Ken, as I stated I HAVE a TT 91 FS with the 15x4W APC and it HAS unlimited vertical. I was not asking if it would, I was stating that it does. You are correct in stating that the 91 4-stroke is not overpowering it, but with the right 91 and right prop you can achieve unlimited vertical if your build is light. A 91 4-stroke is probably the perfect engine for this plane. Trying to put a 120 on it would require some weight in the tail and/or moving the firewall back to get the balance right. The Saito 100 would be the largest engine I would consider since it is as light as a 91.