Mercury MerCruiser - 4.2L TDI

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Captain's Report

Mercury 4.2L TDI (Diesel)


The 4.2-liter TDI is Mercury Marine’s largest inboard and sterndrive diesel power plant. This lightweight, clean burning V8 engine is available in two output configurations, 335 and 370 horsepower. It also meets newer, stricter EPA Tier 3 requirements for smoke and emissions standards.

The Peoples' Diesel. Having moved on from its relationship with Cummins Marine Diesel, Mercury Marine now sources engines from Volkswagen, a global leader in diesel power. This platform is all about technology; from the advanced electronic engine management system to the high-tech metals used in the turbos and block.

Both horsepower versions share a common, light-weight V8, 4-stroke diesel mill that can spin up to 4200 rpm at wide open throttle. Bore and stroke measures 3.27’’ x 3.76’’ with a 16.4:1 compression ratio. Variations of this engine can be found in Audi’s A8 sports cars, which are setting new standards for diesel performance.

2x Turbo Power. The grunt behind much of the power is a pair of water-cooled turbochargers that use variable turbine geometry (VTG). The variable-pitch vanes in the turbo allow it to spool up sooner, providing boost at a lower rpm and greater torque when needed to get the boat on plane. A raw water intercooler then cools the hot boosted air. Mounted at the front of the engine for easy service, this chiller brings the charge air temperature down, making the air denser and thus creating more power.

Weight Reduction is Key

The key to modern marine diesel applications is shedding weight as it directly relates to performance. In an effort to significantly reduce weight, the block is made in a process called Vermicular Graphite Casting (VGC). This process is said to cut weight by 5 to 10% over traditional iron castings, plus is twice as strong. Additionally, the cylinder heads and intake manifold are all made from aluminum, which saves considerable weight and transfers heat better than cast iron. Surprisingly, the dry weight for both horsepower versions is 836 lbs. (379 kg), which is very close to a comparable gasoline engine.


The engine’s closed loop cooling system is routed not just to the block and heads, but to the turbos, manifolds and to the oil chiller as well in an effort to extend corrosion protection -- all with one system. Raw water is fed only to the exhaust manifold risers and the turbo charger’s intercooler. This architecture helps reduce overall maintenance and service costs.

Mercury 4.2L TDI (Diesel)

VesselView monitors provide real-time digital data on the 4.2L TDI.

Engine Management

Bosch supplies the advanced electronic common rail fuel injection. The system’s high tech Piezo injectors can actually sense individual cylinder combustion pressure and relays the information to the engine computer to make adjustments in order to maximize performance. The system is also mated to a SmartCraft module to interface with all the Mercury gauges and monitors for full vessel management.

Mercury 4.2L TDI (Diesel)

A view of the common rail fuel injection manifold.


Although we did not have a chance to test the new 4.2L TDI, we did obtain Mercury’s dyno results. The 370-hp version shows a peak of 572 pound-foot (775 Nm) of torque starting near 2500 rpm and holding it till 3000 rpm. While the 335-hp configuration has a peak torque of 483 pound-foot (655 Nm) at 2000 rpm and is nearly flat until it starts to drop at 3000 rpm.

This power band helps give operators better fuel economy based on how the boat is loaded. The manufacturer states that fuel burn at wide-open throttle is 18.5 gph with the engine turning at 4200 rpm. Considering the power advantage, the 370-hp engine’s wide open throttle fuel burn of 20.5 gph is not that much more than the smaller brother.


Although diesel engines generally require less maintenance than a gas engine, Mercury ensured that basic service points are all within easy access. Situated under a plastic cowling on top of the motor is the self-draining oil filter, the alternator and power steering reservoir. Engine coolant overflow tank and drive lube reservoirs are up front so they just need a quick glance before leaving the dock.

Mercury 4.2L TDI (Diesel)

Shown here is the Bravo 3 sterndrive, one of the more popular choices for diesel power propulsion.

Transmission, Drives and Joystick

Transmission choices for the 4.2L TDI are the ever popular ZF for inboard-propelled craft, or the line of Bravo XR sterndrives. Bravo One XR, Two XR or Three XR’s provide a huge choice of props, so dialing-in the right set-up for boat and engine should prove fairly easy. A big advantage of the Mercury diesel family is the optional digital throttle(s) and shift system, as well as the Axius Joystick controller for sterndrives. These features join the TDI to make for an extremely advanced diesel propulsion set-up.