Engines do not work in isolation. Consider all the possible use cases and the rest of the driveline.

TORONTO, Ont. — How fast do you want to go, how much weight are you planning to pull, and how much are you willing to spend to accomplish your goals? In a roundabout way, those are the three questions you need to ask before deciding on your engine spec’.

Realistically there’s much more to consider, but consider this: If you’re planning to pull what might be called a typical American load (80,000 lb./36,000 kg) on fairly flat ground, an engine producing 350 hp and 1,450 lb-ft of torque is all you really need. That would put you into 11-litre territory, with ratings of 325-425 hp and 1,250-1,550 lb-ft.

On the other hand, if you’re pulling eight-axle B-trains through the Rockies, you’ll be much happier with something in the 15-litre range producing 550-605 hp and 1,850-2,050 lb-ft. That said, plenty of fleets happily pull B-trains over less demanding terrain with 13-litre engines dialed up to 500 hp/1,750-1,850 lb-ft.

There’s really no pat answer to the question, ‘How much engine do I need’? Your engine spec’ depends on what you plan to do with the truck.

Dealer sales reps are usually your best resource when it comes to spec’ing powertrains.


Horsepower is what keeps heavy trucks rolling at highway speed. You don’t necessarily need more of it, you just need to spec the driveline so the horsepower is available when it’s needed.

“The first question we usually ask our customers is, are you looking for best fuel economy, best performance, or a balance of both?” says Mike Stricker, director of component sales, Daimler Trucks North America. “We also ask if they are satisfied with the spec’ of the truck they currently own or drive.” Proprietary Detroit Spec’ Manager software is then used to simulate customer routes and vehicle configurations.

Such spec’ing programs can also reference the experiences of thousands of in-service trucks along with established manufacturer-specific performance metrics like startability and gradeability. “These metrics usually correspond with the best fuel efficiency,” Stricker says.

Torque and horsepower are obviously prime considerations, but you also need to think about the physical size of the engine. They are available today in 11-, 12-, 13-, 15- and 16-liter displacements. Engine rating charts show there’s a lot of crossover in the 400-500hp/1,650-1,850 lb-ft engines available across several displacement ranges.

“The last decade has seen significant migration of on-highway engines from 15-liter to 13-liter with so much power being packaged in a smaller displacement,” says Carl Hergart, director of powertrain and advanced engineering at the Paccar technical center. “A lot of this has to do with the fact that a majority of applications, especially over-the-road, typically are in the range of 400-510 hp and 1,550-1,850 lb-ft. Smaller-displacement engines offer the added benefit of reduced weight.”

Of course, displacement does not affect fuel economy in a like-for-like situation. A 450/1850 13-liter engine will get you the same fuel mileage as a similarly rated 15-liter engine. So, does displacement make a difference?

“There can be distinct differences in displacements that need to be considered,” says RaNae Isaak, powertrain TCO and consultancy leader at Cummins. “A 450/1850 15-liter engine will operate differently from a similar 12-liter engine. As a couple examples, engine life can vary depending on the way the engine is used, and engine brake performance can be different.”

In most cases, the larger displacement engine will be heavier than its smaller cousin. From a budgeting standpoint, the smaller-displacement engines can be less expensive upfront. In many cases, certain electronic features have blurred the distinction in displacement, such as the multi-torque options.

“Multi-torque ratings are a way to get the best of both worlds — torque when you need it, and improved fuel economy when you don’t,” says Isaak. “Torque management strategies allow a user to be better optimized automatically without any change in hardware.”

truck powertrains

Programmable engine parameters can make 400-hp engines perform like 500-hp engines. Ask your dealer for help in setting up your engine.

It depends on the gears

The final decision on engine ratings depends on how the rest of the driveline will be spec’d. The driveline spec’ will determine the engine speed, which affects torque availability and ultimately fuel.

Volvo Trucks’ powertrain marketing manager, John Moore, has an interesting way of looking at it. He says the data collected from in-service customer trucks used to build predictive trip modeling now allows customers to factor trip times into their decision, not just fuel economy and performance.

“Most drivers would tell you there’s a big performance difference between a 450- and 500-hp engine,” Moore says. “In fact, it can be as little as two to three miles per hour on a 6% grade. You can hardly tell the difference, yet you’ll probably have to pay more for a beefier driveline for the 500-hp engine, and maybe even a higher price for the warranty coverage for that driveline. Is the two to three miles per hour really worth all that?”

Moore says that when Volvo runs trip time calculations comparing different drivelines – the higher and lower numeric axle ratios, with higher and lower engine speeds and horsepower — the differences in trip time for specific legs can be mere minutes. But the fuel savings can be significant.

Most dealers can now do those sorts of calculations based on modeling and actual customer data. You tell the dealer sales rep what you plan to do with the truck, and they can come up with options. This can really help make decisions based on data, not seat-of-the-pants impressions or stories from other drivers.

However, buyers need to be upfront about their intent. Don’t tell the sales rep you plan to operate at 105 km/h all the time if you’ll be driving on a lot of two-lane roads at 90 km/h.

Programmable parameters

“There’s a lot more opportunity for improving fuel economy and performance with programming,” says Navistar’s heavy-duty product segment marketing manager, Jim Nachtman. “There are parameters there that can be modified beyond the standard factory settings, for example, setting the cruise control to allow the truck to roll out slightly above the engine-brake-on setting allows you to conserve momentum and save fuel. That’s an adjustable parameter.”

There are dozens, if not hundreds, of engine parameters that can be adjusted to tweak the engine spec’. A knowledgeable sales rep can help set the engine up for optimum performance and fuel economy.

Today’s engine spec’ can hardly be considered in isolation. Powertrains are now fully integrated with engines and transmissions working together for optimum fuel efficiency or performance. Many of the optional parameters, such as predictive cruise control or adaptive cruise control, are available through all manufacturers, but each one does it a bit differently. You may prefer one over another.

“Many new buyers don’t realize all the options available to them,” says Mack’s director of product strategy, Roy Horton. “The dealer will ask the customer a lot of questions in the initial stages of the spec’ing process to determine what they are looking for. When the priorities emerge, they can start drilling down to specific options and parameters.”

To get the best performance from the truck, the whole driveline has to be considered, from the engine to the rear axle ratios. The choice of driveline and rear axle ratio is part of the powertrain specification along with the engine and transmission. All components of the powertrain have to be carefully selected in order to get the optimum performance and efficiency.

If you’re new at this, take full advantage of what the dealer can offer in terms of spec’ing suggestions. They are the experts.



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