The maufacturers or retailers of bolt ons will over state what the gains are. I generally assume its about 70-80% of what they say it can do. I have done the exhaust and underdrive pulley my self. It's still a V6 auto, so I don't expect much (specially with the heavy 00 Cobra R wheels I have) in the HP gains. Between the two, I figure I have about 20 HP gain.