Racing cars in the top echelons of motorsport can accelerate to 100 km/h in little more than a second or two and reach incredible top speeds three times that fast.
They have tons of horsepower, but do you think they could pull a load of firewood out of the bush? Or battle a prairie headwind? No way.
They can barely get themselves moving from a dead stop. That’s why in a ‘CART’
or ‘NASCAR’ race you’ll sometimes see Michael Andretti or Mark Martin being
helped away from a pit stop by their crews – four guys pushing for all they’re
worth while their million-dollar driver punches the revs skyward and slips the clutch to get a lousy 1500 lb in motion.
Compare that to yourself in a truck. The first difference is that you don’t make a
gazillion bucks a year. At least not since you got into trucking. The second is that
you don’t have to feed the engine any fuel at all to get rolling in most cases, and
you’ve only got half the horsepower of that racing car.
What gives? Torque, that’s what. And of course gearing. A racing engine might
produce 900 hp but only 100 lb ft of torque down low on the tach, compared to the
1400-plus lb ft that even a modest diesel churns out at 1200 rpm or so.
And what’s torque? It’s pure twisting force – not a measure of how fast the
engine can do work, which is horsepower – but just the bare potential for work
arising out of that twisting motion. As the torque figure rises, so does the amount
of firewood you could haul with Andretti’s race car.
And the more horsepower you’ve got, the faster you could haul that wood or climb
a grade. It’s a calculated value, directly tied to torque, that measures the rate at which the work gets done. Oddly enough, it has its origins in Scotland.
Nearly two centuries ago, Scottish inventor James Watt decided that the
industrializing world needed a way to measure the output of his steam engine. So he measured how much work a good horse could do, and found it could lift 330 lb 100 ft in one minute. Thus the term, “one horsepower”.
How much torque is involved there? That’s expressed as 33,000 lb ft. We get that
by multiplying 330 lb (the amount the good horse can move in a minute) by 100 ft
(the distance he can move it). Put another way, one horsepower is the ability to do 33,000 lb ft of work in one minute.
Getting a little more technical, a Cummins document says “the torque output of an engine is a measure of the amount of turning force it produces which will move a
load. Torque is a force applied in a circular path and measured in pound feet. One example of torque would be to loosen a screw-type lid from a tightly sealed jar.”
Torque is the amount of force multiplied by the distance at which the force is applied. For example, a torque wrench could be one foot, two feet or four feet long. The bolt head is at the end of the wrench and the distance for determining the torque is measured from the centerline of the bolt head to the point at which the force – or load – is applied. If you apply a load of 50 lb at a distance, or lever arm, of 1 ft, the equation would be: torque = 50 lb x 1 ft = 50 lb ft. Make that a load of 25 lb at a lever arm of 2 ft, and you’d have the same result: 25 lb x 2 ft = 50 lb ft of torque.
In an engine, torque is generated by the pressure load of the expanding gases on the top of the piston times the stroke, meaning how far the piston moves.
Two basic principles apply: 1) torque is stronger at the lower end of an engine’s
operating range, while horsepower is higher at the upper end; and 2) a bigger
displacement engine will produce more power than a smaller one, simply because there’s more area for combustion to force down those pistons.
But, hey, just remember that more torque is a good thing.
We’ve certainly got more torque these days. When Caterpillar and Cummins
announced 600-horse engines two years ago, the drivetrain manufacturers cringed, because with that horsepower came gobs of torque. Both the Cat and Cummins 600 diesels spin 2050 lb ft of torque off the flywheel, and that’s an awful lot of ‘force’ being transmitted through the clutch and on back to the axles. But even in lesser engines, the average torque and horsepower being spec’d is on a rapid rise.
Jeff Romig, Eaton’s mechanical transmission planning manager, says over 90% of his company’s transmission shipments these days are for ratings of 1450 lb ft and over.
Charlie Allen, director of sales and marketing at ZF Meritor, the new transmission
joint venture, has been looking at this phenomenon too. He charted the
transmissions sold over the last 10 years or so, and found that in general the
highest torque rating sold in a given year had become the most popular rating four
or five years later.
