Easy Does It

By on May 1, 2008
ProStar+6

I must admit that I am not the world’s greatest truck driver, nor even second best behind you, my current reader. I will agree that my driving skills are commensurate with my experience. After all, I drive big rigs as much every year as many of my friends do in a week, especially a week with a holiday in it.

Why do I bring up my relative inexperience? Because I found International’s new MaxxForce 13, the subject of my test drive earlier this year, to be a versatile, forgiving engine. It’s great for “newbies” as well as a solid performer for experienced drivers.

Before I describe the driving experience, let’s examine the engine itself. The big-bore MaxxForce engines were developed in conjunction with MAN Nutzfahrzeuge of Germany, one of Europe’s well-established, vertically integrated truck builders. International and MAN started with a clean sheet of paper to create an engine to meet the ever more stringent emissions regulations for 2010 and beyond.

They chose to use compacted graphite iron (CG) instead of the grey iron most engines are currently cast from. CG iron is 70 percent stronger and 40 percent stiffer. It has double the fatigue limit. By designing around the CG iron’s properties and putting buttresses where solid mass was used previously, International got the MaxxForce’s dry weight down to only 2.244 pounds, as much as 600 pounds savings compared to competitive 13-liter engines.

They moved the gear train to the rear of the block, and used gears to drive the air compressor, high pressure fuel pump and power steering pump for reliability. The block was engineered with deeper skirts as support for the crankshaft, and main bearing caps have a unique interlocking design that improves alignment and fastening integrity. The common-rail high pressure electronic fuel system allows multiple injection pulses. The result is greater strength and reduced noise. In fact, the engine is so quiet, you can stand next to an idling MaxxForce and conduct a conversation without raising your voice.

My test drive was a ProStar with a 475 hp MaxxForce 13, actually a 12.4 liter (757 cu.in) engine. It produces 1700 lb-ft of torque at only 1000 rpm. Torque at clutch engagement speed, 800 rpm, is 960 lb-ft. Multiplied by the gears, especially in low range, that’s more than enough to get — or keep — rolling with a full load. I found that out when I drove it.

maxxforce13Let’s examine some more of the engineering features that make the MaxxForce unique. The engine has two turbochargers acting in series, with an inter-stage cooler in between. The output of the second turbo has the normal air-to-air charge air cooler, but the inter-stage cooler uses engine coolant with its own radiator.

The common rail fuel system operates at 26,000 pounds per square inch (psi). The engine computer manages fuel injection for rapid ignition and smooth burning throughout the stroke, rather than the one big squirt that results in the diesel knock we’re all familiar with. Currently, the MaxxForce uses three injection pulses, but that can be expandable using upgraded software. The multiple pulses make a major contribution to the engine’s quiet operation.

Our test drive was over an easy stretch of both Interstate highway and two-lane roads, as befitting a group of full-time professional journalists who are part-time big rig drivers. What made it easy was that it was lightly traveled from late morning to early afternoon. The course itself was challenging for the engine, selected to show its hill-climbing ability.

The trucks were staged in the parking lot of the Las Vegas Motor Speedway, north of the city, on Interstate 15. After exiting the racetrack grounds, I went north on I-15 for several exits. I turned around for one exit, then took Highway 604 past Nellis Air Force Base back to the Speedway.

Most of my “cobwebs” were gone by the time I started up the hill from Speedway Boulevard. I-15 climbs from there, averaging about 3 percent for several miles. We were told that while power was rated at 1900 rpm, this was a low rpm engine, replacing many 2100 rpm engines. Professionals who were assigned trucks during the year of field trials that preceded the product launch told us to use the tach and shift between 1300 and 1500 rpm. With the usual drop between gears, that would mean picking up the next gear between 900 and 1000 rpm.

They advised using the tach because it really is hard to hear the engine. The ProStar I drove was a quiet truck to start with (see my review in the November/December 2006 edition of Road King). With the little bit of road noise and wind noise, I really did have trouble judging engine rpm from sound alone. In fact, I managed (as usual) to miss a shift or two. The engine didn’t seem to mind that when I finally found a gear it was at 700 rpm. I put my foot to the floor and watched as the MaxxForce pulled slowly and smoothly up the hill. When it reached 900, it seemed to kick up a notch and accelerate at an increasing rate until I again upshifted at 1500.

I had no basis to compare fuel mileage, but drivers and fleet managers who participated in field trials reported about a 0.5 to 1.0 mpg improvement compared to other trucks in their operations. Recommended oil and filter change is at 25,000 miles. International’s engineers built all accessories and components to have service intervals in multiples of 25,000 miles, so no scheduled downtime is needed between oil drains.

The MaxxForce is already being delivered in International TransStars and WorkStars, and will be installed in ProStars later this year. The quiet operation coupled with operating economy and light weight should satisfy International fans, while the easy-to-drive torque curve and lower operating rpm may help add many new fans to the fold.

About Paul Abelson

Paul Abelson is the Senior Technical Editor of Road King magazine.

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