The two 8.1L big blocks sit in their respective Chris-Craft Corsair 25s. The Volvo maintenance points are easier to get to. For instance, the MerCruiser fuel filter sits very low in the bilge, while the Volvo’s is higher and easier to reach.

Eavesdrop on a powerboat salesman and his pitch sooner or later focuses on the vessel’s top speed and its horsepower. Speed sells—especially to first-time boat buyers. But in reality, mileage and other performance traits are more important, especially if the difference in top speed between two boats is minimal, as was the case with this very engine comparison.

We initially intended on testing two different brand engines separated by only 5 horsepower—MerCruiser’s 496 Mag HO (425 hp) vs. Volvo’s 8.1 L GXi (420 hp). But when we arrived at the marina, we discovered that the MerCruiser was actually a 496 Mag—not a Mag HO (high output)—and was accordingly rated at 375…