Whether you’re purchasing a new car or building a hot rod in the garage, two factors come into play when considering engine performance: horsepower and torque. If you are like most DIY mechanics or automotive specialist, you likely have a good understanding of the relationship between horsepower and torque, but may struggle on understanding how those “foot pound” numbers are derived. Believe it or not, it’s really not that complex.
Before we go into the too technical part, let’s break down some simple facts and definitions that would make it easier to comprehend why both horsepower and torque are vital factors to consider. We should start by defining the three elements of measuring performance of a combustion engine: speed, torque and horsepower.
Part 1: Understanding the engine speed, torque, and horsepower impact overall performance
In a recent article, one of the greatest mysteries in engine performance was lastly figured out by getting back to basics of how horsepower is basically factored. Most people feel that dynometers (engine dynos) are made to measure the horsepower of an engine.
But in reality, dynometers do not measure horsepower, they actually measure torque. That torque figure is then multiplied by the RPM it is measured at and then divided by 5,252 to give a horsepower figure.
For more than 50 years, the dynometers that are used to measure torque and engine RPM simply could not handle the intense power generated by these engines. In fact, one cylinder on these 500 cubic inch displacement, nitromethane burning Hemis generates an estimated 800 pounds of thrust through a single exhaust pipe.
All vehicles engines, whether combustion or electrically powered, function at different speeds. For the most part, the faster an engine finishes its power stroke or cycle, the more power it generates. In regards to a combustion engine, the three elements that impact the overall performance of that engine are speed, torque and horsepower.
Speed is defined as how fast the motor carries out its work. When we apply the speed of the engine to a number or a unit of measurement, we gauge the engine’s speed in revolutions per minute or RPM. The “work” that the engine does is a force applied over a measured distance. Torque is defined as a special type of work that produces rotation. This happens when a force acts on a radius (or, for a combustion engine, the flywheel) and is often measured in foot pounds.
Horsepower is the rate at which work is accomplished. Back in the old days, if objects had to be moved, humans usually used a horse to move it. It was thought that one horse could move approximately 22,000 foot pounds per minute. This is where the term “horsepower” originated. Different from speed, and torque, horsepower can often be measured in multiple units including: 1 HP = 746 Watts, 1 HP = 2,545 BTUs, and 1 HP = 1,055 joules.
These three elements operate together to produce engine power. As torque stays constant, speed and horsepower remain proportional. However, as the engine’s speed increases, the horsepower equally increases in order to maintain constant torque. Where many people become confused, however, is in how torque and horsepower impact the engine’s speed. Quite simply, as the torque and horsepower increase, so does the engine’s speed. The reverse is equally true: when the torque and horsepower decrease, so does the engine’s speed.