January 2007, TruckingInfo.com - Test Drives
I was really looking forward to an extended drive in the new Peterbilt 389. A brief, surprise drive last year while evaluating an '07 Cummins ISX 600 engine and aftertreatment was enough to convince me that the transformation to the 389 from the older 379 had brought some great things to the party. It was quiet, very comfortable, and the new interior was pleasing to the eye.
For this Drive Test, I would get the chance to live with the 389 (with a 550) for a couple of days, spending an overnight in the inviting sleeper.
Unfortunately, the initial run was terminated when the ISX 550 developed a glitch halfway into the test. I suspected an injector problem from the rough running, so I shut it down for fear there may be a tip floating around inside the motor.
With a Blackberry and my computer out on the slide-out desk in the sleeper, I did find a few extra hours to catch up on work and correspondence while waiting for the wrecker.
It turned out to be electronic gremlins in the electronic control unit (ECU) for the new Holset electronically controlled variable geometry turbo (VGT). I could have limped back. But better safe than sorry.
Electronic problems – if they are to occur – usually happen in the "burn-in" period. And this truck had only 2,000 miles on the odometer. C'est la vie.
Finding an extra couple of days in my schedule to return to Texas for a rerun was tough, but I was glad I did, because the truck lived up to my every expectation on the second go-around. The Test
The Pete was hooked to a loaded demonstration trailer with blocks of concrete bolted to the floor for a gross combination weight of right at 80,000 pounds. With two days available, I headed south from Peterbilt's Denton, Texas, home base on I-35W, down through Fort Worth and on toward Austin. At Georgetown, on the outskirts of Austin I turned west, heading for the beautiful Texas Hill Country, where there are no long grades but a few jump-ups to test the 550.
Route 29 is a reasonable state highway, mostly two-lane, wandering toward Brady. I had passed the previous, unscheduled stopping point and felt good about the strongly pulling 550 as I shut it down for the night in this Hill Country town.
After overnighting there, I headed north for Abilene and then back into Denton via I-20 and I-35W for a total loop of close to 600 miles.
Measurements and photography took some time. One interesting shot (see opposite page) was taken at the control tower at the Alliance Airport, close by the Texas Speedway. (I never expected to be able to drive the rig in to the airport, let alone park a tractor-trailer under the tower and take pictures, but I did. And not a soul came to see what I was doing. So much the security of our ports and airports.)
I had hoped to use the driver display to get fuel numbers, but the device calculated fuel economy in a most curious fashion: It started at zero and improved the further I drove. Usually, these displays start with instantaneous numbers, then settle down to an average that's a good indication of the truck's potential. And fooling with the display when I was shut down overnight meant I zeroed out the first day and had to start over on the second. Luckily I had started with full tanks and the engineers back at the Denton test facility refilled for me, so I was able to get an overall fuel figure.
Average speeds were right around 60 mph on the freeway, but they were significantly slower on the more rural state highways. The Truck
The 389 is new for Peterbilt, representing all the work that had to go into re-engineering the frame and hood to cram the '07 engines and their associated cooling into the chassis. Both Peterbilt and sister Paccar company Kenworth have been very clever in re-doing their premiums for 2007, taking the opportunity to upgrade features, comfort and appearance to help offset the price increases that inevitably come from the 2007 engine technologies. So, while you're likely to pay more, you'll also be getting more.
Most obvious on the 389 are cosmetic changes, many of which add to the aerodynamics and offset some past and potentially new fuel economy issues. The grille surround is now a one-piece unit that looks infinitely better than the old riveted pieces and has a smoother radius for better air flow. Headlamps are smoother and house much better-performing lighting sets that provide more than double the light output of the previous pods and give the bulbs a 60 percent improvement in durability. Directional signals are LEDs for virtually lifetime durability.
Mirrors have been redesigned and hang on the cowl, a mounting borrowed from Kenworth, but a great idea nevertheless. They are more aerodynamic as well as offering better visibility.
The 389 hoods are also different from the 379's – though not enough to upset Peterbilt purists. They are durable, lightweight and feature an anti-blow-down device.
In addition to these standard features, there's an aero package that adds some subtle "tune-up" stuff, such as an air dam behind the front bumper and streamlined tool and battery boxes, some changes to the top fairing and an oval exhaust that can be worth 3/10 mpg. Considering it detracts not one whit from the traditional appearance, this Efficiency Package could be a wise investment.
