Uprating of engine power and torque has benefits and pitfalls. Be sure to check its effect on other components.

"There's good news and bad news about uprating engine horsepower and torque," Jim Neal, executive director-vehicle systems for Cummins Engine, told members of The Maintenance Council (TMC) at its summer meeting in Tucson, Ariz.

"First, the good news: It's pretty easy to do," said Neal. "Just bring out your laptop, plug 'er in, and type in a few instructions. Pretty easy! And you get a lot of benefits from uprating to meet your individual needs. You can test various powers and torques for their impact on your operations. Engine marketing people often use it [ease of uprating] as a sales incentive. And at trade-in time you can step up your power for a higher resale value.

"Now, the bad news: It's really easy to uprate your engine. Just a few keystrokes and there you are."

As an example, Neal told of how a few keystrokes could take a 330-hp. engine with 1,350 lb.-ft. of torque and bring it up to 425 hp. and, probably more important, to 1,550 lb.-ft. of torque, getting past the 1,450 lb.-ft. point that is often very critical for drivelines.

Neal cautioned upraters to pay attention to the effects such changes might have on the clutch, transmission, and driveshaft, and be sure that the vehicle's cooling-system capacity is not exceeded.

Not all electronic engines are the same, Neal explained. "While some engines can be uprated from the bottom of the power range to the very top just by reprogramming, others may require a change of pistons, camshaft, and injectors to uprate to the upper end of the power range."

Uprating's effects on the driveline were examined by Gene Stein, sales and applications engineering manager for Spicer Clutch Div. of Dana Corp., who advised that clutches and driveshafts be spec'd to the highest torque capability of the engine, even though this might not be the most efficient approach from an engineering standpoint. He cautioned against false economy: trying to save dollars by underspec'ing components.

That applies to transmissions, too. "Every transmission supplier specifies a torque range for its product," said Tom Riley, chief engineer, Eaton Transmissions, "and exceeding the range invites disappointment.

"Historically, engine output has not been a variable, and we (transmission builders) have designed our products on that basis. If you increase engine output after you've spec'd the transmission, you're liable to overload the transmission."

Many transmission models are designed for a specific engine, and an increase of only 100 lb.-ft. in engine torque can have a big effect on transmission life as a result of, among other things, reduced bearing capacity and gear-tooth bending resistance, said Riley. That 100 lb.-ft. increase can cut an expected transmission life of, say, 750,000 miles to about 465,000 miles, he added.

An uprating in horsepower, Riley warned, allows an engine to operate at lower speeds, but this increases the risk of destructive driveline resonance. Also, recommended transmission temperatures could be exceeded.

In addressing the effects of uprating on drive axles, Robert "Rosey" Rosenthal, national technical manager, Rockwell Automotive, advised that input torque is the critical factor, and that you must look at two torque levels: the peak gross torque of the engine -- 1,550 lb.-ft. or whatever -- and the peak input torque seen by the pinion, or input shaft.

Most axle manufacturers, said Rosenthal, rate these two torques as having the greatest influence on the drive axle when uprating an engine, followed by the brake horsepower of the engine, the axle ratio vs. gross combined weight, and the static loaded radius of the tire.

"Below 80,000 lb. GCW, most drive axles are pretty tolerant of engine uprating -- within reason," Rosenthal said, "but when you get above 80,000 lb., it's important to discuss those changes with your axle supplier."

As did all the speakers preceding him, Rosenthal urged his audience to "plan for uprate with your initial-purchase specs."