I currently fly mine with a saito 56. I purchased the 91 also with the means of putting it on the sig. I did and it would about shake that thing to death. The tail was moving about 1/2 side to side! Everthing was balanced too. i don't use a spinner on mine. So the prop had to be balanced. The Saito 56 is a great engine for it. it doesn't give it unlimited vert, but it will pull hard, using a 12.25x3.75 apc. I have another that I need to cover that is going to have an os50sx on. That should be a great combo.

In other words, if you have the 46fx use it.