You can use anything from a mild .50 four stroke all the way up to a Saito .80 four stroke in the Four Star .40. However, a lot depends on your flying ability, and what you want to do with the plane. I agree with the others who say they fly REAL nice with something like a Saito .50, or O.S. .52 or YS .53. In my case, I like to fly with my tail on fire, and do unlimited stuff with my 4 * 40 and I have actually flown mine with three different engines over the years. I started out with an old Enya .60 four stroke, then went to a Saito .80 (flew it with floats and snow skiis), and finally settled on an O.S. Surpass .70, which I still fly in it today. The bigger engines require a repositioning of the receiver battery to the rear of the trailing edge of the wing to balance properly, and the larger prop diameter limits ground clearance with the stock LG, but the low end grunt, instant (vertical) acceleration, and ability to do wild tunmbles and flat spins, is what I really like with the bigger four strokes in this plane.

Like I said though, it just depends on what you want...

Lee