Dealer’s Rig Demos Paccar MX Diesel

By Tom Berg, Truck Editor | April 4, 2011

Most semitrailers with product-promotion messages are freight vans, whose broad sides provide a big canvas for words and images. This one’s an end-dump to better suit its audience. Its graphics proclaim, “Powered by Quality...Paccar MX,” and that refers to the MX diesel in the rig’s tractor, a Peterbilt 365.

Tractor and trailer comprise a demonstration rig owned by Las Vegas Peterbilt in southeastern Nevada. It’s for sale, of course, but in the meantime it’s there for truckers to drive and experience the recently introduced engine.

“The Paccar MX tour came through Las Vegas last year with highway tractors, but we wanted something to show to the construction guys,” explains the dealer’s sales manager, Wes Gayhart. So he ordered the vocationally oriented Model 365. It has western specs, including a forward-set steer axle, a long 227-inch wheelbase, and a wet kit to tip the trailer. The MX is mated to an Eaton Fuller 13-speed manual transmission, which helped make it a pleasure to drive.

The dealership, along Interstate 15 in North Las Vegas, also sells Ranco dump trailers, and this one is a 34-foot all-steel Anvil. It’s painted to match the unmarked Pete, which is equipped to please drivers with things like power windows and locks and dark tinting on its windows.

Its mission is to impress truckers with the MX engine’s propulsion ability, even if its displacement, at 12.8 liters, is smaller than what they’re used to. Fifteen or 20 guys had driven the rig by the end of February, and “the first thing they mention is how quiet it is,”  Gayhart said. That’s partly because the MX really is almost noiseless, but also because most of the truckers had come out of 10- and 15-year-old K-whoppers and Petercars with big ole Cat and Cummins power. They also acknowledge that the MX can pull.

Like colleagues at other Pete and KW dealers, Gayhart has taken to heart the factory’s argument that a modern 13-liter-class diesel produces more than enough power and torque to perform many jobs traditionally done by bigger 15-liter diesels. One is pulling heavy aggregate trailers commonly run in the Las Vegas area.

The Great Recession has brought home and commercial construction out here to a standstill. But there were some long dumpers still running in mid-January, when I was there for the World of Concrete show and visited the dealership at the request of factory folks. Those rigs are supporting road building, and some of the projects are being paid for by federal stimulus money.

Attacking the hills

The Anvil had been loaded with river rock to bring the rig’s gross combination weight to about 77,000 pounds, close to the legal limit for a five-axle semi of this length. Peterbilt’s vocational segment marketing manager, Charlie Cook, rode along as I drove the rig up several healthy hills. At hand were 485 horses and torque of 1,650 pounds-feet, the MX’s strongest rating, and these kept the rig moving smartly on upgrades.

The first was on a rural stretch of Las Vegas Boulevard, State Route 604, near the Las Vegas Speedway and Ellis Air Force Base. This two-lane highway climbs out of a wide valley as it approaches its junction with I-15. I started up the hill, maybe a 4 percenter, at 50 mph in 8th-Direct (12th ratio in the 13-speed) with the engine spinning at 1,500 rpm, and the speedometer and tachometer needles stayed right there. A bit later I ran the grade again and stayed at 55 mph all the way up.

Then we swung north on I-15 and headed for Apex Hill, the steepest part of the interstate’s climb out of the Vegas valley, which I’d estimate is 5 to 6 percent, if not very long. I was doing 65 mph and 1,600 rpm in 8th-Overdrive (13th, or top gear) as we hit the hill, and our speed dropped about 5 mph and revs fell about 100. If I had split down to Direct, we might have topped the grade at the same speed we started.

High revs not needed

On these pulls I never wound the engine past 1,700 rpm. I didn’t need to, because there’s where peak horsepower is happening. And I could’ve lugged it way down because peak torque occurs at 1,100 rpm. While accelerating in the lower gears, I often punched the go-pedal there and lower on the rev range. The MX just moves ahead smoothly, with no shuddering or shaking. And as noted, it’s remarkably quiet.

So with this rating, the MX is a real puller, as well as being good at acceleration. Helping was the well set-up tranny linkage, which let me make full-gear changes with little effort and narry a miss, even though I was green on this particular tractor. “It shifts almost as good as a Kenworth,” I told Cook, who winced at the thought. Pete and KW are sister companies, but their respective people are strong competitors.

Back at the dealership, Gayhart said he thinks the MX could also pull double belly dumps, which in Nevada can gross up to 129,000 pounds on nine axles. An MX-485 can’t perform like a big-bore 500- or 550-horse engine, “but it would do the job,” he contended. He did concede that “heavy haulers would want something more,” and for them Pete and KW offer the 15-liter Cummins ISX, which he fully supports as a choice.

Paccar has embraced Cummins as a valued partner, and indeed, the turbo and other parts of the MX came from there. Cummins also supplies the medium-duty Paccar PX-6 and -8 diesels for midrange and Baby 8 Petes and KWs. All 2010-legal diesels found in these trucks and tractors use urea injection to treat exhaust and kill off NOx, so the companies’ philosophies jibe.

The MX, though, is strictly a Paccar design, and is based on an engine built since 2005 by DAF, Paccar’s subsidiary in Holland. Engineers there and here adapted it to meet American emissions standards, which differ somewhat from those in Europe. So, they say, while it’s new here, it’s been proven over there. The North American MX is built at a brand-new plant in Mississippi.

By the way, the surcharge for the ’10-spec MX is $9,250, similar to other manufacturers’ upcharges for these expensive-to-design and -build engines. Stiff pricing and the recession conspired to cripple truck sales for the past two or so years, but the general economy is recovering and truck orders from freight haulers are up sharply. When construction comes back, the MX will become common on streets, highways and jobsites. Maybe you’ll be among those who adopt it.