Back in 1996 MAN had a review of the ST 90 and tested this engine with huge props. I have used a 16x6 prop on this motor but think it is a bit to much. For heavy warbirds I like a bit more pitch, I will not be hanging it on the prop vertical. IMHO 8" pitch at a reasonable dia will give good speed. You should know a good rpm range for your engine and use props that will stay in that range. You probably should try a few different sizes to see what works best for you.