It's no secret that most large commercial trucks and some heavy-duty trucks have diesel engines. Why? Diesel engines generate more torque. Since a diesel engine doesn't have spark plugs and uses compressed air to ignite fuel, the piston (cylinder) has to travel a longer distance to compress enough air for ignition. This cylinder travel distance is called stroke, and more stroke means more torque [source: Mason].
Let's look at the differences between two truck engines. A Dodge Ram truck with a 6.7-liter Cummins Turbo Diesel engine has 350 horsepower and 650 pounds-feet of torque. The same truck with a 5.7-liter Hemi gasoline V8 engine, on the other hand, has 383 horsepower and 400 pounds-feet of torque. See the difference? While the gasoline engine has a little more power, the diesel's torque completely blows it away [source: Dodge].
While the two trucks have very similar towing capacities --15,650 pounds (7098.7 kg) for the gasoline engine and 16,350 pounds (7416.2 kg) for the diesel -- all that low-end power means the diesel engine will have an easier time towing things. Keep in mind that the diesel Dodge engine gets that much torque at only 1,500 rpm, which is just above idle. With a diesel engine, you won't have to rev up to start towing because the power already exists.
Because you're using more energy-laden diesel fuel, diesel engines get better mileage than gasoline ones, so you'll also be able to tow longer without stopping. At the same time, a truck with a gasoline engine is going to have a cheaper base price. Diesel engines almost always cost more than gas ones because they are built to withstand greater forces from a more potent fuel. A vehicle's towing capacity depends on many of its components, including wheels, tires, suspension and transmission [source: Haulmark].
Typically, the bigger the towing job, the larger displacement engine you will need. Towing puts a lot of stress on the engine, so more torque will reduce wear and tear on the vehicle [source: Sunrise Trailer Sales].
We've spent a lot of time and bandwidth comparing horsepower to torque. What's the verdict? In the end, all the evidence points to torque as being more important than horsepower when it comes to towing. Why? The power at low-end rpm provided by high levels of torque lets you move huge loads without much effort. As stated before, some diesel trucks produce twice as much torque as they do horsepower at near-idle RPM levels -- meaning that they can start pulling something like a trailer or a boat with ease.
Horsepower is important because it allows a car to move faster on the highway and at high rpm. However, if you can't get that trailer off the line, all the horsepower in the world won't help you.
For more information on the horsepower vs. torque debate, please see the links on the next page.