ones but these are simply valves that vent to atmosphere rather than the recirculating units that we will all have in our turbocharged cars :-)
