Low@slow,

I get what your saying, I think, and perhaps that works for a class of model sizes such as .40 to 1.0 size models but, engine displacement is not always constant to output. I can show you a class of glow motors that are getting nearly 5 Horse Power for each cubic inch of displacement while, some others are getting less than 1 HP per cubic inch. Gas and four stokes have different outputs from two stroke glow, so it is a engine’s output that is the one constant that can be applied to other constants such as an aircraft’s weight to indicate what to expect.