I have a the TT46 and ST51. My 46 could not pull the 12.25x3.75, but the 51 just rips with the same prop. But I have seen another TT46 rip with that prop and the Tower muffler. I may have damaged mine on breakin, ran it sloppy rich. I'm not positive, but I thought the ST51 was a touch lighter. Both engines have been reliable and easy to tune. It looks like ST has gotten away from the bolt through muffler, I hate that setup. With some file work, it looks like the Tower muffler would fit the ST. Will try that some night.
theradioflyer and Fubar1- Those are both sweet looking planes!

MikeB