Power unit spec’ing

by Lou Smyrlis

The easiest way to control your maintenance costs is to spec the right product to start with. Sage advice but spec’ing a truck right is a science with many intricacies. To help truck specifiers understand some of the crucial details of the power unit spec’ing process, the 39th annual Canadian Fleet Maintenance Seminar picked the brains of some of the industry’s leading experts. Following is a compilation of what they had to say about several key components:


Vocational or over-the-road – you can’t get started on your engine spec’ing until you’ve determined which of those two categories the truck in question falls into. The horsepower and powertrain requirements will be decided by application as will be the model of the truck and warranty classification, advises Ken Clarke, Engine Sales Manager for International Truck and Engine Corp. Canada.

If the truck is an on-highway spec, a key calculation will revolve around demand horsepower. Clarke recommends spec’ing reserve horsepower to reduce shifting.

“Driver efficiency is improved if you build your truck with horsepower to handle grades and headwinds,” he notes.

For example, it takes 258 hp to move an 80,000 lb load down a level road, with no wind, a 0.5% grade factor and a road speed of 62 mph (100 kph). To account for a 5mph headwind, another 16 hp will need to be added to the demand hp; a 10mph headwind calls for adding 36 hp to the demand hp. And to maintain 62 mph on a 1% grade you’ll have to add another 67 hp, which brings the total horsepower up to 361 hp.

The recommended gradeability (ability to maintain engine rpm) minimum for on-highway applications is 0.5%. “This is one area where drivers are very sensitive. If you have poor gradeability you are going to have a problem truck because the driver is going to keep complaining the truck has no power,” cautioned Carl Woodsma – Territory Manager, Eaton Truck Components.

The minimum startability standard is 16%. For vocational applications, 25% startability is generally required or even 30% in the case of mixer applications. (Startability measures the feet of rise per 100 ft. So a 25% startability means that truck fully loaded on a 25% grade will be able to launch itself.)

There are also driveline losses to consider. An engine rated at 430 hp may deliver 430 hp at the flywheel with some small parasitic losses such as the fan and alternators but typically up to 15% of that power is lost at the driveline, between the transmission and the rear axle, depending on the configuration.

“This leaves approximately 362 hp to do the work at the wheels, so if you had calculated that you needed 361 hp (as shown in the example above) 430 hp is what you actually must have to take into account the parasitic losses through the driveline,” Clarke says.

Properly gearing an engine for the most fuel efficient cruise rpm is something that requires consultation with the engine manufacturer. “It’s not quite as simple as we all think it is,” Clarke says. “There are different cruise rpm recommendations for different engine models from the same engine manufacturer. I recommend you check this very carefully if you want to take advantage of the maximum fuel economy available to you.”

For example, a Cummins ISM running at 65 mph should be geared at 1500 rpm for best fuel economy but the recommended cruise for a Cummins ISX engine at that speed would be 1,400-1,450 rpm.

Other things to consider in spec’ing for fuel economy include the fan clutch and air compressor. Clarke advised spec’ing on-off fan clutches. They run on average less than 2% of the operating time while viscous fan clutches draw 4-6 hp at all times and increase fuel consumption by at least 1.5% and by as much as 5%. He also recommended spec’ing 12-13CFM air compressors compared to 15-17CFM to improve fuel consumption by 0.5%.

A few more items included on Clarke’s fuel-saving specs list: Roof fairings can improve fuel savings by up to 15%; a 25″ trailer gap versus a 35-65″ trailer gap can boost fuel economy 5%; 15-inch cab extenders improve fuel economy 4%; a front bumper air dam can generate a 3% savings and tractor sideskirts another 3% savings.

Clarke was also more optimistic than some about how the engines meeting the pull-ahead October 2002 emissions standards will fare on fuel economy.

“Many are expecting fuel economy reductions on engines equipped with new emissions strategies, however, in the last few months I’ve noticed that engine manufacturers are claiming that fuel economy after October will pretty well equal fuel economy today,” he said. “There is a great deal of research that has been done. For example, the new turbo technologies will compensate for any performance and fuel efficiency drop off.”


Woodsma spoke favorably of spec’ing soft rate dampeners over standard rate dampeners. “If you do have to replace a clutch we strongly recommend that you do stick with the OEM specified product due to the fact that if the engine and transmission did come with a soft rate dampener and you put in a standard rate dampener you are going to have some problems and it will probably start off with the driver saying he is getting transmission noise,” he said.


