I'm not sure about what they mean by the stock muffler dosen't have enough volume, but maybe it has to do with back pressure to the tank for fuel draw? I have never dealt with a ST stock muffler, but my Dad has a pitts muffler on his ST(it's the biggest engine ST makes) and it ran fine. Don't know much about them, so i'll leave it to the "experts" on those engines.

However, we do have a Razzle with a YS 91AC 4 stroke, and it pulls out of a TR awesome. A 90 2 stroke would pull it no problem once it is tuned, plus the Razzle is a fairly light airplane.

Brandon