RE: 4* Engine question
Yes its the difference in engine torque.
I would expect he used a 10X6 or 11X5 with the .40 and the 11X6 or 12X5 with the .52 4-stroke. 11X6 is a bit much for a .40 2-stroke to be swinging (even the O.S. SF, which I think is a better engine than the FX.)