Well the .61 size engines will fly it the way the designer meant it to fly.... [img]i/expressions/face-icon-small-wink.gif[/img]

A .72 4-stroke gives about the same useable power as a .61 2-stroke... so a .91 4-stroke would be a reasonable increase. The lower RPM, larger prop will give FAR superieor pull at low airspeeds too. (14X4 or 14X6 is what I use on my .91 4-stroke)

Really... its a matter of personal preference. what type performance are you after? Unlimited vertical climb (definitely non-scale) get the 1.20 4-stroke. It'll swing a nearly scale size prop.