Torque vs Horsepower 101 February 09 2021
What are torque and horsepower, and which number is the more relevant number for assessing performance? There are plenty of articles on the internet about the subject, but most seem to focus on the relationship of Torque X RPM = Power. We are going do something different and talk about power and torque in a more fundamental way. What exactly is torque, and what even is power?
Torque is a measurement of a force acting at a given distance from the center of rotation. Apply 10 pounds of force to the end of a 1-foot long wrench, and you have 10 foot-pounds of torque. Simple enough, right?
Horsepower is a measurement of power. But what does that mean? Power is a unit of measurement to describe how quickly energy can be unleashed, whether that energy is mechanical energy, thermal energy or electrical energy. Watts, horsepower and British Thermal Units are all units of power. That 1000 watt stereo system? Yep, that is how much electrical energy can be converted into acoustic energy in a given time. That 25,000 British Thermal Unit (BTU) stove? Yep, that's how much chemical energy can be converted into thermal energy in a given time. When talking about mechanical power, power is usually measured by how much force you can exert over what distance and in what amount of time.
Moving half the weight twice the distance in the same amount of time uses the same amount of power.
Imagine two people lifting bricks up to the top of a building by using a pulley system as shown above. Fred takes the strategy of loading up only 10 bricks at a time and then relying on speed to lift them to the top of the 4-story building. Andy is stronger and takes the strategy of loading up 100 bricks at a time, but each load is heavy, and it takes him much longer to get 1 load to the top. Power allows us to figure out who is getting more bricks to the top in a given amount of time, despite their differences in strength and speed.
Power can describe the rate of linear mechanical work, but can also be adapted to describe the rate of rotational work as well. In the diagram above, imagine the pulling force was coming from a winch. You could look at the linear rate of work that is being done on the weight, or you could look at the rotational rate of work of the winch, and they would be the same. We can convert the linear force into a torque if we know the diameter of the winch. We can convert the linear distance the weight travels to the number of rotations of the winch if we know the circumference of the winch. Rotational power is the same concept as linear power, but wrapped around a shaft. For rotating shafts, the linear power equation is rewritten to say, "How hard can you twist, and at what rotational speed can you produce this twist?"
For this reason, horsepower gives a better estimate of how quickly a car can accelerate than torque does. Torque is somewhat ambiguous because torque tells us how hard the twist is, but not how fast the twist is. Power tells us what the combined effect of how hard the twist is and how fast the twist is. It allows us to compare a cyclist pedaling hard but cranking slow versus another cyclist that is pedaling lighter but cranking faster. Power says I don't care how you accomplish your work, I just want to know how much work you can accomplish in a given amount of time.
In a thought experiment that takes this concept to the extreme, what if I lined up against a Ford Fiesta on a quarter mile drag strip with a wooden Conestoga wagon? On the back axle, I would attach a 10-foot-long, ratcheting breaker bar, and I would crank it to produce over 1,500 foot-pounds of torque to the rear wheels. The torque would be so high but the rpm would be so slow, and I would lose the race by nearly a quarter mile. This is an extreme example of high torque and low horsepower, but it illustrates how torque can be a poor metric for describing how fast a vehicle can accelerate. The outcome could have been much more accurately predicted by comparing power figures.
In another example, imagine two cars with continuously variable transmissions that can operate their engines at peak horsepower all the way down a drag strip. Car 1 has more torque at peak horsepower, while car 2 has less torque but higher horsepower. All other factors being equal, it would all come down to which car has more horsepower, not which car has more torque. Again, horsepower looks at the combined effect of torque and rotational speed. Torque matters, but you have to factor in the rotational speed to get the whole picture. Power just cares about how much work is getting done in a given amount of time, and not what combination or torque and rotational speed you use to achieve it.
That being said, there is some inaccuracy with using a max power figure to estimate acceleration potential, because power is inconsistent across different engine speeds. An engine will make different power at different rpm, so you have to account for the variance in power across the rpm range. Some engines have to be spun up to high rpm before they can make good power, which means they can have a weak start off the line. This is why some people say that horsepower sells cars, but torque wins races. What they really mean to say is a power curve that ramps up quickly wins races. However, we stand by our opinion that power that tells a more complete story than torque.
The max torque figure isn't completely useless, as it can tell us is a little about the shape of the power curve. We can compare the max torque to the max power and get a general idea for whether the engine makes good power at low rpm or whether it has to rev higher to make its power. Because of math, the larger the torque number is compared to the power number, the lower in the rev range the engine reaches max power. So an engine with 400 ft-lbs and 250 horsepower would reach max power very quickly at low rpm, and an engine with 400 ft-lbs and 1500 horsepower would need to rev up to very high rpm to reach max power.
Hopefully you now have a good understanding of torque and horsepower. The two are interrelated, but as we discussed, horsepower tells a much more complete story than torque. Therefore, horsepower is the more relevant performance figure in our opinion.