I found a lighter engine for my plane that I would like to install. I know the weight and arm distance from the firewall to the CG of the old engine so I thought I would take that moment and divide it by the new engine weight to give me the arm for the new engine CG. Then to test my theory I moved the "firewall" back 10" and ran the same numbers and it gave me an answer much greater than 10" more for the new engine. Shouldn't the result been the same? Am I overthinking this?

Is there a simple formula to figure the new engine arm? Any help will be greatly appriciated.

Rick