Horsepower vs. Torque

Understanding What Horsepower & Torque Measure














Engines in the United States are rated by there horsepower and torque ratings. The focus of this article is to provide a better understanding of the differences between horsepower and torque. Additionally, we'll explore many age old questions surrounding these measurements and relativity to diesel engines. The truth of the matter is that while horsepower and torque are related, they have significantly different meanings when used as metrics of performance.


Important Definitions:

In order to understand the technical side of this article, it is important that we define some key engineering terms.
Work: Work is a measurement of force acting through a distance. Example: lb-ft "pound-feet" or ft-lb "foot-pounds" (lb = force, ft = distance).
Power: Power is a measurement of the rate in which work is performed. The units are therefore force through a distance per a unit of time. Example: lb-ft/sec (lb = force, ft = distance, sec = time).
Horsepower: Horsepower is a measure of power. It is defined under the assumption that a horse can move 33,000 lbs 1 foot per minute (1 horsepower = 33,000 lb-ft/min). Since there are 60 seconds per minute, horsepower can be reduced to 550 lb-ft/sec. Horsepower measures how much work is being done in a given time period.
Torque: In engineering terms, torque is the measure of an objects tendency to rotate about a point. It is a measure of work, and in terms of engine output can be simply described as twisting force. Torque is used to measure the output of an engine per crankshaft revolution.


Horsepower vs. Torque

The difference between torque (work) and horsepower (power) is that horsepower is measured over a period of time, where as torque is a measure of the force an engine produces per revolution. Horsepower is dependent on time, where as torque is independent of time. Though they are fundementally different, horsepower can be calculated if torque is known, and vica versa.

Calculating Torque Using Horsepower: Torque = (Horsepower x 5,252) / RPM

Calculating Horsepower Using Torque: Horsepower = (Torque x RPM) / 5,252


Dynamometers actually measure torque and calculate horsepower using the formula above. Don't get confused by the 5,252, it is a mathematical constant derived from the fact that a 1 foot circle has a circumference of 6.2832 feet. Dividing 33,000 by 6.2832 gives you 5,252, which helps to simplify the equation. This constant is the reason that horsepower and torque ALWAYS cross paths at 5,252 rpm (irrelevant to diesels, which do not operate at this engine speed).


Which is More Important, Horsepower or Torque?

In terms of rating an engine, the application regulates whether torque or horsepower is the more important measurement. For example, construction equipment, small engines, and generators are typically rated by engine horsepower, not torque. This engines are designed to run at a specific duty cycle (constant load/rpm) for extended periods of time and therefore it is more beneficial to know the amount work the engine is capable of over a period of time (work over a period of time = power). Racing engines are another example where horsepower is the favored metric of engine performance. When speed counts, it is more beneficial to know how much work an engine performs per second than per crankshaft revolution. In this situation, higher horsepower dictates an engine that can perform more work per second than a lower horsepower engine.


When discussing truck engines, whether gasoline or diesel, torque measurements are typically given more importance than horsepower ratings (though both are obviously used). This is because torque better measures the pulling force of the truck. Diesel owners are especially concerned with the amount of torque an engine can deliver off idle, since this gives a measure of the engines ability to move a large load from a stop. Horsepower gives the amount of work the engine performs over a given period of time, which could be misleading considering a diesel engine typically operates in a narrow range of engine speeds (rpm) and the engine speed changes frequently. Torque can also be used to describe an engines ability to resist obstacles that threaten to slow the truck down, such as steep grades.


Horsepower, Torque, And Mathematics:

To further your understand of the relationship between horsepower and torque, we'll take a closer look using a mathematical example.


The new high output 6.7L Cummins produces 800 lb-ft or torque at 1,600 rpm. We can therefore calculate the horsepower at 1,600 rpm.


HP = (Torque x RPM) / 5,252 , so HP = (800 lb-ft x 1,600 rpm) / 5,252 = 244 hp @ 1,600 rpm. Looking at the torque curves for the 6.7L Cummins confirms that this figure is correct.



Given that our oil burners are known for low end performance and that peak horsepower is created at engine speeds that are seen infrequently, torque is the favored measure of towing capability, especially in diesels.



































































































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