Cat C13
As engine makers transition to lower emissions, they are adopting varying approaches on horsepower and displacement. One trend, however, is better torque curves that often help produce more peak torque. This means that choosing the correct axle ratio for the tire size, transmission top gear ratio and engine torque curve is more important than ever. “Simply put, torque does all the work, and horsepower determines at what speed that amount of work will be done,” says David McKenna, Mack’s powertrain marketing manager.
“Some people with long experience in the industry still have not fully embraced ‘gear fast, run slow,’ ” says Mike Powers, product development manager at Caterpillar. “Because of the fuel economy penalty related to meeting emission standards, the sweet spot is smaller than it was.”
Powers says nothing has changed for 2007: Caterpillar owners should follow the same ACERT gearing recommendations that have been in place since 2004. In that year, for example, Cat’s 475 hp rating went from 1,650 lb.-ft. of torque to 1,850, typical of the change that “allows a lower rpm at cruise,” he says. Match carefully, and be prepared to use a faster axle ratio than what you’re used to with many engines. C15 engines with less than 1,750 lb.-ft. of torque should cruise at 65 mph at 1,400 rpm, while those with 1,750 lb.-ft. and above should cruise at 65 mph at 1,325 rpm, Powers advises.
Owners of Cummins ISX engines, meanwhile, should cruise at 1,450 rpm at 65 mph, whether it’s a pre-2007 or a 2007 engine, says Lou Wenzler, on-highway market communications director at Cummins. “The 2007 ISX has a family of ratings that are similar, if not identical, to pre-’07 ratings. All ratings provide a robust power curve, which enables great lugback performance. The ISX ratings are fully compatible with a full range of automated and fully automated transmissions.”
In the Volvo D13, the sweet spot has moved downward, says Ed Saxman, Volvo’s product manager for drivetrains. On the D13, peak horsepower has increased from 465 to 485 while torque remains at 1,650 lb.-ft. But the sweet spot is now 1,300 to 1,500 rpm, down from 1,400 to 1,600. “You get the best fuel economy by staying within that rpm band,” Saxman says. “The torque is so good, a 1,450 rpm cruise will feel good even at 69 mph.” Gear accordingly.
The D13 is a 12.8-liter engine rather than a 12.1-liter machine with the same bore but a slightly longer stroke, intended to allow the engine to meet the 2007 emissions standards with “the same durability, and staying within the same design parameters, such as peak cylinder pressure, by lowering the density of horsepower a bit,” Saxman says.
