My testing on NA 500 CI was the Sport Tubes made about 15 HP more than race E-Tops (5" OD) at higher RPM. The standard CMI Sport Tubes are available with 4.0" ID X 4.5" ID stepped tails as an option if you want 5" OD pipes at the transom. On a cost per HP, $1500 for 15 HP isn't out of line on engines that are already at 625 HP.