my favorite engine is the ys 63. Physical size wise it is smaller then an os 70 but it has immense power for its size. On my 40 size profile it has unlimited vertical performance. Hand launches by flying out of hand are possible. run one tyank on the ground to get it tuned in right then run it a little rich in the air to finish breaking it in. they have a tendency to overheat if run on the ground too much for break in. they work great if broken in in the air and are extremely reliable