A bunch of guys at our field all built 4-Star 40's and had a pylon race with them. Only rules are: kit or ARF, airframe must be stock (no clipped wings or 0 dihedral, must retain stock landing gear), and engines must be no larger than .46 2-stroke or .56 4-stroke. Of course, if your rules allow it, you can make a 4-Star really sizzle with a little imagination, and they are relatively cheap. We're making it a 3 x a year club event it was so much fun.