The ys.63 WITH muffler is 1/4 pound heavier than the os .50 WITH muffler. I dont want another 1/4 pound on my plane no matter what it does. The one drawback to the 2 stroke is the muffler points right at the wing and its unbelievable how much slime collects on the wing on every single flight. But of course when your 3d'ing alot ALL the oil goes onto the wing were normally some of it would blow away. So far i love the os