Torque Versus Horsepower - What Do They Do and Which One Makes Your Car Go Fast?

Both scientific terminologies existed long before cars and motorized vehicles in general, so we are going to have to use a little physics in our story.

Horsepower

First of all, let’s go back to the person who decided how to measure power. His name is James Watt, and he was a Scottish engineer whose name gave the standardized unit for measuring power. The Watt is used to measure power, but horsepower was adopted to bring a substantial equivalent. One metric horsepower equals 735,5 watts.

What is a horsepower? Well, the description refers to the ability to lift 75 kilograms (165 pounds) by one meter (3.28 feet) in one-second (thank God there’s no metric/imperial on this one). Horsepower stands for how quickly work gets done.

Torque

Meanwhile, torque or moment refers to a particular force that has the tendency to rotate an object around an axis. In layman's terms, torque is a measure of the force needed to turn a bolt or a wheel. When you twist the cap off a plastic bottle, you use torque.

For another example, the machine that fits caps on plastic containers in a factory to ensure they do not leak has a torque specification. The latter term refers to how hard the machine has to twist the cap on the container to make sure it is airtight, without any damage to the object or the assembly. If the torque specification is not met, the product inside the container could leak or could be inaccessible to its user if they do not have enough torque available in their wrists to open it.

To put the two terms into perspective, imagine that torque means that you are making homemade jam at your house, and you need to put it into jars. You need torque to seal the caps on the jars, but horsepower is necessary to lift the container with the filled jars to get them into your storage cabinet.

Torque and horsepower in internal combustion engines

Here’s the part you were looking for. How does torque work with power in internal combustion engines? Well, the two work hand in hand for your vehicle’s engine to deliver its performance to the road.

The formula that explains this is HP=Torque x RPM /5,252. This equation goes for every internal combustion engine and can be verified at any RPM that unit is capable of, and 5,252 is a constant.

A simple explanation of this fact would be that an engine produces power using a rotating shaft (crankshaft), which can apply an amount of torque to a load at a given RPM. Therefore, power is calculated from torque and RPM. At 5,252 rpm, power and torque will be equal. Meanwhile, at lower values, torque will be higher in value than horsepower, while at higher values it will be vice-versa. This statement applies to all combustion engines.

So, whenever an engine’s horsepower is measured, a dynamometer is used. The machine measures torque and RPMs, multiplies the values together and then divides them to 5,252 and obtains the man-made value of horsepower.

Power or torque, which one is best?

Well, this is a silly question, but we must address it. A car with a high power level is typically faster when accelerating, attains a higher top speed, and can carry higher weights. However, a car with high torque numbers will have better in-gear acceleration, lower engine speeds at given load (crucial when it comes to fuel economy), and the ability to go faster off the line.

Since horsepower goes up with torque, a "torquey" engine can deliver a high HP figure as well if it is capable of exceeding 5,252 rpm and configured to achieve that particular task.

What’s a power band?

This term is the RPM range between the maximum torque value of an engine and its maximum power value. In between those two values, the engine operates efficiently and provides solid performance and fuel economy.

Electric motors have enormous power bands, as they can produce their maximum torque from the first revolution of their axle up to high rpm values, and their maximum power is even greater than production internal combustion units.

Diesel engines have a narrower power band, as their peak torque comes lower than in gasoline engines, and their peak power is also achieved in lower RPMs. Gasoline engines have wider power bands, and are preferred for performance applications because of this. Modern gasoline engines with turbochargers, direct injection, variable valve timing and other smart engineering solutions provide awesome power bands with incredible elasticity.

Why are high-torque vehicles often out-accelerated by high-power cars?

It is all in the gearing, because the cogs in the transmission control RPMs, and, therefore, horsepower. Gearing magnifies torque, improving acceleration in first gear and bringing an advantage to vehicles that have low-end torque. When the gears get shifted up, power comes into play and torque drops once the RPMs get higher.

That is why diesel-engined vehicles might be faster off-the-line than their gasoline-engined counterparts, but eventually get outpaced by the latter category. There’s a small difference in weight between them, but it is always mainly about the gearing and the torque.

Why are high-horsepower cars used in racing?

Because cars with high horsepower figures have specific gearing to benefit from the ability of their engines to reach high RPMs. Since all motorized competition takes place at high RPMs, the wider power band of those power plants (usually gasoline engines) gets put to use.

However, there are events when diesel-powered vehicles are successful, like the 24 Hours of Le Mans, where Audi has won the big prize with its TDI racecars on numerous occasions. In the latter case, the Audi team won through their increased fuel efficiency, which allowed fewer fuel stops.

For a more in-depth explanation of torque and horsepower, you can view this video below from Engineering Explained: