I hope its OK for me to respond in this support forum.... (please move if not OK)

The Boxxer prototypes flew on the following combinations----

OS 50 SX with Std muffler
OS 50 SX with Mac Pipe
OS 46AX with Mac Pipe
Irvine .53 with Tower Muffler

There is also one with a Saito .72 on it[>:]

The one in the vid, and my personal fav is the OS .50 SX with the Mac pipe. That is my setup and the one that I feel provides the best of all things. I run an APC 12-4 and 15%Morgans fuel or 10%Coopers. (10% Coopers in the vid) With the 50SX/pipe, I have more than enough power to punch out of absolutely any situation, and the spool up from the prop is instant. Matter of fact, I have easily beaten many monoplane profiles in vertical drag races with my setup.

I flew the APC 12.75x3.75w, and did not like it as much on this particular engine. Spool up is pretty slow and transistion was not quite right. The same APC was however great on the Irvine .53 with the tower muffler.

Any good .46 (such as the AX, TTPro, etc) should be fine, especially if run with a tower muffler or mac pipe.