Just my 2 cents, but I've just finished a Hanger 9 P51-D. A friend in my club suggested a Saito 100 (same size at an OS91 but with a little more punch). The plane came out at 8lbs 5 ounces, probably because I added on-board glow and a second battery pack just for the on-board glow. The point is that the Saito does a great job of pulling this plane. I know they are pretty expensive engines, but it runs great and does give me an extra 500-700 RPM over the OS 91.