I have a 46 FX with a Tower Muffler in my Sig Somethin' Extra and I get pretty good results using an APC 12x4. Basically buy a bunch of props of varying sizes like an 11x5, 11x4, 12x5, 12x4, 12.5x3.75, etc. and try them out when you get the engine broken in and see what you like and what works best.