I am just going by what the guy told me to use. He has been using this prop set up for a while and it is working the best.

Also I have tried a prop close to the one you mentioned 7.5x8. But the one I used was a 7.2 x 8.0 and It did not come up the pipe as i mentioned before. That is when I had a constant 16,000 or so rpm's. But now that I noticed that the pipe kicks in at about 17,000. I can mess around with the 7.2 x 8 maybe shave some off and try to get some more out of it. As odd as the 8.5 x 5 sounds this is supposed to be a good point to start at. Like the guy said he has been running this prop for years on his DD and likes the setup. (speed wise also) So for now I am going to take his advice being that he has the exact plane,engine and pipe combo I do.