Sweet ‘N’ Low

Watching the tach is key to shifting progressively. Shifts between 1,200 to 1,400 rpm are appropriate on level roads in the lowest gears, while shifts at 1,400 to 1,600 are fine at higher speeds. But on steep grades, acceleration at low speeds may mean you need governed rpm of 1,800 and up to get a clean, clash-free shift.

You can often tell an experienced driver from a novice because the old hands are comfortable with progressive shifting. Drivers who shift progressively slip through the lower gears with less noise and fuss, and much more quickly, than newbies. Progressive shifting also saves fuel, and it’s well-suited to the new low-emissions engines.

It comes down to one simple idea: The rpm that’s best for shifting depends upon the load on the engine. Maximum rpm is appropriate only under maximum load. Low loads require much less rpm.

Lower-gear ratios multiply engine torque eight times or more. That means you need to use very little throttle, or you’re going to feel harsh acceleration and create a lot of strain and roughness in the drivetrain. So, when getting through the lowest gears, touch the throttle lightly and use minimum rpm. That’s progressive shifting.

“The basic concept of progressive shifting is to save fuel by shifting at some point below peak rpm,” says Tom Freiwald, vice president of marketing at Detroit Diesel. “Running the engine up to peak rpm in every gear wastes fuel. In addition, it generates more engine noise.”

Mack’s David McKenna, a marketing manager, says progressive shifting, while efficient and economical, “is largely limited to highway and suburban use, however. Vocational chassis … demand higher rpms due to grades and startability issues and so don’t lend themselves to a progressive shift process.”

Outside of vocational applications, though, progressive shifting offers a lot. Phil Hook, Caterpillar’s senior truck engine applications specialist, says, “Shift as early as possible in the rpm rise – 1,000 to 1,200 in the lower gears and progressively taking the rpm a little higher for each upshift – but almost never going above 1,500 unless trying to make an upshift on an extreme upgrade. Just think of a set of stair steps going up as you go through the gears.”

Hook explains that fuel is saved because early upshifting “helps to reduce the frictional horsepower loss that goes with high rpm.” When climbing a long mountain grade at full throttle, the friction generated at 1,600 to 1,800 rpm becomes a small portion of the 450 hp to 550 hp it is making, and the engine works efficiently. But, when going through the gears at partial throttle, the friction generated up near governed rpm eats up a lot of fuel compared with what it takes to rotate the engine at only 1,000 to 1,200 rpm.

Progressive shifting, contrary to what some drivers might think, does not slow you down. “One purpose of progressive shifting is to get the truck up to speed a little quicker,” Hook says. “Once a person learns to shift progressively, I think the truck actually shifts easier. Since transmissions shift on a percent of engine rpm, you actually put the gears closer together when you upshift as early as possible, thereby spending less time in neutral and more time pulling the load.”

As Hook says, the number of rpms the engine must drop becomes lower as you shift earlier. With most engines, the rpm will drop so quickly at the lower shift points that you will likely need to rush just to time the shift correctly and not clash gears, and those faster shifts enable quicker acceleration.

Progressive shifting is particularly appropriate with the recent changes in engine design, Hook says. “Our line of ACERT engines responds favorably to low rpms,” he says. “With the increased displacement of the heavy-duty ACERT engines and the higher boost pressure associated with it, these engines are able to pull better and run smoother than ever before. What we are asking drivers to focus on with ACERT is upshifting as early as possible and ‘lugging’ the engine as much as the situation will let them – that is, adapting to the load and terrain.”

Even before the 2002 changes in engine technology, manufacturers began utilizing the engine control module to shift progressively. Freiwald says, “Modern engines, like the Series 60, offer electronic controls that can predetermine the progressive shift points. Older engines relied entirely on the driver to gradually build both rpms and vehicle speeds as the driver worked through the gears.” The vehicle owner can activate progressive shift points at default values or adjust those points. The engine is limited to 1,400 rpm at 12 mph and below, and 1,600 rpm between 12 and 27 mph unless you adjust those values.

Cummins’ Load Based Speed Control differs from ECM-enabled progressive shifting systems because it adapts to varying conditions, says Steve Bellinger, a Cummins technical adviser.

Of any system that always restricts governed speed to the same rpm at low vehicle speeds, Bellinger says, “For level road launch, you could accelerate just fine. But, what happens if you slide down into a dock or stop on a mountain pass with an 8 percent grade for a brake check? When you attempt to make one to two to three shifts, the vehicle speed falls so much during the torque interruption that you end up without enough torque to accelerate in the next gear.”

Load Based Speed Control works as a function of required power rather than road speed. Bellinger claims drivers appreciate that it helps them shift at just the right time on level roads when they are watching traffic, but adapts and allows plenty of rpm when grunting up a steep grade in the lowest gears. Cummins features it as standard equipment on the ISX because this engine produces gobs of torque at low rpm and gives optimum efficiency in the same range.

For on-highway drivers making their own shift decisions, progressive shifting has clear benefits for fuel economy and equipment wear. Just remember to delay your shifts at low speeds on steep grades.

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