The mechanic who rebuilt my C-75-12 last year doesn't call it an "upgrade" when one is modified into a C-85-12. In his opinion we did it right, rebuilding the 75. I paid for tangible benefits, to him there weren't any in the "upgrade."

Please, correct me if I'm wrong. But the 85 eeks 10 more hp out of the same displacement by using bigger jets in the carb, to burn more fuel, to spin 300 rpm faster but with less torque at maximum rpm.

It spins a prop that's four percent shorter (and likely very slightly lighter, although the catalog says it's same weight) with a nine percent smaller arc and eleven percent greater pitch so if you figure it all out mathematically you are moving the same amount of air per unit of fuel by my calculations.

Other than 300 more rpm being available at takeoff is there a benefit?

I agree with your mechanic. The extra ten horsepower is optimum and not guaranteed. It turns faster and runs hotter and makes some extra RPM, but unless overhauled the little engines like to move as that have been in the past. Take a plane from living at high altitude to lower altitude and it will not turn considerably faster.

Props are high art. Sounds like you have some math experience. You can probably understand the variables better than I. My old crop duster mechanic always placed the bent prop with the bent airplane when we crashed. He said the prop goes with the airframe not the engine. Props were bent back and tracked and airframes were rebuilt. Engines generally just went onto another airplane after being dialed and light gears checked. Different world.

Bird in hand worth however many in bush.