They remove the tailshaft houseing and put in a shorter output shaft, on mine anyway. I use low gear for no wake zones otherweise it'l idle about 15 MPH, also has reverse. They use the 350's and 400's for the heavier boats like cruisers. The Powerglides are not high HP trannys stock, they put in Hardened input shafts and stainles clutch houseings etc.