Horsepower and Torque Limits

Red letter QWith HD pickup manufacturers locked into the horsepower and torque race, I’m guessing they’re primarily retuning the same engines and pushing ever closer to the limits of what the parts will handle. At what point will the drive for more horsepower and torque affect engine longevity? To what extent can reduced longevity be mitigated by taking it easy and not burying the go pedal in the floor?
Michael Brown

Green letter AIt’s true that the vehicle and engine manufacturers leave a little “on the table” when the first of a new generation of a specific gas or diesel engine comes out. And why not? They spend bucketloads of money on R&D bringing a new engine to market. So it’s only common business sense that they have some room left to push the power envelope over the next three to five years (usually with software modifications) before they get close to what the powertrain engineers see as the “limit” for those particular parts in that particular powerplant.

The engine/transmission development teams know exactly where those limits are because they put millions of miles on the engines in dyno rooms and computer simulations, and often push components until they break. So they know at what point they need to reevaluate parts and processes in the development of the next generation — or the development of a completely new engine design.

Ford, Cummins, VM Motori and GM have done just that for the past 20 years with the engines and transmissions we find in our trucks. As torque numbers advance beyond 1,000 lbs-ft, we’ll see new materials and technology emerging that are designed to handle demands placed on heads, engine blocks and all the rotating parts. As engines get stronger, so too does everything else, downstream to the tires. Transmissions and differentials are changing right along with the engines.

Powertrain warranties are good indicators of how much faith the manufacturers have in their products. We expect owners who are vigilant about preventive maintenance, have a lighter right foot and leave the engines stock will reap the benefits of a relatively trouble-free truck long after the powertrain warranty has expired.

Aerial view of white Sierra truck towing a Fuzion fifth-wheel
Photo: GMC

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Bruce W. SmithA respected automotive and RV journalist and longtime Trailer Life contributor, Bruce W. Smith has held numerous editorial titles at automotive and boating magazines, and authored more than 1,000 articles, from tech to trailering. He considers his home state of Oregon a paradise for RVing and outdoor adventure.

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  1. I have worked with engines of the gas and diesel variety and have done lots of mods. In so doing to a diesel, I found that the common mistake people used to make was to increase fuel flow by modified pump changes, not realizing the diesel engine runs hotter in ratio to the amount of fuel added. That’s because a diesel runs on the lean side of peak EGT when set up as per factory specs. The EGT will be in the 650-750 F range, which is very safe. I had a 1997 Freightliner FL 60 with a B series Cummins rated at 250 hp. With my Teton hooked up, weighing in 21,000 pounds, it would not shift into sixth gear and just was an underpowered heap of metal.

    I had a shop do the typical changes and it woke up, but the pyrometer would hit 1,200 F on a good hill, and I knew that would not last. The fix was a new 4-inch exhaust system with a high-flow muffler that was quiet. The exhaust temps dropped into the 675 F range with no loss in power; it may have actually had more power. The last I saw the truck, it had 250,000 miles on it and was running great.

    The biggest threat to a diesel is high exhaust temps, the factories know this, and now the B series Cummins is rated at 360 hp with EPA certification. Turbocharging is far less stressful than increased RPM is to attain more power. The loads internally go up by the square as RPM increases. Turbocharging merely adds load to the cylinders and bearings, which is easy to calculate.

  2. I like that you said that powertrain warranties are good indicators of how good your engine is going to perform. One of my friends has an old Ford truck, and he is thinking about installing a new motor to it. I’m going to check online to see if it’s possible installing one of the new models.


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