Cummins MerCruiser diesel QSC 8.3-500

The Cummins MerCruiser diesel QSC 8.3-500 is a turbocharged-aftercooled direct injection engine with a 114mm cylinder bore and 135mm piston stroke. Removable wet cylinder liners are fitted for maximum reliability and the one-piece crossflow head has replaceable valve guides and seat inserts, while valve actuation is via rockers and pushrods. The aftercooler is seawater-cooled for effective reduction of air intake temperature and the heat exchanger cooling system normally operates at 85 to 95 degrees. The starter motor and alternator are high-mounted, away from bilge water.

For years the 6CTA 8.3 was available in mechanically-injected form and, recreationally-rated, was known as the 450C, which developed (at 1hp equalling 746W) 430.3hp at 2600rpm based on wide open throttle operation of one hour in every eight and a maximum of 300 operating hours per year.

The next stage of development was the 480C-E with electronic management and an in-line fuel distribution pump, which developed 459.8hp at 2600rpm and 1634Nm of torque at a usefully-low 1600rpm. The bobtail (sans gearbox) weight increased from 856 to 858kg. In 2004, Cummins released the QSC versions of this engine, featuring common rail injection and four valve heads and a warranty coverage extended to 1000 hours or two years. Initially, the engine was offered as the QSC8.3-540, which developed 532.2hp at 2600rpm and 1689Nm at 1800rpm, while the '500' version developed 483.9hp at the same revs. A torque figure is not available but it should be around 1650Nm at 1700rpm. Both engines have bobtail weights of around 895kg or 969 with a ZF 280 Series single-speed hydraulically-operated gearbox. The V-drive version weighs 1001kg.

The QSC8.3 engines have bobtail measurements of 1141mm long, 954 wide and 981mm high, compared to 1173 x 849 x 954mm for the 450C and 1141x945x 950 for the 480C-E.

Cummins rates all its recreational marine diesels on a fuel density of 839gm/lt, the same as poor quality Aussie distillate. The QSC8.3 complies with US EPA Tier 2 exhaust emission regulations for recreational marine diesel engines and is available with Mercury Marine's SmartCraft system, which monitors fluid levels, vessel speed, fuel flow, range to empty, rudder angle, and water depth. Cummins also offers a 'C-Cruise' package, which includes engine synchronisation, slow idle for manoeuvring, and electronic throttle/gearshift control.

As standard, the SeaRay 44 Sundancer, which measures 13.72 x 4.27m, is powered by twin MerCruiser 8.1 Horizon V-drive petrol V8s, which develop 370hp at 4400rpm. The optional petrol power is twin 8.1S HO V8s, which develop 419.6hp at 4600rpm.

But, as the 44 Sundancer has a dry weight of 10.2 tonnes compared to 8.7 for the 40 Sundancer, even the 8.1S HO engines would provide inadequate planing performance. So my local SeaRay dealer, Lifestyle Marine opted for twin QSC8.3-500 V-drive engines in its demo 44 and the difference in performance between this boat and the 40 was astounding.

With the engine and genset upgrades, the fitting of a bowthruster, half fuel and water tankage and two adults, the demo 44 displaced 12.4 tonnes, compared to 9.5 for the petrol-powered 40. Four-bladed 22 x 27-inch Nibral props operating in SeaRay's clever flat-topped half tunnels were fitted and the reduction ratio was 1.56:1.

The Cummins engines started instantly from cold and even with the exhausts exiting above the waterline the noise and vibration levels were only marginally higher than the petrol MerCruisers. Using the slow idle setting we slid out from Lifestyle's marina at 550rpm averaging 5.4kts and 1.9lt/h per engine, whereas the petrol engines used 6.1lt/h at 3.7kts and idle revs of 700. Using the one ahead/one astern technique without increasing throttle opening or using the bowthruster, we were able to swing the 44 in its own length.

At 1000rpm, where the exhaust system switches to underwater and outboard of the tunnels, we averaged 8.2kts and 8.7lt/h per engine, compared to 4.7 and 8.4 for the MerCruisers. Whereas the 40 averaged 8.8kts and 20.2lt/h, each at 2000rpm, the 44 averaged 11.7kts and 27.6lt/h each at 1500rpm, where turbo whine was apparent.

Amazingly, considering the additional 980kg of the Cummins engines over the MerCruisers, we planed cleanly at 16kts on 1750rpm using 41.6lt/h. But the 40 planed at 14.7kts on 3000rpm using 46.8lt/h each engine. An offshore cruise of 21.2kts was achieved on 2000rpm using 54.9lt/h each compared to 18kts, 3400rpm and 57.6lt/h each for the MerCruisers. The most efficient planing speed was 25.5kts on 2200rpm, and the Cummins engines used 64.3lt/h each, while at 24.2kts and 4000rpm the MerCruisers used 88lt/h each.

At the maximum continuous revs of 2400 the Cummins engines used 75.7lt/h each for an average of 28.1kts, whereas the WOT average for the 40 was 28.9kts on 4750 revs using 132.8lt/h each engine. At WOT and 2670rpm, the Cummins engines used 94.6lt/h each for a 31.4kt average and, surprisingly, noise levels were lower than the MerCruisers at WOT. When the throttles were 'floored' from fast idle black smoke appeared only at 1500 to 1600rpm, where the hull was starting to plane and, through full-lock figure of eight turns at 2000rpm, the speed dropped only one knot with no lost revs or increase in fuel flow.

In my opinion, from testing various petrol MerCruisers in 2005 compared to the QSC8.3-500, petrol engines are fine for infrequently-used sportscruisers to 40ft. But for larger hulls and owners who enjoy offshore passage-making, I would opt for electronically-managed diesel power.

For more on the QSC8.3-500, contact your local CMD dealer or email Murray Clifford at murray.clifford@cummins.com