We toss the words “horsepower” and “torque” around pretty often in the automotive industry, but perhaps the meaning of both gets lost along the way. Never fear, Engineering Explained is here.
In a new video, EE host Jason Fenske helps explain in the simplest of ways what power and torque are, and why the old saying of “Horsepower is how fast you hit the wall, torque is how far you move the wall” is incorrect. It helps to have a basic understanding of what both things actually are. Torque is a force multiplied by distance, and the easiest way to understand it is by using a wrench. As an individual provides force to the wrench, it travels a distance and provides torque to tighten a bolt.
Combustion provides the force in a cylinder to press a piston down, which then presses down on a crankshaft at a specific distance. This is where the phrase “units of twist” comes from, since the piston and crankshaft provide a twisting force.
Horsepower, on the other hand, is the rate at which work is done. Torque multiplied by rpm returns horsepower. Basically, the faster the crankshaft spins with the same amount of force, the more power an engine will make. A car with more hp than torque will always be quicker since this gives a car acceleration and speed.
Jason uses two hypothetical cars to illustrate all of this. Both have the exact same gear ratio, but one uses a diesel engine with 200 pound-feet of torque, and the other uses a gasoline engine with 100 lb-ft of torque. The diesel engine with twice the torque will accelerate quicker initially because it has a greater sustained force to make more power. However, it only revs to 2,626 rpm. Meanwhile, the gasoline engine will rev to 5,252 rpm. It won’t accelerate as quickly initially, but it won’t have to shift gears while the diesel will. The two will offer the same amount of speed, but the diesel will accelerate more quickly. This is why low-end torque becomes important for better acceleration in many scenarios.
Higher torque doesn’t mean one vehicle will necessarily be faster than another, though. For example, a Ford F-250 makes 925 lb-ft of torque, while a Honda S2000 has just 162 lb-ft. The S2000 is quicker, even with less horsepower, because of other factors, the most important of which is its power-to-weight ratio. The 2,800-pound S2000 hits 60 mph in 5.7 seconds, while the 8,300-pound F-250 does the deed in 6.9 seconds, which illustrates that power-to-weight ratio is more important than weight-to-torque ratio for acceleration. That doesn’t mean the S2000 is ready to tow a 5,000-pound trailer, though, as Jason explains that weight and torque are very important for towing figures as well.