I'm one of those few (?) who like the smaller displacement engines. If there is a 40 and a 46, I would usually buy the 40. There might not be any difference, but I have a feeling that the thicker metal in the smaller engine will provide better cooling and a more "stable environment". In addition, it produces less power, but has the same crankshaft and bearings, so it will most likely last longer. It would be great if someone with a little more knowledge than me could comment on this. I also have to say that I usually fly on the wing, I'm not into 3D and hanging by the prop. Several of my planes have/had smaller engines than "required". An Irvine 25 on a 40-size trainer was fun, OS 46LA or FS-52S on the 60-90 size BTE Flyin' King works great etc...