Recent fluctuations in diesel fuel pricing, and the new technologies being introduced to the diesel engine market, place increased emphasis on spec'ing the most efficient powertrain for your medium- and heavy-duty truck applications.

"Obviously, with the price of diesel fluctuating over the past year, we probably see a little more emphasis on getting the right engine, horsepower wise," says David McKenna, Mack Trucks powertrain marketing manager.

Efficiency is achieved by finding the best powerplant that matches your requirements. "First and foremost, never buy more horsepower or torque than you need," says McKenna.

Recently, however, more trucks have been spec'd with additional power. This can be attributed in part to competition for drivers. "With the driver shortage, there has been a horsepower creep," McKenna notes.

"Driver retention is always a big issue," says Matt Stevenson, manager of product strategy, Sterling Truck Corp. "A lot of times, guys like engines that are probably more powerful than they need. Drivers always like a little more power."

This can become a balancing act. "Giving the driver too much power, then taking the dump truck into deep mud or loose soil, does create its own problems in terms of tearing up driveline components or rear ends," says Stevenson.

Similarly, an under-sized engine creates productivity issues. "The last thing you want to do is get an engine that is under powered [in order] to get a lower weight," says Stevenson. "Then you just have poor driver impression of the vehicle. Maybe it is slow to get up hills, slow to start and then slow on the return trips."

Assess horsepower and torque needs

It's important to truly understand your power requirements. "If 325 hp and 1,300 or 1,400 lb.-ft. of torque will do it, don't go out and buy 425 hp with 1,660 lb.-ft. of torque. If you can match your engine, transmission and rear axle ratios, you can optimize that truck for a particular duty cycle," says McKenna.

This requires an understanding of the relationship between torque and horsepower. "Basically, torque is the ability to perform work," says McKenna. "Horsepower is the rate of speed [at which] you can do that amount of work."

Torque ratings too often take a back seat to horsepower ratings. "Most of the customers, unfortunately, still talk horsepower," says Stevenson. But torque is what actually gets the load moving. "In any application, it is best to figure out what your torque needs are, and then cruise speed to determine your horsepower needs."

The transmission choice will have a major impact on horsepower and torque requirements. "On the vocational side, we have certainly seen a growth in the powershift automated-type transmission," says McKenna. "The skill set of a lot of drivers today doesn't lend itself well to a fully manual transmission."

But there are trade-offs. "The fuel economy tends to suffer a little bit with the fully automated transmission," says McKenna. "The reason they call it a powershift is you don't have to lift your foot off the throttle for it to power up through the gears. The powershift manufacturer claims some productivity improvement, where you can get from Point A to Point B significantly faster."

And not all applications are candidates for automatic transmissions. "The automatics, for the most part, only go up to 1,650 ft.-lbs. of torque," says Stevenson. "With a large engine that has 1,850 lb.-ft. of torque, usually you can't get an automatic on a general construction rating."

However, the increased shifting precision may allow you to make more efficient use of the available torque and horsepower. "With an automatic transmission, you can usually use a smaller displacement and have it geared just right," says Stevenson. You can back off the horsepower rating a little bit because it is shifting more accurately than the average driver would. "There are fleets that will downsize to a 370-hp engine and run Allison automatic transmissions because they get as good a performance, better fuel economy and less wear and tear on the components."