I had SCX uppers and SC lowers on 39 Donzi, 900 + power. All was good for 2 seasons. End of 2nd season through prop blades on both sides back to back weekends and then froze a lower. Changed to the SCX-4, gained 7+ mph with the same pitch and there is more there. Boat likes them, feels better. Blows out quite a bit getting on step, but no issues unless you are drag racing or pulling skiers. Only have a few weekends on them but this seems to be a good set up vs. rebuilding the boat for #6's. Should hold up better the gears are massive. Plus I like the rooster....