I think it depends on just what .40 you are using. They all don't put out the same power. I run my LA ,FP,& GP engines on a 11X4 prop. The ball bearing ones like a 11 X 5 or 6 better. You may also need to up the nitro some . All up weight is also a factor. Best to give it a try ,you can up grade to suit your tastes. Good luck & have fun. Max H.
