The engine decides when it is broken in. If the machining and tollerances are within spec and cross hatching applied correctly on cylinders, piston rods, etc, they break in within a few hours, meaning good mating surfaces, cylinder pressure, oil and compression rings, and valve seating. Kawi and Kohler motors don't show any appreciable difference in performance when breaking in. The small Vans do a little, and many of the large vans actually come alive after 10 hours. The industry standard is a 4% variation in power with exact motors. All having to do with all the little variable that make an engine run. So, if you have the same engine and same boats/loads, don't be surprised if one runs differently than the other. What can you do to help? The four biggest factors in engine performance are lubrication, fuel, throttle and governor settings. There are a long list of component quality issues, but you have no control over that. After running awhile, plugs and valve settings,(kohler excluded, they have hydraulic valve lifters) should be your focus. After the first season, how well you prepared your motor for storage can dictate how your motor runs season after season.