My Mach Racer balanced out perfectly at the 8 inches with the os 55 ax and Ultrathrust muffler WITHOUT taking into account the amendment to the extra 3/4 inches or so of distance between the firewall and spinner backplate.
Incidentally, I changed to a 9x9 APC prop (From a 10x7) and the rpm stayed the same at 16,900 and speed at about 130mph
I managed to break that prop when I ran out of fuel and the prop stopped at top dead center and I had to make a downwind landing. No other damage though so was lucky there.
I changed to a 9x10 APC prop and again, the RPM stayed at 16,900 but the top speed went up to 143 mph level flight. I still have the flutter at top speed but I suspect it is the elevon pushrods flexing as there is no movement or slop in the control surfaces. (I did change to the hole right next to the center of the servo horn which helped until I went to the 9x10 prop.)
Next will be to change to maybe a carbon fiber pushrod to rule out the flexing. I'll have to let yous guys know how that works out! (The flutter combined with the RPM scream makes it sound like a hotrod flying past. It sounds neat but has an ominous tone to it)
One of the guys out the field took some video and with his permission, I'll put it out when I am able to for comments from the peanut gallery!
George