Volvo Penta D6

Released in Australia at the 2003 Sydney International Boat Show, the D6 is part of a new family of electronically-managed diesel inboards and sterndrives from Volvo Penta. Comprising the four-cylinder D4 and six-cylinder D6, the engines have DOHC valve actuation with four valves per cylinder and are designed to be used in conjunction with Volvo Penta's EVC (Electronic Vessel Control), which provides electronic throttle and gearshift operation, power trim control for Aquamatic drive units, twin engine synchronisation and data link gauges where information is displayed in easily-read text messages. Functions such as engine revs, coolant temperature and fuel consumption are displayed next to the helm.

FAREWELL KAD300
Now Volvo Penta's most powerful diesel Aquamatic unit, the 5.5lt straight-six D6 replaces the straight-six 3.6lt KAD300, which started its life in 1977 as the TMD40, developing 130hp. This engine was upgraded to 165hp by fitting turbocharging and aftercooling, and was very popular in hulls such as the Riviera 30.

In 1991, fitting a mechanical compressor in addition to turbocharging and aftercooling lifted the power output to 230hp - and in 1997, electronic engine management was added, delivering another 30hp. Two years ago the final stage of engine development was completed, resulting in the KAD300 delivering 282hp (at 1.0hp = 746W) at the crankshaft.

But as Eastern Engine explained to me, the 3.6lt powerhead was fairly stressed in KAD300 form, and to increase engine lifespan over a wide range of applications the engine was dropped in favour of the D6 - which has the same 103mm bore and 110mm stroke as the 3.7lt D4.

Hydraulic valve lash adjusters were fitted, but the replaceable wet cylinder liners have been superseded in favour of integrated cylinder liners - although the oil-cooled pistons still have two compression rings and one oil scraper ring.

The engines also have protection features including a "Limp home" mode - should the EDC (Electronic Diesel Control) system fail - and a rev limiter, depending on engine temperature. For example, even if the helmsperson floors the throttles when the engines are cold, the revs will increase to about 1200 and increase in steps as the engines approach normal operating temperature. And in really cold climates the idle revs are set a little higher than the standard 600 revs, reducing exhaust emissions and enabling the engines to warm faster.

EMISSIONS COMPLIANCE
The 2006 EU (European Union) and US EPA regulations demand a significant reduction in the levels of oxides of nitrogen from diesel or petrol engines used in leisure craft up to 24m. Volvo Penta has achieved this compliance by fitting common-rail, high-pressure fuel injection systems - similar to what other European manufacturers such as Fiat Iveco have done - which are apparently far superior to the unit injectors used by some US manufacturers.

STRONGER, BIGGER
Instead of developing 282hp at 3800 revs, as did the KAD300, the D6 develops 306 crankshaft horsepower at 3500 revs, achieved through the 53 per cent greater piston displacement, and without the complication of supercharging.

Even so, the D6 has rapid torque revs from fast idle, producing 740Nm at 2000 revs and peaking at 745Nm at 2500 revs. This torque delivery gives the D6 the edge over Yanmar's 4.2lt mechanically-controlled 6LP-STZE, which develops 296hp at 3800 revs and 700Nm from 2500 to 3000 revs.

Of course at 750kg complete with Duoprop sterndrive, the D6-310/DP is significantly heavier than the 6LP-STZE - which weighs 526kg with MerCruiser Bravo Three sterndrive, similar to the Duoprop concept. The KAD300/DP weighed 576kg with a Duoprop drive. The D6 engine dimensions are also greater, measuring 1218mm long, 753mm wide and 780mm high for the bobtail (minus gearbox) sterndrive engine. The Yanmar measures 1145 x 699 x 812mm, and the KAD300 measured 1051 x 788 x 734mm.

The D6 produces significantly more power at any given revs compared to the Yanmar. For example at 2000 revs it develops a maximum output of 208hp, whereas the Yanmar develops 161hp. At 2500 revs the outputs are 255hp and 244hp, and at 3000 revs the outputs are 302hp and 292hp.

DRIVING THE D6
During the Sydney Boat Show, Eastern Engine provided a Mariner 430 sportsboat fitted with twin D6 units with Duoprop drives for me to test. The boat displaced an estimated 12.3 tonnes including four adults and full tankage - a fair ask for engines of this output and torque to handle.

The engines started instantly from cold without touching the throttles, and emitted no visible exhaust smoke. The vibration levels at 600 revs were no greater than comparable output V8 petrol units, but manoeuvring out of Darling Harbour there was a lot more bottom-end torque. The electronic single lever throttle/gearshift controls were a delight to use, with positive throttle response and barely a clunk into ahead or astern gears - and the electronic synchronisation eliminated the need to play with the throttles to balance the engines.

Across the entire rev range the noise and vibration levels were a little greater than petrol V8s and at Wide Open Throttle we could hold a normal conversation at the helm. Turbine whine was quite muffled and far less noticeable than other diesels of this output I've tested.

Although a GPS was not fitted, the performance figures supplied by Eastern Engine - determined by a test in calm water and with seven adults aboard (estimated 12.5 tonne) - gave a good indication of the capabilities of these engines. At 1000 revs the Mariner averaged 6.7kt using a total of 7.5lt/h, while a clean plane was achieved at 18.1kt using a total 72lt/h. Cruising at 3000 revs the Mariner averaged 26.4kt using a total of 92lt/h, and at 3430 revs and WOT averaged 35.3kt and a total of 125lt/h.

The hull planed at similar revs with four adults aboard and the drives trimmed right in, but across a very sloppy harbour the WOT average was 32kt on 3400 revs with the drives at "Plus 2" on the trim indicator. Through tight turns at 3000 revs, the props refused to ventilate with the drives trimmed out - and despite the engine loading, no smoke appeared. Even when we floored the throttles from fast idle, the engines refused to smoke - very unusual for turbo-aftercooled diesels.

Volvo Penta has succeeded in combining driveability, low noise levels and excellent fuel efficiency into a very user-friendly package. For more information on the D6, contact Eastern Engine, tel (07) 3902 5444 or email graeme.avers@volvopenta.com.au.