I have a Mag .91 4s in my VF Extra66 and it flys just fine... My Extra weighed out right on 8 pounds.. The mag is swinging a 14X6 APC prop at 9700 rpm on 15 percent PowerMaster... The engine is still breaking in.. I also have the GP 40 size ARF Extra with a Mag .91 4s in it... A real fun plane and flys pretty good... Going from it to the VF Extra 66 is like going from a VW to a Roles Royce though...