Horsepower is for the birds. If you want to sufficiently test numbers on each of these engines then you either need to measure the static thrust with the best prop combo for each engine or throw the 46 and 72 into the plane and fly it to compare. Horsepower numbers are always being thrown around when power is the issue. Prop size versus rpm is the only accurate way to measure the power of our glow engines when they are in the same displacement category. If they differ in size as above then I guess the only way to answer the question at hand is to throw them in the plane and compare.

Just as a note, I was able to get 83 mph out of mine with the saito 72 running on 30% nitro.