Paccar’s MX-11 Pulls Like Something Bigger

Oct. 23, 2015

"There’s no replacement for displacement.” That’s an old hot-rodders’ saying, and truckers use it, too. All other things being equal, a larger engine will outperform a smaller one. But things aren’t as equal as they were. Modern electronics and combustion technology have wrought engines far more powerful than was possible even a few years ago. The latest example is the MX-11 from Paccar, parent of Kenworth and Peterbilt. Like its big brother, the MX-13, the new engine is based on a proven design from DAF, Paccar’s Dutch parent.

A couple of months ago, I had the privilege of listening to a pre-introduction briefing on the MX-11 diesel provided by Kenworth representatives at its research and development center in Renton, Wash. This thing has me believing another more recent saying: “Smaller is better.” For starters, its dry weight is 2,200 pounds, which is 400 less than Paccar’s MX-13 and the Cummins ISX12. That prompted Kurt Swihart, KW’s marketing director, to comment, “We expect the new Paccar MX-11 will gradually build a following among vocational-truck operators and bulk haulers, and anyone who is weight-conscious, because lower tare weight means more capacity for payload.”

Sitting in an engine compartment, the MX-11 looks like almost any inline six-cylinder diesel, and it’s painted a conservative dark gray, as are all Paccar engines. It might look like an MX-13, but almost no parts are interchangeable between the two engines. Compacted graphite iron forms the MX-11’s block, double overhead camshafts actuate the valves, a “lube module” consisting of two filters cleanses the motor oil, and a composite plastic material is used for the oil pan. Coincidently, its bore and stroke are 123 by 152 millimeters, the same as Volvo’s D11 and Mack’s MP11, so it has the same displacement of 10.8 liters or 660 cubic inches. The MX-11’s B-10 life is expected to be 1 million miles, meaning 90 percent of them should still be running at that point with few repairs. Time will tell.

After the briefing, I drove two MX-11-powered vehicles: a four-axle T880 dumper configured for northeastern states with a single pusher-type lift axle ahead of the tandem; and a T680 tandem-rear-axle daycab tractor pulling a long semitrailer. Both had MX-11s with the strongest rating: 430 horsepower and 1,550 lb.-ft.

With 10 tons of sand in the box, the dumper grossed about 40,000 pounds. Driving it was a breeze, because with an Allison automatic there was no work to do but point the truck where it was supposed to go and enjoy the ride. Mostly fair skies and mild temperatures made this almost a pleasure cruise; OK, it was a pleasure, as a T880 is currently one of the finest rigs to drive. It’s quiet, smooth riding and comfortable, and the wider cab (about 8 inches more than the T800’s cabin) offers spaciousness that makes a guy appreciate his time behind the wheel.

The Allison worked almost flawlessly, with smooth starts from dead stops and steady acceleration as the speedometer needle climbed toward a desired cruising speed. For us, it was usually 60 mph or so on two-lane state highways that are wide and smoothly surfaced. The Allison wasn’t perfect, and a couple of times at low speeds going through smaller towns it up- and downshifted with a thump. Overall, its smoothness makes it good at certain work; in construction that includes concrete mixers that start and stop often and sometimes must creep while placing their loads, and yes, in dump trucks. But the Allison softened the feel of the engine’s output.

The tractor and its trailer grossed about 70,000 pounds (the photo shows a flatbed, but we pulled a van). It was a lot more work, as it had a Fuller 13-speed manual that wasn’t the easiest-shifting setup that I’ve ever driven. I had trouble getting it out of gear and into neutral while shifting, something I recall happening during my first drive of a T880 several years ago. What’s with this? I griped. Now I wonder if I was failing to push the clutch pedal far enough toward the floor, allowing the clutch facing, and therefore transmission shafts, to be dragged by the flywheel. Next time I drive a Kenworth T-series with a manual transmission I’ll concentrate on finding out.

But balky or not, the manual tranny gave a better sensation of the engine’s strong output, as power flowed through solid gears and out into the driveline, except of course while shifting. There was no seat-of-the-pants feel that the MX-11 is a lesser engine (compared to the MX-13), and propulsion actually felt like it was coming from something bigger. My stint with the T680 was mainly along two-lane roads in the Cascade Mountains east of Seattle-Tacoma. One was Highway 18, which we followed southwest as it crossed several tall hills that presented steep grades to traverse. Downhills were easily and safely handled by the engine brake, which produces as much as 430 retarding horsepower (in all ratings, even in the under-400 ones), and I seldom needed to use the service brakes.

The engine pulled strongly on the uphill portions and didn’t need high revs to do it. One of the briefers had said that it was good at lugging, and it certainly was. On two hills, I let it hammer away at 1,100 to 1,200 rpm and it didn’t falter. On the third such hill, I decided to see if the old axiom, “for performance you need rpm,” was true with the MX-11. It wasn’t. When the tach needle said 1,100, I downshifted a full gear, from 7th-direct to 6th-direct, raising the needle by about 300 rpm. But I lost some momentum while making the downshift and had to get on the accelerator to return us to the previous road speed, which as I recall was about 33 mph, and couldn’t accelerate beyond there. If I had left the shift lever alone, the engine would have kept pounding away, and rather smoothly, I should add. The lesson here: Let it lug.

With the larger MX-13 and some more power and torque, this rig might have climbed those hills a little faster, maybe by 5 or so mph. Would that have made much difference in running time? Perhaps 15 to 20 minutes over a full day’s driving. There are some runs where hills are a constant, but even on those there’s some level pavement to cruise on, and a bigger engine won’t move a sane driver and his rig over those miles any faster. Meanwhile, the MX-11’s weight advantage theoretically allows a rig to carry 400 pounds more payload; if the shipper will actually pay for the extra cargo, there’s a tad more revenue to be had. Even if not, an engine must always pull the truck itself, and the lighter it is, the less fuel it needs.

Here’s another argument for a slightly smaller engine like the MX-11 for Class 8 trucks. Yes, it works harder than something with larger displacement, but that can be a good thing. Like all diesels these days, this one has a particulate filter that needs heat to operate efficiently. The more heat generated by the engine and sent through the exhaust, the better the DPF likes it. Lack of heat leads to problems with soot plugging, and that’s more likely to happen with trucks that start and stop a lot, because their engines often run at far less than full output, allowing their exhausts to cool. This point came up at a recent maintenance meeting, and I’d never thought of it before.

In any case, the Paccar MX-11 seems to deliver more performance than its modest displacement gives it a right to. That and its lighter weight are likely to make it a popular choice among thoughtful Kenworth and Peterbilt customers. For those who feel better with more liters (or cubic inches) in the cylinders, there’s still the Cummins ISX12, then the Paccar MX-13, and then the Cummins ISX15, in that order of ascension. Ain’t choice grand?