The reality is that most people buy engines on the basis of horsepower and
displacement, not torque; they think of transmissions in terms of number of gears
and maybe steps between gears, not torque capacities. But the many engineers
who contributed their thoughts to this article also agreed, to a man, that torque is not well understood and that people spec’ing trucks would do well to give it much
Driveability is the issue, but it’s not just the amount of torque that determines how
well an engine responds to the driver. It’s a matter of the complex relationship
between torque and horsepower and gear ratios. There are spec’ing questions like what happens after a given downshift? Where do you leave yourself in terms
of the engine’s torque output?
How about what Volvo engineer Ed Saxman calls “torque response” – meaning how quickly can you get back to maximum power after a shift?
All of this demands that truck buyers study engine power and torque curves
carefully. For now, let’s concentrate on driveline issues.
The Cost of Torque
Obviously, it’s a whole lot easier to drive a 460-horse engine with 1650 lb ft of
torque than a 350 with 1350 lb ft, but it’s also a whole lot easier to pay for the
lesser diesel. Beyond the incremental cost of the bigger engine, you also need a more capable clutch, transmission, driveshaft, and rear axle. It can easily mount up to several thousand dollars.
Can you get more torque without writing a big cheque? Yes, with qualifications.
For several years now Caterpillar, Cummins, and Detroit Diesel have offered
engines with dual ratings – the Multi-Torque, ESP, and Cruise Power models
respectively. Each of them, in broadly similar fashion, gives you two power curves
and will switch to the higher one automatically as the demand arises – in certain
gears, at certain rpm, and perhaps only in cruise-control mode, depending on the maker.
Cat Multi-Torque diesels, for example, do so by calculating the ratio of vehicle speed to engine speed. When that ratio falls below a certain level, implying a greater load on the engine, you might find yourself with an extra 100 lb ft of torque and maybe 50 hp. Enough to beat that hill, maybe without dropping a gear. You still need to uprate the driveline, but there are compensations.
“You have to be careful how you spec the truck out with Multi-Torque ratings,” says
Cat engineer Mike Whitledge. “If you don’t spec your driveline to the higher torque rating, then you’re leaving yourself open to a breakdown on that component.
That’s a no-brainer.
“Spec the truck properly, for the maximum torque, and then everything’s covered. Over the long run, the difference will be paid for by the increased fuel economy and driving ease.”
A new approach to the idea of dual ratings was introduced last year. Called ‘Smart Torque’, it’s the patented brainchild of Cummins engineer Steve Bellinger.
The basic ‘ST’ idea is to tailor the engine’s torque output to the specific input limit of each gear ratio in the transmission – for example, to take a given engine and transmission combination and add 100 or 200 lb ft of torque in the top two gears only, where drivers typically spend 95% or more of their time. It’s done by controlling fuelling rates gear by gear.
You would buy a 460-horse engine that normally delivers 1650 lb ft of torque, for
example, but in the ST version it would actually give you 1850 lb ft in the top two gears. You’d also get extra horsepower – a maximum of 485 in those upper gears, as opposed to 475 in the lower cogs.
The extra torque comes on automatically, the object being threefold: to improve
grade-climbing ability; to minimize shifting; and to reduce average rpm per mile.
There’s also a financial advantage because in this case you can get the extra
torque without buying a ‘bigger’ engine (there’s a very small upcharge for an ‘ST’) and a heavier drivetrain – you buy the drivetrain that matches the lesser of your engine’s two torque ratings. You’d buy a much lighter and much less expensive transmission and/or axle. Only the clutch must be spec’d to match the engine’s
maximum torque output.
How can you add torque without beefing up U-joints and the like? Well, Bellinger
says that additional torque in the top two gears is pretty benign. It’s at startup and in the lower gears where torque can do driveline damage because it’s multiplied many times over by the transmission (see ‘The Challenge of Direct Drive’,
highwaySTAR, December 1999, p. 30).
