Hill Climbing's Easy for mDrive and 'Downsped' MP8 Diesel

Mack’s automated transmission is a fine piece of work and now, with low-ratio gears added, it gains real startability for Granites.

October 2016, - Test Drives

by Tom Berg, Senior Contributing Editor - Also by this author

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C1 gear easily started this Granite dumper on a 15% upgrade. So did the mDrive's normal 1st gear, but with slightly more effort. Photo: Tom Berg 
C1 gear easily started this Granite dumper on a 15% upgrade. So did the mDrive's normal 1st gear, but with slightly more effort. Photo: Tom Berg

Mack’s mDrive automated manual transmission is a fine piece of work and has gained considerable popularity with buyers of highway trucks since its introduction seven years ago. Now, with low-ratio gears added, it’s even more capable and should become more desirable for people who run vocational trucks.

That was my takeaway after demonstrations and driving sessions at the builder’s Customer Center and nearby public highways in Allentown, Pa. Also shown off last week were some of Mack’s 2017 diesels, which are smooth and gutsy, though any performance differences with current engines are not obvious.

The mDrive is Mack’s version of the Volvo I-Shift, but it’s tuned to match the operating characteristics of Mack Power diesels, whose mechanicals are also shared with the sister company. Electronic calibration sets the engines and transmissions apart and carefully deliver what Mack customers want, executives say.

Creeper ratios within a 5-inch-long gearbox added to the front of a basic 12-speed mDrive HD convert it to a 13- or 14-speed unit. That gives a truck greater startability, especially on upgrades, and/or the ability to move out with extra heavy loaded rigs, such as with long combination vehicles on turnpikes and elsewhere in the United States and Canada. 

Startability was graphically demonstrated with a loaded Granite dump truck on a steep grade near the center’s oval track. Tim Wrinkle, vocational segment marketing manager, acted as an instructor as I tried out the vehicle. First, we ran around a short circle shot with potholes, humps and rough stone pavement. There was a whole lot of shakin’ goin’ on, and we were both grateful for our air-suspended seats.

The hill was meant to be the real test for the 14-speed mDrive. Wrinkle directed me to stop on the steepest part of the slope, which he estimated at 15%, then start out using the transmission’s normal 1st gear, whose ratio is 15 to 1. The mDrive’s readout said it could do it, and it did, with little hopping or other protesting from the clutch or gearbox.

We rolled back down the hill and he had me switch to manual, or M on the selector keypad, and punch down to the crawler ratios, through C2, with a 19 to 1 ratio, to C1, with 32 to 1 – or twice as low as 1st gear. “In C1 the truck will move at only about a half a mile an hour,” he said. The system’s hill-holder function kept the brakes applied for 3 seconds after I took my foot off the brake pedal; I put a little pressure on the accelerator pedal and we then moved out smoothly.

The engine raced as we slowly ascended the hill, staying in C1 until we topped it. At that point it began upshifting through C2 and into 1st and 2nd. Normally, the transmission will quickly begin skip-shifting, going to 4th or 5th and upward through the range of ratios, depending on conditions.

However, at Wrinkle’s direction, I stopped, backed up, turned and headed down the hill until we again reached the steepest part. I punched the selector’s R button, putting us into reverse, and let off the brakes. With a bit of fuel the engine, working through the low-low ratios, easily pushed us back upgrade.

At the top, I stayed in reverse and backed toward the circle trail. I gave ‘er some more gas and punched the up button to manually upshift as we picked up speed, using all four reverse ratios. I forgot to look at the speedometer but figure we might have been going 10 mph or so when I eased off the accelerator.

Keypad selector includes the usual go positions. But there's also a button at lower-left that causes the transmission to aggressively downshift to maximize engine braking.  Photo: Tom Berg 
Keypad selector includes the usual go positions. But there's also a button at lower-left that causes the transmission to aggressively downshift to maximize engine braking.  Photo: Tom Berg

Gear jammers know that with a manual transmission in reverse gear, you can shift from low range to high range while on the move and gain some speed. But it’s far easier with an mDrive HD, and with the creeper gears you’ve got four speeds instead of two. As I said, the mDrive also makes it very easy to start out on a hill, where a guy with a manual transmission must really know what he’s doing with the gearshift lever and the clutch pedal.

