If you're in Europe, you can check out MTM: http://www.mtm-online.de/en/index.php?nav=3 or in the US Hoppen Motorsports: http://www.hoppenmotorsport.com/Audi.S4.B7.Tuning.htm
There's not too much you can do to the 4.2 other than taking the speed limiter off and getting an exhaust (not sure of the hp gains there) unless you go for the supercharger - 450hp, but check out the cost!
APR is supposed to be coming out with a supercharger for the 4.2 someday, and it will likely be considerably cheaper - though I'm sure NOT cheap - than the MTM unit.
There's not too much you can do to the 4.2 other than taking the speed limiter off and getting an exhaust (not sure of the hp gains there) unless you go for the supercharger - 450hp, but check out the cost!
APR is supposed to be coming out with a supercharger for the 4.2 someday, and it will likely be considerably cheaper - though I'm sure NOT cheap - than the MTM unit.
