Leave it to the engineers at O.S. to concoct a sure-fire recipe for marine racing success! First, they started with the 21XM, O.S.' exceptional outboard engine and winner of 11 championships in a 4-year span. They then upgraded the cylinder head and muffler. The result: the 21XM VII, an air-cooled powerhouse that's even faster than the original! Cooling the cylinder head with air is more reliable than water cooling, and prevents overcooling. It also adds the benefits of simpler construction and less maintenance.