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Power Trip

By on March 1, 2012
RoadKing Mag

As soon as I heard the new MaxxForce 15 engine was available, I contacted Navistar’s communications manager, Steve Schrier, to arrange a test drive of International’s ProStar. ProStar is an “old” truck, introduced at the Mid America trucking show in 2006. I test drove an early pilot-built ProStar. It was an impressive truck then, and it still is today. But I wasn’t interested in the truck, only in its new motor. That (I thought) was the big news. The 2012 Prostar is not a new platform, but it has been tweaked and refined considerably since its introduction.

Designed for efficiency

When it came out, the ProStar was wind-tunnel tested to be one of the most aerodynamic trucks on the road. But its claims to aerodynamic supremacy were soon challenged by its chief competitor, Freightliner. Their Cascadia was also developed in a wind tunnel; their own, specifically engineered for trucks. Claims and counter claims went back and forth in advertising and at trade shows.

According to a back-channel report, I learned that another truck maker decided to benchmark its truck against the two claimed leaders. It found the two virtually identical in aerodynamic efficiency, but its own trucks fell a few percentage points short. They, of course, tuned their design and all three are now among the top aerodynamic trucks available.

Aerodynamics can be described as keeping air flowing smoothly, without creating drag or turbulence. International achieves this by design and by using fairings, and also by sealing as many gaps as possible. Space between bumpers and body will create turbulence, and the resulting drag affects fuel economy. Although each tweak is small, taken collectively they could add up to a tenth or two mpg.

Bumper design was also a result of International’s research into driver and fleet needs, wants and complaints. For example, when doing pre-trip inspections or performing underhood maintenance, drivers and technicians complained of climbing over bumpers to get the job done. In bad weather, especially ice and snow, this could be challenging and dangerous. The ProStar’s bumper pivots to open up the space in front of the steer tire for easier and safer access. The bumper also has seals that prevent turbulence in narrow gaps.

Inside the cab, comfort and safety features abound. The automobile-like steering wheel has a large integrated hub and spoke design that incorporates often-used controls. No need to reach right for the switch panel to set and resume cruise control, work the radio or flash lights to signal other trucks. The switches are right under your thumbs. The dashboard has an elegant look, with black matte-finish panels set into wood grain trim. Gauges have white faces and chrome surround rings with bright red indicators.

One impressive safety features comes from the ProStar’s aerodynamic design. Even with the 125-inch BBC (bumper-to-back-of-cab) dimension needed to house the 15-liter engine, the hood slopes enough to see the top of a 3-ft 6-in. child’s head standing less than two feet in front of the bumper. The ground can be seen just 12-ft 1-in. ahead. That’s outstanding visibility. Sight lines to the sides are equally good.

The Diamond Logic information and communications system includes diagnostic tools and a feature that drivers will love. With CSA assigning points to drivers for missing or burned-out lamps, Diamond Logic has a light check function that identifies problems with any of the headlights, stop/turn/tail lamps or clearance/marker lights.

The sleeper has a traditional layout, but a number of new features improve its function and livability. Up to 150 percent more storage room is available, depending on how the cab is optioned. If you don’t need the storage, an optional slide-out cooler replaces the cabinet behind the passenger seat. It’s convenient for the driver. For those who want more storage, overhead airline-style cabinets are optional in some sleepers. For those who want more walk-around space in their sleeper, there’s even a swing down Murphy bed option.

One option I did not get a chance to try was the MaxxPower HVAC system. It’s battery powered, and International claims it has the longest running time and quickest recharge available.

Gone are the days of truckers having to shout to be heard in a truck. My “navigator” for the trip was Shane Spencer, director of advanced programs, reliability and quality. He came along to answer my questions and to get feedback about other trucks I’ve driven. Schrier sat on the bunk. Even on some fairly rough roads, we were able to converse at normal levels.

To the Maxx

Of course, one of the things we spoke about was the new MaxxForce 15 engine, the reason I wanted to drive the truck. Like all International engines, it uses Advanced EGR to achieve EPA 2010 emissions limits, rather than selective catalytic reduction (SCR). If that was the only difference between the MaxxForce and other 15-liter engines, there would be no point in testing it. But there are differences in the way the truck drives.

To fully appreciate the differences and their effect on the driver, let’s go back to when most trucks had Cummins 290 and Detroit Diesel 318 (hp) engines, and owner-operators thought they were in heaven with a “four-and-a-quarter” (425 hp) Caterpillar. Engines then were governed at 2,100 rpm, and most delivered peak torque at near 1,800 rpm. That left a usable rpm range of about 300 rpm, or 14 percent of maximum engine rpm. Above that, the governor limited operations. Below it, the engine stalled for lack of power.

Then engines were improved with low speed power. Maximum torque-to-peak-power range was about one-third, from 1,200 to 1,800 rpm. Trucks were much more drivable. Drivers didn’t need to shift as much and didn’t lose torque.

The new MaxxForce 15 extends the drivability range even farther. Torque at clutch engagement speed (800 rpm) is a whopping 1,150 lb-ft. Peak torque comes on at only 1,000 rpm and stays at peak through 1,500 rpm. Then it falls off gradually.

My test truck had just 450 hp but it drove like a 500 because of its 1750 lb-ft or torque. Horsepower increases to 1,300 rpm and stays flat through 1,800. Then it drops gradually to the engine’s governed speed at 2,100 rpm. The truck is drivable all the way down to 800 rpm as I found out quite unintentionally. I missed a shift entering I-55. The truck slowed to about 800 before I managed to get it back in gear. I guess I’ve been driving too many trucks with automated transmissions and I lost the few skills I had with manuals.

Once I found a gear, I was at just over 800 rpm. The engine pulled smoothly all the way back up to about 1,600, my next progressive shift point. When I checked the math, I found the engine is smooth across 62 percent of its governed rpm. It makes the 15-liter engine very flexible, forgiving and easy to drive.

There is still a dispute raging about the best way to achieve EPA’s 2010 standards for NOx. That is for the engineers and executives to decide. My evaluation is from the driver’s viewpoint. All we need worry about is if the engine has been certified for sale, which the MaxxForce 15 has. From my perspective, this is a very easy-to-drive truck that cruises at an economical 1,225 rpm at 60 mph and 1,300 at 65 mph, at the middle of the engine’s sweet spot, for economical operations.


Wheelbase: 226-in.
BBC: 125-in.
Sleeper: 56-in. low roof
Engine: MaxxForce 15 450 hp 1750 lb-ft.
Transmission: Eaton Fuller 13 speed overdrive
Drive axle: 3.36:1
GCW as tested: 76,000 lbs.
Safety Systems: ABS, Automatic Traction Control, Electronic Stability Program

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