I told Wrinkle that I never cease to be amazed at the smooth clutch engagement with automated transmissions, no matter the driving circumstances. That goes for the mDrive as well as other automated products I’ve driven in recent years. I wish I could comprehend the code writing for the electronic controls done by engineers that makes this possible.

The only thing better for startability under repeatedly rough conditions is a torque converter automatic, namely an Allison. Mack will sell you one of those if you’d prefer, but it costs more and won’t be as necessary now, with mDrive’s creeper-gear availability. The 13-speed mDrive HD is now standard in Granites and the 14-speed is optional. 

On the Road

The demonstration included several Pinnacle highway tractors, and I drove both axle-back and axle-forward versions around the smoothly paved track (which was a test track when this facility was Mack’s tech center).

The axle-back model had mDrive with EasyShift, which changes gears more slowly and applies power more gently. This is meant for hauling tanker-borne liquids, where cargo surge can be bothersome, and livestock, which can lose their footing and be injured if shifting is abrupt. This tractor purposely ran bobtail, where shifting can be jerky without a trailer and load to settle things own. But EasyShift kept things smooth.

The track speed limit is 40 mph, but nearby streets and highways were there for those of us with CDLs. I chose the Pinnacle axle-forward, which has high-hooded traditional styling and is my favorite Mack (for looks, anyway.)

This one was set up with a “downsped” MP8 455-hp Super Econodyne diesel and a 13-speed mDrive, which made it suitable for somewhat heavy hauling because the extra low ratio would aid starting out on upgrades. Turns out we never needed it, but as always, the automated transmission removed most of the work from driving the tractor.

Handsome Pinnacle axle-forward tractor had a "downsped" MP8-455 whose strong torque propelled it up stiff hills on I-78. PowerLeash engine brake held it back on downgrades. Photo: Tom Berg 
Handsome Pinnacle axle-forward tractor had a "downsped" MP8-455 whose strong torque propelled it up stiff hills on I-78. PowerLeash engine brake held it back on downgrades. Photo: Tom Berg

The macho-looking tractor with a low-height sleeper was hitched to a spread-tandem flatbed for on-site running, but they switched it to a loaded 53-foot van for out on the highway. My guide for this was Scott Barraclough, technology product manager, who knew a lot about mDrive and its applications. He directed me out of the facility to some city streets that led us to Interstate 78. We followed that west for about 40 miles before turning around.

He said rolling terrain along the patch of Interstate would be a good test of the powertrain’s lugging ability, and it was. Downspeeding had the engine cruising at 1,300 rpm at both 60 and 65 mph; 60 was in 11th gear, which is 1 to 1 direct, and 65 was in top gear, which is overdrive-12th. (The 13th ratio is a creeper, and is not counted as part of the normal 12 speeds.)

Engine cruised at 1,300 rpm at 60 and 65 mph in 11th-direct and 12th-overdrive gears, respectively. Photo: Tom Berg 
Engine cruised at 1,300 rpm at 60 and 65 mph in 11th-direct and 12th-overdrive gears, respectively. Photo: Tom Berg

Barraclough said the rig weighed close to 80,000 pounds. So how did it pull the hills at such low engine speeds? With 1,860 lb-ft of torque. It worked very well, with no more speed loss than one would find with higher revving engines, my experience told me, and it seemed quieter and smoother, too.  

As the engine dropped to 1,000 rpm or so, the mDrive quickly and smoothly downshifted into one or two lower gears to maintain momentum. It does take a bit of getting accustomed to, as you’d expect to need more revs to climb hills with any sense of liveliness.

With cruise control set and the PowerLeash engine brake switched on, the MP8 and mDrive worked together to try to keep road speed close to the set number – not exactly, but within 5 mph or so on downgrades. The retarder was fairly quiet and you could probably use it even where there are signs saying “No Engine Brake.”

That's unless you punched a special button on the mDrive’s selector, which Barraclough pointed out. This caused it to downshift more aggressively and the brake to make extra power and somewhat more noise. I got to like that button, especially at lower speeds and where a red light waited at the end of a long off-ramp and the engine brake slowed us with little need for service brakes. 

Mack’s 2017 diesels have many mechanical advances designed for greater fuel economy, lower carbon emissions and lighter weight (see a story here) . But except for quietness – especially noticeable when standing next to trucks whose 2016 and 2017 engines were idling – they don’t seem to behave much differently from previous models. The power and torque are there, and pulling power is as good as ever.

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