I installed a Magnum 91 Four stroke and agree that there is a limit to motor size and weight. I made taller gear to allow for the 15x4w prop. It will hover on a dime with plenty of vertical power. But you do loose alot of other characteristics. I've tried other props and the 4w works great for the hover but acts like a speed brake when power is cut. There is no more gliding in on landing, too low of speed and she stalls and tips over on the nose.