I have flown a Knife for a long time, and I'm telling you that the 4-stroke tork is what you want. The fact that the 13-14 inch props give you a much bigger thrust tube is icing on the cake! Both of these factors add up to a much more responsive plane in the hover position. I like a y.s. 63 and a 13-7 apc. If there was a better engine, I'd buy one!! I understand Saito .72 is a good choice as well. I have the Saito .72 but its on a G.P. Extra. Haven't tried it on a knife yet. I used to think my 46.fx and Rossi had good transition untill I tried a Y.S. Don'y knock it until you try it. I'm a very firm believer now! Good Luck!!!!!