The horsepower gain unfortunatly is going to be very negligble, especially just being an axle back. This exhuast is very nice from SLP, but do not expect any considerable gains and horsepower.
You will achieve more power, horsepower and torque from a set of long tube headers, and connecting pipes (X-pipe or H-pipe), the axle back is really to give the car some tone!
Regards
-Brandon
http://www.ipsmotorsports.net