RE: O.S. 46 FX on a pipe
Yeah..open header will give you a rough idea of what to expect. The important thing is to have a prop that is a good match for the plane, engine and exhaust. The prop is like the wedding ring for the other 3 components.
I think you will find the narrow blade MAS props in that size range to work pretty good and of course the APC 9x8 should give that combo impressive speed too. The 9x7 is good to start with since it will be easier for the engine to turn.
Save 1 or 2 of the little hunks of exhaust tuning should you ever want to lengthen the system a little..to accomodate a different prop.
I never have seen the Mach Racer in person, but it looks like a pretty good model if the factory glued it together well.