After reading ddawson's post about gaining hp with just a change in catback. I did the same thing. lats dyno run with 04 stock cat's and S/W turbo exhaust we got max hp 337 on 6/7/06 with the knock senser's kicking in. not sure why. know went with 05' stock cat's and 05' magnaflow cat back and with no knock at all max hp 346 and max torque345. picked up 9 hp & 11 lbs torque, not much of a gain but at least i'am going the right direction. next LT's.