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Torque Vs. Horsepower

The aim of this article is to present a comparison between horsepower and torque, to clear the confusion that often arises between the meanings of these terms.
Omkar Phatak
If you have recently started checking out car engine specifications, then you must have noticed two main items on the list, which are horsepower and torque. Automobile magazines often use them, to compare car engines, which may lead to confusion regarding what these terms actually mean.
How are Torque and Horsepower Different?
In the language of physics, torque is known as a moment of force. It is a twisting force, as opposed to a linear force that acts to displace any object in a straight line. Suppose, you are tightening a nut, using a wrench. Torque is the vector product of linear force, that is applied at the end of the wrench, multiplied by length of the wrench from point of contact with nut, to wrench end. For example, a force of 100 pound, exerted at the end of a 1 foot length wrench, will translate into a torque of 100 pound feet (lb-ft).
An engine crankshaft revolves in a circular fashion due to the torque generated by displacement of engine cylinders. Thus, torque is the twisting force exerted by the engine, on the crankshaft. This exerted torque ultimately gets translated into linear motion of the car moving ahead, as it is applied to the wheels via transmission.
Horsepower is a unit of power. It is used to measure the total power output delivered by the engine or work done by engine per unit time. There are many types of horsepower units but the one that is used in car specifications is mechanical horsepower. The equivalent unit of horsepower is ft-lb/min and 1 Hp (horsepower) is quantitatively equal to 33,000 ft-lb/min. This should be remembered as the mechanical horsepower formula.
If you know the torque value, then horsepower at a specific engine RPM value can be calculated, using the following formula:
Horsepower = (Torque x Engine RPM)/5250
If you put in some RPM values in this formula, you can get an idea about the range of power that your car engine can provide.
Thus, horsepower measures the forward thrusting power of a car. It is a numerical estimate of work done by the engine per unit time, as opposed to the torque, which is a measure of the twisting force exerted by the engine.
When designing an engine that can deliver a specific amount of horsepower, an automobile engineer needs to calculate the amount of power, that is created from the generated torque. The horsepower delivered by an engine is directly proportional to the overall torque generated in the crankshaft. Further, the generated torque is dependent on the total displacement capacity of the engine.
So, there is a limitation set by maximum possible displacement (measured in cubic centimeters), that is generated by the engine. There is a maximum constraint on the torque that can be generated by an engine, which automatically sets a limit on the maximum horsepower that it can generate. A car with an engine of high torque value will pick up speed faster, while a car with high horsepower will attain a higher maximum speed.
In short, both these concepts are deeply related to each other and need to be taken into consideration while designing an engine or reviewing its performance.