It all depends on what you want to do. If you plan to race the airplane then the ST may fall slightly short of silver speeds. A YS 91 AC would be minimum for silver, a 110 puts you in the zone. I tried the piped 2 stroke path for a while. Had a purpose built airplane running an enclosed pipe Rossi 61Setup. This combo would run an APC 11x11 but was only a fast silver airplane. If you want to go gold speed the 115 is the only real option and then if you are going ARF then strengthening mods ars nesessary.