I cut another 1/4" off and I think I'll stop there. Length of my header is now 5 3/4" and Mac's web site suggests 6" for this engine.

Temp: 70°
Powermaster GMA 5/22
OS #8 plug
APC 11x10
12,337
1.68hp
9.02lbs. static thrust

This is probably about the maximum power this engine will produce with this prop. Going up to 15% nitro would add a bit, too. Though, nine pounds of thrust is a lot for an 11" prop, and the calculated speed is 101.5mph.

I have an extra backplate with a pressure tap and a pump. If I ever come across an #86 pump carb I'll see if that adds anything, but those carbs are extremely rare unless you buy the whole engine.[]

David