Daboo, I am currently flying a Seagull Zero with a Magnum .91 4-stroke. I have used a 14-8 two blade prop and a MA 13-8 3-blade prop. I cannot tell the differance in performance. The .91 will turn either prop at just over 9000 rpm on 15% Cool power. Take-offs and climb-outs are spectacular and speed is right up there. The engine seems happy and is not running hot. The 3-blade just looks RIGHT on the Zero, and would look the same on your P-40. I've heard all of the efficency stories, but in this application I doubt there is any large differance in performance. Someday when a radar gun is at the field, I'll try a speed run with both props. Our field is at 5000' altitude.

Best regards from old bird.