Really, the most economical way to BOOST your B7 RS4 is to Supercharge it. There is really no room in the enginebay on these cars to do it effieciently and effectively without spending more than the car would be worth unless money is no object. Check out EURO Tuner magizine they have been following a Twin Turbo build on a 08 RS4. In my opinion the way to go with this vehicle is a supercharger. I like the APR.... After owning a RS6 I wish there was a better way to Boost our RS's.