If you were to look closely at how a 4*40 is made, you would probabily reconsider the motor choices. Some small basswood spars,No leading edge sheeting, 9 ribs to each wing half, and some spar webs make up the wing. There just isn't that much holding the plane together, so In my opinon any .40 size motor will do. My club holds 5-6 pylon races a year for 4*40's with Super Tiger GS40 engines. With 9-7 or 10-6 props and these $50 dollar motors these planes are really quite fast.