And that's why I said HP is not scaleable because mechanical stresses (centrifugal forces, piston accelerations etc) increase at a far greater rate than the simply linear engine size. What should be understood about F2A engines though is that they're unlike any other engine because they're taken to the absolute limit and are terribly critical. It's because they're so critical that they can make that kind of HP. The 1.5HP you calculated for a .15 would make that a total beginner's engine but probably about right if it was only turning the 25K of that 4.1HP engine.