Torque Vs. Horsepower

If you have recently started checking out car engine specifications, then you must have noticed two main items in the list, which are horsepower and torque. Automobile magazines often use values of horsepower and torque to compare car engines which may lead to confusion regarding what these terms actually mean. The comparison made in this article is aimed at eliminating this confusion and set the record straight in this matter.

How are Torque and Horsepower Different?

It will help to clarify what is the difference between torque and horsepower if we define both concepts right at the start. After that, I will resolve confusion regarding units and ultimately consider how these two parameters are important in engine design.

Definition
In the language of physics, torque is known as a moment of force. It is a twisting force, as opposed to 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 liner force applied at the end of the wrench, multiplied by length of 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 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 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 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 torque which is a measure of the twisting force exerted by the engine.

Significance
When designing an engine that can deliver a specific amount of horsepower, an automobile engineer needs to calculate the amount of power generated from the engine created torque. The horsepower delivered by a car engine is directly proportional to the overall torque generated in the crankshaft. Further, torque generated is dependent on the total displacement capacity of the car engine.

So there is a limitation set by maximum possible displacement (measured in cubic centimeter), 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 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.

Torque and horsepower are both deeply related to each other and need to be taken into consideration while designing an engine or reviewing its performance.