It’s the foundation of any Class 8 vehicle and a frame that is too light can cost an owner increased down time for repairs, body failure or component failures while a frame that is too heavy will result in reduced payload capacity, advises Randy Fleming, Manager Vocational Sales for Volvo Trucks Canada. What you should be shooting for is “structural integrity”, a term used to describe a structure that behaves as intended under all predictable operating conditions. As applied to a truck frame, structural integrity will allow for some flexing and twisting with the pounding of loads, through dips and crowns in the road.

“Section modulus” is used as a basis for strength comparison. It is a mathematically derived index of the capacity of a member to resist any bending force. Shape is one of the key factors to consider when determining the bend-resistance of a frame rail, Fleming points out.

This is the formula used for calculating the section modulus of a typical “C” channel frame rail:

WD3 – wd3


By increasing dimension “D” one can significantly increase the section modulus of a frame rail given that the height of a frame rail is cubed in the context of this formula. Frame thickness and width have a less significant effect on the section modulus of a frame rail.

Fleming pointed to two examples to show that there is more than one way to provide virtually identical frame strengths using differing configurations: A single channel frame rail section with a rail that’s 11.81″ deep, 4.13″ wide and .44″ thick and weighs in at 27.7 pounds per foot has a section modulus of approximately 28.01 inches cubed. A double channel frame design with a frame section that measures 10.75″ deep, 3.10″ wide and .38″ thick and is coupled with a .25″ inner channel frame reinforcement, weighs in at about 32.4 pounds per foot with a section modulus of approximately 27.87 inches cubed.

“Both these frame configurations have merit. The single channel option provides reduced weight and the elimination of corrosion between a main rail and inner liner combination and may have merit in a snowplow application. On the other hand, the double channel frame could provide increased torsional rigidity that may be more appropriate for an application that has a high percentage of off-road operation, such as an excavator dump,” Fleming said.

He added that shape isn’t the only factor in determining a frame rail’s resistance to bending. The material itself plays a significant role in determining frame strength. Metal strength is usually tested to determine the “yield strength”, which is a measure of the maximum load a material can withstand before permanently deforming.

“Generally speaking most Class 8 frame rails are formed using heat-treated alloy steel with a yield strength of 110,000 PSI, however, frame steels used may vary, with yield strengths measuring high
er or lower than the typical 110,000 PSI,” Fleming explained. “This is an important point because in order to make an accurate index of strength, one needs to combine both section modulus and yield strength in order to create a measurement of frame strength known as ‘resistance bending moment’ or ‘RBM’. RBM is simply a product of section modulus multiplied by yield strength.”

Fleming said that although it’s essentially the OEM’s responsibility to provide a structurally sound frame, it also requires the purchaser of the vehicle being forthright in his conveyance of the intended vehicle application.

“This information needs to be provided to your sales representative who in turn will inform the appropriate OEM engineering personnel. Certainly, misapplied vehicles run the risk of performing poorly or having warranty claims denied,” he cautioned.


Generally, the cab selections available to a purchaser are limited to each OEM’s individual cab design.

So what does this mean to the potential purchaser of a Class 8 vehicle at least so far as the cab is concerned?

“I think the purchaser would find it beneficial to create a list of those attributes that would be of significant importance to their vehicles’ respective operation and application. This list can then be used as a basis for rating each OEM’s perspective cab offerings,” Fleming advised.

Areas worth considering include:

How the cab is actually assembled: welded or rivetted, hand or robotically welded, in-house assembly or outsourced?

To what tolerances is the cab built?

How are cabs inspected from a quality of assembly perspective?

From a cab design standpoint, Fleming’s list of items worth considering includes:

The materials used in the production of the cab and their relative merits.

How structurally sound a cab is, from a safety perspective.

Cab egress and entry may be a concern as it relates to a driver’s ability to enter and exit the vehicle without risking injury.

Are the door openings wide enough?

Are grab handles positioned to allow for a controlled cab entry or exit?

Are the steps visible to the driver from his seated position?

Is the visibility from the driver’s side of the cab consistent with the safe operation of the vehicle in its intended application?

The cab paint process is an area which should certainly warrant some investigation given that invariably the process and materials used to paint cabs can affect not only the appearance of the vehicle but also the useful life of the cab itself. Consider:

How is the cab prepped, cleaned, primed, painted?

Type of primer?

Type of paint? How is the primer or paint applied? Human hand or robotics?

What types of factory inspections are performed to ensure a high quality paint finish? Is gloss measured or for that matter paint thickness? And to what standards are these measurements made?

Moving to the interior of the cab, Fleming would give consideration to:

How well a cab is sealed, as this will invariably have an impact on the noise level a driver will experience when operating the vehicle for many hours at a time.

“Developing a vehicle specification is not an easy task but with the right mix of experience, and open mindedness, along with a healthy dose of research, one can most certainly create an optimum Class 8 truck specification for most applications,” Fleming said.

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