ZF Meritor’s Charlie Allen agrees, explaining it this way: “You have to get the truck moving, so you need a lot of torque and a transmission with a series of gears. For instance, let’s pick a direct-drive transmission with a reduction of 15.02:1 in first gear. You would take the peak-torque rating of the engine and multiply it by 15.02 to get the output torque in that low gear.
“You can see some pretty high torque values at the output shaft. If you took 15.02
times, say, 1550 lb ft, you’d see 23,281 lb ft of torque in startup mode. That’s what
the driveline and axle see as you try to get the vehicle moving. When you get into
high gear, then the driveline torque becomes the engine’s peak torque because the transmission ratio in use is 1:1.
“So we have to design the system for good component life in the worst set of
conditions it could conceivably see,” Allen continues. “With most of the
components that’s going to be first gear.”
Dana’s axle product manager, Steve Slesinski, is a little less comfortable with the
notion that torque in the upper gears is more or less harmless. There are still
stresses, he says. In an overdrive transmission the challenge is a little easier
anyway, because your startup ratio is going to be something like 11:1 and in high gear perhaps 0.74:1. In each case the output torque is lower than with a
In practice, you can buy ZF Meritor transmissions (Torq2 models) that are intended to handle an extra 200 lb ft in the upper gears, and Eaton Fullers that will take 100 lb ft, without going to a more expensive transmission.
The Future of Torque
Where’s it all going? Higher, that’s where. It’s inevitable that engine makers will
offer more torque in the near future dependent only on the market’s desires and the driveline makers’ willingness to design for it. Now that Eaton has introduced a
transmission – the new 18-speed RTLO-22918B, rated for a whopping 2250 lb ft,
available this month – we’ll see engines go there soon.
But given the high costs involved, on both user and manufacturer fronts, we’ll see more torque management as well. Steve Bellinger’s patent contains 24 separate
claims to technology that will move us further into the realm of protecting the
drivetrain from the various shocks and spikes and forces that conspire to break
things, allowing truck buyers more spec’ing flexibility at less cost and drivers an
For example, Dana and Meritor axle people are concerned about coast-mode loading of the drivetrain, especially the ‘reverse’ torque created by ever more powerful engine brakes. But if that engine-brake torque can be controlled or limited in certain gears or at certain engine speeds, the problem can be made to disappear.
This past year Cummins introduced a variation on the Smart Torque theme to do
exactly that and more. Remembering that torque is progressively less difficult to handle as the transmission is shifted closer to top gear, Bellinger’s patent speaks of three separate “fuelling-rate signals” – meaning an engine/transmission
combination that offers multiple torque steps, at least three of them.
That just came to market as an electronic torque management technology called
‘STx’, first on Signature 600 and 565 engines and then on the ISX 600 and 565. It
lets you spec lighter – and less expensive – drive axles with ‘big’ engines.
Meritor, for example, offers its RT-140 40,000- and RT-160 46,000-lb tandem drive axles with the STx version of the 600-horse Signature. These pairings are possible because the STx strategy selectively limits the engine’s 2050 lb ft of torque. Weight savings over the heavier axles that would be required without STx
can be several hundred pounds, and the cost saving is as much as US$3000 at
data book prices.
While the basic ST technology provides extra torque in the top two gears only, this
new STx version essentially does the reverse. Instead of adding torque, it limits
torque output in the lower gears and in retarding mode where an axle could be
In the STx Signature 600, torque output is limited to 1650 in the first three gears, then 1850, and by the time you shift into high range you’ve got all 2050 lb ft. It’s similar in the Signature 565 and the ISX 565 and 600. As for controlling ‘coast mode’ torque, in STx engines with the powerful ‘Intebrake’ engine brake, drivers get the brake’s full 600 retarding horsepower in the upper gears, then progressively less in the lower gears.
It’s all about optimizing the complete engine and drivetrain package, making the most of the componentry in terms of initial cost, driveability, durability, and fuel economy. And it’s going to lead us into very interesting territory in the next few years. For the torque junkies among us, it means we’ll be getting much more cake – and eating it too.