Under that impressive red hood sat an equally impressive red Cummins ISX. This one was rated 550 horsepower and 1,850 pounds-feet for hill-stomping performance with the best of civility and manners.
The ISX is truly a tour de force, created to take over from the old N14 when emissions requirements got too much for it. So the ISX has all the building blocks to sail through emissions reductions called for in 2004, 2007 and 2010 – in fact, likely whatever comes in the 2014 timeframe. There were some issues with exhaust gas recirculation valves in early EGR ISXs, and it is worthy of note that the EGR valve has been relocated to the cool side of the engine for 2007. There are also a few subtle changes to the block that improve coolant flow. But the bigger changes include the electronic controls for the variable geometry Holset turbo, and the diesel particulate filter.
Electronic control allows for closed-loop control of the VGT. Instead of the engine controller guessing what is needed, now the control circuit knows what the VGT has to do. The result is even better throttle response – a major plus that came from the VGT in the October '02 engines, and, likely, this time, an additional opportunity to address fuel economy.
The diesel particulate filter is part and parcel with the engine as we go into 2007, and Peterbilt does a very elegant job of integrating this bulky component behind the passenger-side toolbox/step. The downside is, it reduces the size of the box. But from this engineer's perspective, it does increase the Wow factor of the 389.
Up behind the big red motor was the dream-team 18-speed, a transmission every gearjammer should want. Refinements to the age-old Eaton Fuller double countershaft transmission make it a sweet-shifting and relatively quiet box that just seems right with a long-nose Pete. I mostly drive 'em as a 13-speed but bless the opportunity to split the lower side when in a bind – a sharp turn off an uphill offramp, for instance. And the manual 18-speed is bulletproof.
Axles under the truck were Eaton/Dana, which is no surprise, as these partners are the preferred Paccar drivetrain suppliers. The ratios in the rear ends were 3.70s, which were likely specified for performance rather than economy. The last ISX 550 I drove with 22.5-inch rubber had 3.55 ratios that work better in most applications. As they say in the ads, ask your dealer.
Running the numbers for the Bridgestone drive tires, we get a calculated engine speed of 1,350 rpm at 60 mph in top gear and a downsplit to 1,600 rpm, which is very much in line with the observed speeds. Personally, I'd rather cruise at 1,300 at 60 mph or even 1,400 at 70 mph, which would call for either 3.55 or even 3.25 ratios. But then there would be less flexibility in the hills. And there was no question, once this Pete was in the big hole, that was where you left it even running across the hill country at the posted 55 mph.
The suspension was conventional parabolic leaf up front and the familiar low air leaf at the rear. Again, it's a personal thing, but I have been so impressed with the Peterbilt Flex Air suspension that it would be my first choice for both ride and handling. As with previous 379s, this 389 felt like it was settling on the Air Leaf in corners, adding some turn in that needed steering correction. However, as the tires were new Bridgestones with 32/32nd,tread depth, the turn could be caused by shuffle of the tread blocks.
Steering was TRW TAS65, which is an older system that has been re-engineered for the International ProStar and is a much more road-sensitive gear.
And we have to consider front axle brakes on this Pete. As with other Peterbilt demo chassis I have driven, it was spec'd with Bendix disc brakes. They are fine products with fantastic fade resistance – no bad thing on a truck. But they have no self-energizing as a leading shoe drum brake has, so they take a prodigious shove of the brake treadle to get the truck stopped, even in a relatively slow-speed stop. Drivers need to be aware of the new characteristic so they are able to make the transition to the much safer brakes. The Cab
As I climbed into the driver seat, I was blown away by the visibility and by the luxury of the interior trim. It was like sitting in a well-upholstered goldfish bowl.
The removal of the wing windows, the slope to the side glass, the big viz window in the passenger door, the relocation of the mirrors, all add up to a conventional Peterbilt that looks great from the outside but works so well from the inside.
The hood still intrudes – the closest sight to the ground in the measurements panel in the chart on page 46 shows that – but in all other respects, visibility out of the 389 is outstanding.
The visibility is enhanced by the narrowness of the cab, a factor that also brings penalties. It's still tough to get back into the sleeper (Pete should browbeat sister division Kenworth into letting it have the superb fold-away armrests that are on new KW seats) but it's easy to see why Pete drivers like the close-shouldered feel of the cabs.
