Wagon on Water Yeah: Talking Torque with Andrew Norton

It didn’t take long for VW to start promoting its 3.0 litre diesel in the Amarok 4WD. Originally the twin turbo 2.0 litre diesel was promoted but why stop at that when the 3.0 litre engine is so good? This high revving (for a diesel) engine suits performance hull applications, especially when mated with the MerCruiser Bravo 3 leg. 

In fact, this engine was marinised for marine usage way before VW offered it in the Amarok, unusual as normally automotive versions of engines appear before their marine counterparts.

The Nitty Gritty

When checking the specs the first thing I noticed about the TDI 3.0L is that its piston stroke is bigger than the cylinder bore, unusual in V6 engines which traditionally are way over square, having larger bore dimensions than stroke. However VW was smart in doing this as it allowed for more torque at lower rpm whereas bottom end torque is normally lacking in V6 engines.

Another aspect was VW’s conservative approach to engineering. The TDI 3.0L is rated to 500 hours of running per year compared to around 250 for the automotive-origin diesel competition. And the 3.0L can be run at Wide Open Throttle for one hour in every eight or a total of 12.5 percent. Not bad for a high revving engine which was obviously designed for Autobahn driving!

Once the plastic shroud covering the engine has been removed, access to normal servicing points is very good. The engine sump oil dipstick is forward with the power steering pump reservoir and coolant overflow tank (fresh water cooling is standard) to port and the water separating fuel filter to starboard. Neatly grouped for twin engine installations...

Although the electronically managed common rail fuel injection system is normally self-bleeding a hand pump is fitted for initial priming. Fuel is injected at 26,000 psi or 1770 bar, right at the top end of injection pressures, so maintaining fuel lines in good condition is essential with this engine. 

Servicing intervals are every 100 running hours or annually after the first 20 hours which should be adhered to as turbo intercooled diesels don’t like dirty oil. Personally, if using the engine infrequently I’d change the engine oil and filter every six months. Frequent oil changes are the secret to gaining longevity from any engine. If in doubt about the right oil to use, consult Mercury Marine’s recommendations as modern turbo intercooled diesels can use surprisingly low viscosity oils.

The engine-only dimensions are 927 mm long, 813 wide and 853 mm high, slightly longer and wider than the MerCruiser 6.2 MPI but substantially taller. The 6.2 measures 813 x 762 x 559mm though at 423 KG with Bravo 3 drive the 3.0L is 63 KG or 13 percent lighter than the MerCruiser 6.2 MPI with Bravo 3 drive.

On the Water

Though slightly over-propped the demo 3.0L in the 250 SLX appeared a very good match. There was no smoke or diesel smell on cold start up or at any other time, even when climbing out of the hole. The single Bravo 3 drive gave excellent control compared to twin Bravo 3 drives, where using one ahead/one astern simply crabs a hull sideways. Personally I think single-prop Bravo 2 drives are better for twin installations because they allow for true one ahead/one astern handling. The shift action of Mercury’s DTS with “drive-by-wire” technology was smoother than expected with no “clunk” into ahead or astern. However the usual diesel rumble meant noise levels when trolling were slightly higher than a petrol V8.

Trolling at 800rpm the 3.0L used only 40 percent of what the 6.2 MPI 320 HP model uses when idling. The performance table shows how fuel efficient the 3.0L was up to 1000rpm.

Only when the hull started to plane with the engine under a fair amount of load was the characteristic V6 exhaust note apparent and once planing, the only real noise was from the drive unit with its two sets of right angle gears. Mercury Marine now needs to work on reducing NVH or Noise Vibration and Harshness levels in its stern drives to make them as quiet as the engines driving through them. 

The 250 SLX planed cleanly below where maximum torque is developed – always good as once planing the engine is operating in its peak fuel efficiency zone. 

I’ve tested inboard diesels in some hulls where planing occurred above where maximum torque was developed and this seriously affected planing fuel efficiency. The 3.0L planed us around 1000rpm lower than a comparable-torque V8 and used 16.7 L/hr at 2000rpm whereas at a 6.2 MPI I tested planed a hull at 3000rpm using 41 L/hr. Again the 3.0L used only 40 percent of the fuel. Cruising at 2500rpm compared to 3500 for the 6.2 MPI the 3.0L used 44 percent of the fuel. 

Obviously developing 320 HP (and now 350 HP) the 6.2 MPI would give higher top end speeds but the performance of the 3.0L sure wasn’t disappointing. Through tight turns at 3000rpm the 3.0 maintained its revs without us touching the throttle. At WOT the 3.0L used 59 percent of the fuel of the 6.2 MPI at WOT, indicating the gap narrows the higher the revs are. This is a typical situation as petrol engines have maximum fuel efficiency higher up in their rpm ranges than comparable-output diesels.

Even at WOT the 3.0L had way lower exhaust noise levels than a four stroke petrol outboard of similar output and showed just how far diesels have come in reduction of noise. Vibration levels were also low and for fatties like me out there, the 3.0L sure won’t reduce any flab!

The Wrap

The mating of the 3.0L and Bravo 3 for single installations is excellent. Based on price alone the 250 SLX would be powered by more 6.2 MPI engines than the TDI 3.0, but for boaters serious about conserving fuel the diesel is very appealing. Obviously maintenance and servicing costs will be higher and despite being electronically managed the 3.0L won’t like pottering around bays and inlets for extended periods, whereas the fresh water cooled 6.2 MPI would be right at home. 

Freshwater-cooled petrol engines don’t suffer cylinder bore glazing as can diesels, unlike raw or seawater-cooled four strokes which run relatively cold, these suffer blow-by past the rings from the combustion chamber to the sump during extended low-load periods, diluting engine oil and reducing its lubricating effectiveness. Engine wear is also accelerated and one owner of a Caribbean 26 powered by twin raw water-cooled MerCruiser 4.3 litre sterndrives told me the engines had “worn out” by 1400 hours. 

However the point is that a diesel can provide comparable low to midrange performance as a petrol engine of similar torque. Top marks to Sea Ray for providing an alternative power source to the ubiquitous petrol V8!

For your nearest MerCruiser dealer Google Mercury Marine Diesels, click on Find a Dealer then enter your suburb and postcode. The closest dealers will be displayed with distance as the crow flies.

SPECS:

MerCruiser TDI 3.0L/Bravo 3

Engine type 90 degree V6 turbo intercooled diesel

Rated BHP/MHP* 256/260 at 4000rpm

Maximum torque 566 NM at 2250rpm

Displacement 3000cc

Bore x stroke 83 x 91.4 mm

Dry weight (423kg) w/Bravo 3

*Brake horsepower/metric horsepower or PS.