Horsepower vs. Torque: Understanding the Difference

February 22, 2012

Torque and horsepower are orthogonal concepts that are often confused with each other. Get the full explanation on horsepower vs torque.

Measuring Torque With A Dynamometer

Understanding horsepower and torque can be difficult as these relate to work, power and force but in different ways. They are related but the relationship is not straightforward. Many people read about these two measurements and see specifications for them in engines but have little understanding of what they really mean. This relationship between torque and horsepower can be used to figure out the horsepower acceleration, as the torque is responsible for going from zero to moving.

Torque

The amount of force needed to turn a bolt or screw is the torque. There are many different units of measurement for this, but the most common is foot-pounds. A simple torque calculation is the amount of power (pounds) needed to turn a 1-foot-long wrench to tighten a bolt. So as the length of the wrench changes the amount of pounds needed to turn it will change. Normally the longer the wrench, the less amount of pounds needed.

Horsepower

The unit horsepower was actually created by James Watt. This was also the gentleman that discovered the watt found in the 60-watt light bulb. In his time animals, mainly horses, were used for work. He calculated how much work, in foot-pounds, a horse could achieve in a minute. This is really very arbitrary, as all horses will do a different amount of work. However, it has become the standard for engines and other machines. Thus, 1hp equates to a horse moving a 330-pound load 100 feet in a minute.  So 1 hp = 33,000 foot-pounds.

Different Units

Horsepower has many measurements besides the foot-pound. It can also be calculated as 1hp = 746 watts. This is the amount of power a horse could generate by continuous motion. Over the course of an hour 1 hp = 2,545 BTU, in which BTU is a thermal measurement. From here BTUs can be converted to gram-calories, joules and more. Torque is also measured in degrees, using the torque angle as well as Newton meters which can be converted into lb-foot.

Measuring Torque and Horsepower

Torque is typically measured using a torque wrench. The user applies a force to the wrench. There are many different torque wrenches and tools that can calibrate torque as well as be used to ensure the bolts are tightened to the correct spot. Horsepower is measured using a dynamometer which will put a load on the engine and measure how much power it needs for the engine to lift or move the load.

The Relationship

There is a relationship between torque and horsepower. However, a conversion is needed as horsepower measures the force needed to move a load over a distance, and the torque measures the force needed to move a load around a rotation or fixed point. Using several different conversions the equation for this relationship is 1 hp = torque* (rpm/5,252). An online torque calculator is an easy way to figure out these calculations. Each engine will have its peak point where the maximum power is produced. Additionally, if the horsepower and torque are graphed, there is a point where each will be the same, 5,252 rpm. So an engine that is described as having a lot of low end torque means that the maximum peak value for the torque occurs at a low rpm value. Maximum torque will always occur under 5,252 rpm, and maximum horsepower will occur at greater than 5,252 rpm.

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