The new trims are different for the traditional truck maker. By all accounts, Peterbilt was charged with coming up with a heavy-duty equivalent to the Lexus and, by George, they've done it. The interiors look spectacular, the dash is fantastic to look at. The whole interior is a joy.
The fact that the dash doesn't work terribly well is a bit of a downer, though. When I set the wheel via the foot-operated column plunge and tilt to the just-right position, most of the engine gauges were hidden under the steering wheel rim. And the really useful driver display in the center of the main dash was totally obscured. So I had to compromise by either setting the wheel at a less optimal angle or dodging my head around to read the instruments.
Having said that, I found myself sitting in this truly gorgeous cab at night in Brady, looking out at the night streets with the dash illuminated and thinking what a work of art this Peterbilt interior is.
And the reason I was sitting and looking out is that I was trying to make sense of the navigation system. I am old. My children would probably take to it in a moment. But I have struggled with the same system in three Kenworths and Peterbilts and find it a most unintuitive device.
Unlike my children, I looked for a manual, and there was none in the test truck. But I did manage to program it to get me back to Denton via Abilene. Here's a thought: If you are going to put these in your trucks – and you absolutely should for driver retention – then make sure someone understands all the ins and outs and educates drivers on the system and its benefits. On The Road
The 389 lived up to all my expectations on the road. The 550 pulled like a locomotive. Once in top gear, it never needed a downshift unless traffic or speed limits dictated. The 15-liter Cummins pulls down to 1,100 rpm with the throttle to the floor and lower rpms – if you back out of it – without a protest. It has great throttle response that makes it feel even bigger than an 1,850-pounds-feet engine, and it has no idiosyncrasies to spoil the experience of driving it.
As noted earlier, I was not able to get average fuel economies from the dash display, but instantaneous cruise readings were often in the 7-mpg range. However, at the end of the test, refueling showed overall only 5.3 mpg for the trip, which I considered a bit of a disappointment. But considering the very low mileage on the engine and the previous problems with it, there is likely a lot better fuel economy to be had from the 550 than what I experienced.
The first time I drove the truck, I had no problem with the throttle, but my colleague Jim Park of HighwayStar
(a sister publication in Canada) was critical of the foot-feed. We had talked in the meantime, but by the time I came back to retest, the Cummins techs had reprogrammed the electronic governor. In its new state it responded like a Cat's, which is more linear with the throttle position calling for road speed rather than engine torque. To be honest, I didn't notice much difference, but the engineers were excited about the deliverables for the Peterbilt/Cummins installation.
What I did notice was the quiet cab. If it were not for the paradigm-shifting noise abatement of the new International ProStar, this 389 would be the quietest truck by far. Full load noise levels in the 60 dB range were unheard of only a few years ago and make these modern trucks a joy to live in and drive day by day. I didn't have a riding partner, but conversation would have been easy. The noise quality makes driving easier and more satisfying and makes the Concert Class radio a joy. Operational Factors
The engine manufacturers and the OEMs have gotten pretty savvy with daily checkpoints around the truck. Peterbilt has become very refined in the systems around the chassis that allow for electrical service and troubleshooting. And you have to be impressed by the long-established clutch lubrication points on the frame rail, left side.
The databus dashes are an enormous leap forward in terms of the packaging of the wiring, the weight, and the diagnostics of the electrical system. The electronic backbone of the truck is second to no other in letting the dealer or the shop operator know what is happening with the vehicle.
But even given all this, the problem we had with the turbo control did not throw a fault code.
I have to believe that Cummins is working on this. In fact, with the variable geometry turbo (VGT) such a fundamental component in the emissions system – and the upcoming on-board-diagnostics regulations that EPA has just put out for comment (see story page 60) – VGT operation is a critical part of the emissions puzzle.
Meanwhile, a walkaround of the recent show at the American Trucking Associations convention revealed that the Volvo and Mack engines that use the same Holset turbo mount the electronics off the turbo, and also run engine coolant through the module. In Conclusion
The 389 is just a great truck. It has to be top-of-the-line cost, but the 389 is going to be one of the best driver retention tools in your toolkit.
To make it even better, carefully consider your specifications. The Flex Air should be considered. Tall gears without compromising the driving flexibility should be a major consideration. And I truly believe – even given the turbo controller glitch – that Cummins has the best handle on 2007. Why else would the other engine manufacturers use the Cummins Holset turbo?
The ISX is complex without being complicated, the 389 is superb. What a great combo.