Review: Cummins QSC8.3 engine

By: Andrew Norton

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  • Trade-A-Boat

No petrol inboard comes close to the Cummins QSC8.3 – 500 marine engine for torque.

Review: Cummins QSC8.3 engine
The Cummins QSC8.3 – 500 diesel inboard shows that marinised truck engines are more than up for the task.

I love the sound of a big block petrol V8 boat engine being worked hard. Multiply that by two and I get shivers down my spine.

But the sad fact is big blocks are fuel guzzlers on boats. And with their love of premium unleaded they’re damned expensive to keep going. Even the most powerful petrol V8 motor can only handle hulls to 13m, not because they lack power but the torque simply isn’t there. An 8.1lt V8 produces only about 700Nm of torque at around 3200rpm, about the same as a 4.2lt turbo-aftercooled diesel at 2500rpm.

Diesels also score because they can be run continuously at only a couple of hundred rpm below maximum, whereas automotive-derivative V8s shouldn’t be run at more than 3500rpm for any length of time. So you’re stuck with diesels for hulls over 13m, running on a fuel that’s great for firing up damp barbecue timber and cleaning rust scale from steel decks. It also does a great job of rotting planking in timber boats.

 

CUMMINS QSC8 MARINE ENGINE

The direct injection Cummins QSC8.3-500 is a great example of the diesel alternative for boats. Developing 493 brake horsepower at 2600rpm, this straight-six is the least powerful of a range of 8.3lt engines. But who gives a damn when it puts out 1799Nm at only 1800rpm. And it can be run continuously at 2400rpm.

Unlike its European competition, Cummins rates maximum output running on diesel having a specific gravity of 839 grams per litre, the same as our crappy fuel sourced from Asia. So when Cummins says 493bhp it means just that. Diesel fuel density makes a big difference to engine performance. And unlike some of the direct recreationally-rated competition the 500 is designed to be used up to 500 hours per year compared to 250.

Forget the complexities of DOHC valve actuation. The 500 has a crossflow head and simple pushrod and rocker design with replaceable wet cylinder liners. Of course heat exchanger cooling is standard with seawater taking away heat from the aftercooler.

The 500 has electronically-controlled common rail fuel injection and has slow-troll rpm to reduce idle speeds in marinas. There is an engine synchronisation facility and the drive-by-wire throttle/gearshift controls eliminate the need to tug hard at the levers during panic stops. Not that I panic because my nerdy nature ensures I approach all jetties slowly, waking up passengers when we’re docked.

Servicing the Cummins QSC8.3 - 500 is straightforward and spares are easily sourced around Oz. With a hydraulically-operated straight drive gearbox the 500 has a dry weight of 969kg or 1001kg with a V-drive.

 

CUMMINS MARINE ENGINE PERFORMANCE

The 13.7m Sea Ray 44 sportscruiser I tested the 500s in, came as standard with twin 8.1lt petrol V8s developing 420hp each. Fine for the old Sea Ray 40 but not the 44 which during trials displaced 12.4 tonnes including two adults, half fuel and water tankage and a bowthruster.

The 500s drove through 1.56:1 reductions to four-bladed 22 x 27in props swinging in Sea Ray’s innovative flat-topped half tunnels.

The demo 500s started instantly from cold with no white smoke appearing. Using the one ahead/one astern technique the hull could be spun in its own length without using the bowthruster. Vibration levels were very low, though unfortunately none of the spine tingling I get when firing up V8s.

Using the slow-troll setting we maintained a reasonably low in-gear idle speed and the only time black exhaust smoke appeared was when we floored the throttle levers to get the 44 out of the hole. This occurred at 1500 to 1600rpm, just before the hull planed. But when we put
 the 44 through hard turns at 2000rpm the hull speed dropped by only one knot with no decrease in engine rpm or any black smoke appearing. As I prefer to see how a hull trims without aids the trim tabs were left at neutral.

Like most moored demo hulls that have been sitting around for a while before delivery there was some slime on the hull which probably robbed us of about a knot at Wide Open Throttle. However, the 500s were slightly under propped to allow for this and full fuel and water tankage, plus extra passengers. At or near WOT the 500s were quieter than the 8.1lt V8s I tested in a Sea Ray 40 with similar vibration levels.

 

THE TRADE-A-BOAT VERDICT

I’ve never had a problem with marinised truck engines being used in boats and the Cummins QSC8.3 - 500 is a fine example of a complete recreational package that, with regular maintenance, should last the life of the boat.

Thanks Lifestyle Marine, Toronto, NSW, phone +61 2 4959 1444, for supplying well-prepared review engines.

 

TWIN CUMMINS QSC8.3-500 PERFORMANCE

Twin Cummins QSC8.3-500s with half fuel and water and two adults. Average of two-way runs in calm water using standard instrumentation and GPS

RPM

SPEED (KTS)

FUEL BURN (LT/H)

550 (slow troll)

5.4

1.9

1000 (fast idle)

8.2

8.7

1500 (fast offshore troll)

11.7

27.6

1750 (clean plane)

16

41.6

2000 (slow cruise)

21.2

54.9

2200 (most efficient cruise)

25.5

64.3

2400 (max cruise)

28.1

75.7

2670 (WOT)

31.4

94.6

*Sea-trial data supplied by the author. Fuel burn is per engine.

 

CUMMINS QSC8.3 – 500 SPECIFICATIONS

TYPE Straight-six turbodiesel

RATED HP 493 at 2600rpm

MAX TORQUE 1799Nm at 1800rpm

DISPLACEMENT 8.3lt

BORE x STROKE 114 x 135mm

WEIGHT 969kg (dry)

 

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Originally published in Trade-A-Boat #451, April / May 2014. Why not subscribe today?

 


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