Review: Cummins QSCD8.2 500hp

REVIEW: CUMMINS QSC8.3 500HP

THE NITTY GRITTY

Unlike most automotive-origin V8s, the 500 is built to last. Though it has the same push rod valve actuation from a single camshaft, this is gear and not chain driven. The long stroke crossflow power head also has replaceable wet cylinder liners for a longer service life than dry liners that ain’t cooled as effectively. The 500 is rated to a maximum of 500 running hours per year, with Wide Open Throttle being able to be used one hour in every eight (or 12.5 percent of the total running time).

Apart from the massive torque output, the biggest difference between diesels and petrol engines is that they can be operated continuously only a couple of hundred rpm below peak rpm, whereas the V8s should not be run continuously over around 3500rpm, or 1500 below peak rpm. The best running efficiency of the 500 is only 400rpm less than the peak rpm, enabling a sports cruiser to cruise comfortably well on the plane while returning relatively-good fuel efficiency.

Unlike European diesel manufacturers that quote maximum torque and output using distillate of 860 grams-per litre, Cummins uses a figure of 839 grams; this is the same as the substandard fuel sold in Oz (because our governments are too weak to insist on better-quality distillate from oil companies). Fuel density plays a big role in diesel engine torque and peak outputs, so when you buy a Cummins you know the engine will develop the same outputs in Oz as it does in the US. It’s just a shame that Cummins went down the metric horsepower route for engine outputs, whereas old farts like me still use BHP measurements. Still, Cummins is not alone in this, as other US diesel manufacturers also use MHP to make an engine appear more powerful than it really is.

The 500 has heat exchanger cooling as standard, with seawater cooling for the turbocharger after-cooler. Using seawater instead of fresh means more effective charge air cooling, with the downside being possible seawater ingress to the intake manifold should seals fail — whereas fresh water getting into combustion chambers won’t cause as much damage. As with all engineering, there are pros and cons in which directions engine manufacturers take.

Servicing the 500 is straightforward and I recommend changing the sump oil and filter every 100 to 200 running hours, or annually depending on how much low-speed operation is done. Even electronically-managed diesels don’t like a lot of idling, and with the thermostat set to open at a lower temperature than in most automotive engines, blow-by of unburned fuel past the piston rings may dilute the sump oil. For temperate climates, I suggest using an FCW (Four-cycle water-cooled) SAE30 mono-grade oil and a diesel-specific SAE15W40 oil for tropical conditions. Be careful when using this viscosity of oil, as some of them are designed only for light-duty petrol engines.

ON THE WATER

The twin 500s installed in a Sea Ray 44 sports cruiser were a perfect match for this hull and planed the hull below the peak torque band, which improves planing fuel efficiency. I have tested hulls where the planing is achieved above the torque band and this means the engines have to work too hard on the plane. Also, there’s limited rpm range between planing and Wide Open Throttle, whereas with the 500s there was a range of 14kt for real cruising flexibility.

The 44 displaced 12.4 tonnes with two adults aboard and half fuel and water capacity (600 and 190 litres respectively), plus the optional bow thruster. Frankly, with twin engines I believe a bow thruster is unnecessary, but being able to move sideways out from a jetty with the wind blowing against it is very handy.

To provide effective conversion of engine torque to prop thrust, the 500s had 1.56:1 V-drive reduction ratios, bringing rpm for the four-bladed 22 x 27-inch props down to 1700 at Wide Open Throttle. The props swung in Sea Ray’s innovative half-tunnels with flat tops to improve lift out of the hole, while reducing hull draft and eliminating the need to use trim tabs. In my opinion, no properly balanced hull should need tabs to plane, and when down they only create drag.

When firing up the engines there was a purposeful rumble from beneath the cockpit floor, but unfortunately none of that delightful V8 rumble found in petrol sports cruisers. And even at 550rpm, the minimum idle speed of 5.4kt meant some throttle jockeying to limit speed in tight marina confines. I found slipping the engines in and out of gear kept our idle speed down to a more manageable 3kt — easily done with the “drive by wire” throttle and gear shift control. And how nice to have single lever controls. Why manufacturers such as Caribbean still use twin levers is beyond me, especially when single-lever Morse Teleflex controls have been around since the sixties.

Once away from the marina with the engines warmed up, opening the throttles planed us at 1750rpm, with some black exhaust smoke only appearing at 1500 to 1600rpm where the engines were under maximum load. Planing cleanly at 2000rpm, the engines maintained set rpm through tight turns with no sign of exhaust smoke. Noise levels from here out to WOT were relatively low, and we could talk normally at the helm.

I must confess, being a yachtie I prefer sports cruisers to flybridge hulls. The lower centre of gravity makes them better sea boats, while having cockpit lounges close to the helm gives the boat a more social feel than everyone crowding onto a flybridge. It’s also easier to run forward to drop an anchor than being on a flybridge, and the boat has less windage when manoeuvring in tight marinas.

THE WRAP

Frankly, I love the relatively simple truck engineering of the 500s refined by using electronic management. The 500s are under-stressed and, with regular maintenance, should return thousands of hours of reliable motoring; this is unlike the Cummins QSC550 and 600, both of which rev to 3000 and, according to US owner reports, can have a short service life if over-worked or over-propped.

For the location of your nearest Cummins agent head to their website, click on Find a Cummins Service Location and enter your postcode. 

SEA TRIALS CUMMINS QSC8.3 500HP 

Twin Cummins 500 diesels in a Sea Ray 44 sports cruiser, total displacement 12.4 tonnes. Average of two way runs on Lake Macquarie NSW, calm water. Range is for half fuel tankage of 600 litres in nautical miles with a 10% reserve.

Check out the full feature in issue #507 of Trade-a-Boat magazine. Subscribe today for all the latest boat news, reviews and travel inspiration.