Mercury TDI 4.2-liter 370 Tier III Diesel

Mercury's eco-friendlier TDI 4.2-liter 370 Tier III diesel features increased torque and horsepower in a refined package.

Mercury Marine (mercurymarine.com) has been busy refining a fresh series of diesels, and while much effort has gone into meeting the Environmental Protection Agency's Tier III emissions regulations, it's evident from our testing that Merc has not forsaken its penchant for speed.

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.

The Mercury TDI 4.2-liter 370 updates include compatibility with Mercury’s Digital Throttle and Shift system and Axius joystick docking control. The Tier III generates a peak torque of 577 foot-pounds at 2,750 rpm against the Tier II’s max torque of 522 foot-pounds. This begs the question: How did Mercury boost the Tier III torque by 10 percent and horsepower by 6 percent?

For one, the timing chain moved from the front of the engine to the back, says Reinhard Burk, diesel category director. “This allows for ever-more precise timing of the cam shaft and delivery of fuel.”

Merc also recalibrated the boost of the twin turbochargers and adjusted the activation sequence of the variable-pitch vanes in the turbos, Burk says.

The difference shined in our second test of the Statement, the center-console that in 2012 set a Bermuda ­Challenge record (boatingmag.com/bermuda-challenge). At wide-open throttle, the new Mercury 4.2 TDI 370s attained an eye-tearing 55 mph at 4,300 rpm, a 4 mph increase over the Tier II diesels that ran 51 mph at 4,160 rpm. We also experienced an improvement in cruise-speed fuel efficiency with the newer diesels achieving 2.74 mpg at 2,000 rpm and 24 mph versus 2.55 mpg at 2,700 rpm and 24.4 mph with the Tier IIs.

Diesels aren’t exactly rockets out of the hole. Yet once the turbos spool up, hang on. In our test, the twin 4.2-liter TDI 370s began their romp at 1,600 rpm and then pinned us in the seats as the tach needle crossed 2,500, close to peak torque. Our time to plane was around six seconds, and we hit 30 mph in a tick over nine seconds. The Tier IIs posted a 9.6-second 0-to-30 mph time.

Mercury’s line of eight Tier III diesels includes a 2.0-liter in-line-four (115 to 170 hp), TDI 3.0-liter V-6 (230 to 260 hp) and the TDI 4.2-liter (335 to 370 hp). Merc offers them with a variety of MerCruiser drives. The 4.2, for example, is available with the Bravo One, Two and Three XR drives, as well as inboard versions with ZF transmissions. Manufacturer’s suggested retail price for the Mercury TDI 4.2 370 ranges from $44,765 for a sterndrive version to $46,268 for an inboard bobtail.