Whenever we see advertisements of cars or trucks on televisions, we mostly see words like horsepower or torque. While some of us know the terms individually, majority don’t. Conventionally one says the bigger the numbers the better. But does anyone really know what they are or how they are related? Well, let’s find out!
As the term suggests, horsepower was first coined by James Watt in the 18th century, which literally means the number of horses needed to pull a car (steam engine in that time). In layman words, if 5 horses are required to pull a car, thus the power would be 5 horsepower (or HP). It is a term that is used to quantify the work an engine undergoes. That is in terms of horses, the amount of work a horse can do per minute. One horsepower is the equivalent amount of work that it takes to move 33,000 pounds for a distance of one foot in one minute. For cars, HP is a constant supply of power. It is that high end power which does not initiate or start a car but, when it reaches up to speed, it keeps it there. A greater HP means higher acceleration and a smoother ride. It has been standardized to be 550ft-lb/sec or 745watts. In cars, horsepower is still the preferred mode of measuring the power output of an engine, but cars have a specialized method of measurement and the result is in brake horsepower. It is taken by measuring how much power an engine can put out without all the other intervening mechanisms like the alternator, generator, gearbox, and other devices that are needed in the car. It is usually measured with the aid of a dynamometer, a device that is used to determine an engines performance. HP is usually measured in ft-lb/sec, however, the electrical equivalent of one horsepower is 746 watts in the International System of Units (SI), and the heat equivalent is 2,545 BTU (British Thermal Units) per hour.
Torque is another term used by car enthusiasts to define the output of their car. Torque is basically the force required to rotate an object. Or, in car terms, the force exerted on the ground to get the vehicle off and running. As previously stated, it is not HP but torque that initiates the car and gives it the necessary power to move from rest. Hence it can be defined as the low RPM power of the machine, as opposed to horsepower. The unit newton meter is properly denoted N-m and is used to measure torque in system international (SI). The SI unit for energy or work is the joule. It is dimensionally equivalent to a force of one newton acting over a distance of one meter, but it is not used for torque.
Difference between Horsepower and Torque:
Many of you have probably ridden dirt bikes or street bikes, and probably know that 2-stroke dirt bikes are really fast as long as they are in the “power band”. When they are below the power band though, they perform really inefficiently, especially if there is a rider on the back. A 4-stroke bike will make much more torque and won’t be as inefficient on hills of steep roads and the power comes-on smooth and at a much lower RPM than a 2-stroke bike. Think of the 2-stroke bike as a high horsepower bike, and the big 4-stroke bike as a high torque bike, and that is pretty much the difference between horsepower and torque.
In racing where there is an accident, remember – Horsepower is how fast the car hits the wall. Torque is how far it moves that wall. If a 300 pound race bike crashes into the wall at 100 mph, horsepower got it to hit the wall at that speed, and the wall probably won’t move much. Hit the same wall with a 75,000 pound truck traveling at just half that speed, and the wall will get knocked down and pushed off the race course a few hundred feet. This is because of its higher torque.