I think it is a mistake using 170 mph at a 45 degree dive to calculate the power from gravity. At that point my calculations show the prop is acting as a break and the real speed from gravity is unknown. I also think it is a mistake trying to use HP to calculate speed in this case.

The conservative #'s in my earlier post show a terminal velocity of of just over 200 mph. Forget starting the engine and take the prop off and drop it from a high altitude. I am not sure why you want to complicate terminal velocity with engine power.

We know the engine is turning the 10x8 prop at 15,500 rpm static and can make an educated guess that the unload rpm is around 17,000. Trying to incorporate HP ratings into these facts only complicates things and could hurt accuracy. Engine and pipe timing should keep it from going much faster even though the 9x10 prop has less load using widely accepted methods to calculate prop load. Using the adjusted pitch #'s, 12.5" because of the props airfoil shape, and 17,000 rpm we come up with just over 200 mph. Considering the terminal velocity speed and the gravity assisted dive there is no reason to not achieve 0 slippage with the prop.

My prediction stays at 201 mph. Now if Speedboy would just do his thing to prove me right or wrong.