As a matter of reference I once owned a 4-60 with a 61FX. Ran a MAS 12X6 on 15% Nitro with an OS #8 plug. The verticle was truly unlimited but not enough power to pull out from a hover but then again this is not a 3D air frame - its an everyday good second plane. I think any good 60 size engine will fly the 4-60 well enough for most to enjoy.