John- you could be right. I pretty much followed Macs pipe tuning info. The length from the plug to the fat part of the pipe is 10.75” The last two 1/4” cuts off the header resulted in no rpm gain so I stopped. Maybe I should cut until the rpms drop? The OS32 I had on the other plane I lost got “on” the pipe nicely. IIRC the header was ½” shorter than I’m running on the ST now.
Chris- I did the break-in and initial needle settings with a 9x6 & muffler. Peak rpm was 15,4. Open exhaust it was 15,8. I put on the 8x8 and still got 15,8 with open exhaust so I figured the props has similar loading. So if I assume the 8x8 would also run around 15,4 with the muffler I do have a gain of 1000 rpms at 16,4
Without getting into all the intricacies of prop efficiency, induced drag etc. etc. (it is a spad after all) an 8 pitch prop turning 16,4 has a theoretical speed of 124mph. A 7 pitch prop would have to turn 18,7. Now if I could get the 8 pitch prop to turn 18,7 we might be getting somewhere.
I believe the q500 props are 8.75x9 and turning at 19k+ [X(]