*In my opinion there are 3 good .40ish size engines. The Thunder Tiger 46pro, OS 46fx and ST.45. ST has been a ghost lately so apparently the Magnum is taking its place for a while.
I have all three and happy with them.
*The TT & OS are for the most part ,the same engine all the way around. Now lets use thew 46fx. You install this engine on a trainer like a Sig LT-40. Pulls it around pretty good. Nothing spectacular but good enough for training.
* Now you pull that engine out and put it in a small .40 size warbird like a P-51. Aero dynamics are better and speed is significantly increased.
* Now you take this same engine and put it in a .40 size Shrike or Diamond Dust and your looking at around 125mph. It all comes down to what plane you are going to use. This topic will open up a great can of worms.