You’ll be living with your transmission choices for a long time.
Personal preference plays a large part in the decision, but today, engine performance and fuel economy have to be considered. Before emissions controls came along, those factors weren’t that critical.
If you’re replacing any trucks more than three years old, you’ll need to take a much closer look at your transmission options than you did last time.
The relationship between engine speed and road speed cannot be over-emphasized. Each engine brand and model will be optimized to run at a certain speed, and your drivetrain specs have to reflect that. You select gear ratios so that the engine will run at x rpm at x mph at cruise, where the engine spends most of its time in a highway application. Your gearbox is right in the middle of it all.
Fleets have historically spec’d 9- and 10-speed transmissions, and for a number of reasons: they’re lighter, they cost less than multi-speed boxes (13 and 18), and they’re more driver-proof. When you’re buying hundreds of them at a time, the savings can be substantial. Operationally, the 9- and 10-speeds are adequate for most lightweight applications (80,000 lb or less).
How adequate they are depends a lot on geography, says Ed Saxman, powertrain product manager, Volvo Trucks. “A linehaul truck on flat ground will be absolutely fine with a wide-step transmission [9- or 10-speed]. Fuel economy is always lousy in any gear lower than top gear, so the sooner you get into top gear, the better. And the fastest way to top gear is with fewer shifts,” Saxman notes.
“On the other hand, the multi-speed boxes let you match engine speed more precisely to road speed,” he says. “Take California, with its 55 mph speed limit. If you’re geared to run 65 or 70 and you have to drop back to 55, you’re going to have to run at least one gear down from the top. That will really hurt your fuel economy because there’s a less efficient transmission of power through the gearbox.”
Chuck Blake, special projects manager at Detroit Diesel Corp. says the wide-step transmissions can force an undisciplined driver down into the higher torque range of the engine before making a downshift. “The temptation to split gears on a small grade to keep the speed up can be great,” he says. “You’re better off pulling that hill at a lower engine speed.”
Blake also points out that a sharp driver can use a 13- or 18-speed to some advantage by splitting gears to keep the engine closer to its optimum operating speed.
Mack is an interesting study in engine/transmission mating because it builds different “personalities” into its engines. Mack’s “Maxicruise” engines feature a distinct hump in the horsepower curve. They reach peak horsepower in the 1,400-1,600 rpm range. The “Econodyne” engines have a more traditional power curve, hitting peak horsepower in the 1,600-1,700 range.
What this means, explains Dave McKenna, Mack’s powertrain product marketing manager, is that with the Maxicruise engine, the driver will be at 90 percent of the available horsepower after making an upshift. If you upshift at 1,700 rpm with a 10-speed, by the time you get it back into gear, you’ll be at 1,200 or so, and just coming into the peak horsepower range.
“The chief cause of low power complaints is bad shift points,” he says. “If you aren’t careful in matching the power curves to the gear steps, you’ll find yourself running outside the curve. The Maxicruise engine is very forgiving, and well matched to wide-step 6-, 9-, or 10 speeds.”
Mack’s Econodyne models are better suited to multi-speed transmissions where the splitter can be used to keep the engine closer to its optimum operating speed.
Canadian weights and road configurations almost demand a multi-speed transmission. We don’t have vast stretches of level, well-groomed Interstate highway, so engines don’t run at as steady a speed. The closer you can keep the engine to its so-called sweet spot, the more efficient it will be.
Canadian weights give rise to driveability issues. More weight demands more horsepower, and that means higher engine speeds, generally. Pulling hills with heavier loads means more downshifting, so you need more gears — specifically, closer steps between the gears — to keep the engine running efficiently, especially today with engine speed being such a factor in fuel economy.
And with higher horsepower comes higher torque. That’s where the driveline people start putting limits on various components based on torque capacity. You can’t spec a 10-speed with Volvo’s high-torque D16 engine, for example. Neither Eaton nor Meritor builds a 10-speed to handle 2,250 lb ft of torque. Engines with moderate torque, say 1,650 lb ft, will mate with dozens of different transmissions in any speed range.
Given the finicky nature of today’s engines, matching those characteristics to your intended application is extremely important. Intended cruise speed, GVW, terrain, and even driver skill and discipline are factors that need to be considered in a transmission spec.
The one-size-fits-all days are behind us now, and your choice will help you or haunt you for the life of the trucks.
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