Here’s one of the most common technical mistakes in automotive publishing. Today, we fix it. Ha.
In your automotive reading down through the years, you may have noticed a small but curious thing. You will find engine torque quoted in either foot-pounds or pound-feet with apparent interchangeability, almost as if the two terms have the same meaning. Some sources use foot-pounds; some use pound-feet; some never quite make up their minds.
However, these two similar-looking units (and their abbreviations, ft.-lb. and lb.-ft.) are not interchangeable. While it may seem trivial, one is the correct term for torque in English units, and the other is a measure of something else altogether. For those fussy people who want to get it right, here is a brief rundown.
The foot-pound (also and originally known as foot-pound force) is a traditional English unit of work. It is equal to the work done by one pound of force acting through a distance of one foot. For example, when James Watt determined that a horse could lift 550 lbs. at a rate of one foot per second, he declared it one horsepower. The SI or international equivalent of the foot-pound is the Joule (J).
The pound-foot (also and originally known as pound-force foot) is a traditional English unit of torque. The angular equivalent of linear force, torque is the tendency of a force to produce a rotation. Torque is the product of the force and the distance from the center of rotation to the point where the force is applied. For example, if a one-pound force is exerted on a wrench with an effective length of one foot, one pound-foot of torque is applied to the fastener. The SI or international equivalent of the pound-foot is, naturally enough, the Newton meter (Nm).
In a similar way, when a brake and lever are fixed to the output shaft of an engine, if the measured resistance force is 100 pounds and the lever is one foot long, 100 pound-feet of torque is indicated. And that, in a nutshell, is how engine torque is actually measured. And this is the value you will find quoted as torque in car magazines and sales literature.
The takeaway: The proper term for torque in English units is the pound-foot, which we can find abbreviated any number of ways, such as lb.-ft., lb-ft, lb/ft, and so on. However, a foot-pound is a unit of work. Engines certainly do produce work as well as power, but in this case torque is the property in reference.
(For an amazing exploration of the wonderful world of measures, see A Dictionary of Units of Measurement by Russ Rowlett of the University of North Carolina at Chapel Hill. Be prepared to burn a few hours.)