I know this thread is a bit old now but I'll post anyway.
Got this engine for a replica 70's pattern ship so as was a previous post from Finland not really interested in stump pulling big prop torque, these things have to sound right as well as go fast.
Run in on six tanks of FAI fuel on 11x7 at about 11000 rpm rich then switched to 18% KL100 10% nitro.
Baseline with Macs Black muffler which apart from being nice and light is not very quiet and I think knocks the perfomance a bit too, I didn't have a stock Webra muffler.
14x6 MA 11,900
11x9 APC 12,600
I wasn't blown away by those numbers but we'll press on with a Macs 1190 pipe and see what happens, well !
APC 14x8 11,600
APC 13x8 13,200 (narrow sport prop)
APC 11x10 14,200 Yee Hah
APC 11x9 14,300 (the pipe needed to be shorter but I think I would err on the side of loading this engine higher rather than lower).
I didn't have a 12x10 APC with me on this test but my gut feeling is it is THE prop for this engine in a fast 70's style pattern ship like the new Hanger9 Phoenix 7.
This one is going in a Funkey Speeda which is like an MK Blue Angel.
Well done Macs I love your pipes they make serious power, not sure about your mufflers though.
Cheers
Greg Hede
Australia