We had a chance to run a set of the new Mercury TDI 4.2-liter 370s and to draw some comparisons from an earlier test of the Cummins MerCruiser TDI 4.2-liter 350 HO (high output) Tier II diesels in a similarly equipped 37 SUV from Statement Marine (boatingmag.com/statement). Both engines are based on Volkswagen's 4.2-liter V-8 compressed-cast block, incorporating the TDI common-rail, direct-fuel-injection system with twin variable-geometry turbochargers designed to eliminate turbo lag. Bore, stroke and maximum rpm are identical, and both feature closed cooling. Compression ratios varied — the Tier III was 16.4:1 versus the Tier II's 17.5:1. Dry weight of the Tier III is 836 pounds versus 811 pounds for the Tier II. Also, the first test boat was fitted with extra fuel tanks and, most notably, an active in-deck air suspension system, so it weighed about 2,000 pounds more than a 37 SUV in standard trim.