When I was looking to buy a new tractor, one of the decisions that came up was whether to buy an E model versus an M model. Same horsepower rating for the tractors but different engines. Nearly identical displacement but 5 cylinder versus 3 cylinder.
One comparison tool was a basic formula that would calculate a relative efficiency number with which to compare different engines. Went something like Hp x Rpm / constant = efficiency value
Question... ? Does anyone know this equation?