Horsepower And Torque Increase - Playing The Numbers - Tech

Horsepower and torque are the two common units of measurement to describe the capabilities of an engine. Of course, if a little of anything is good for most hot rodders, a bunch is better, so the larger the horsepower and torque numbers are the more we like them. That, however, usually leads to the debate about which is more important.

Defining The Differences
The first definition we have to get out of the way is work, which is defined as changing the position of an object against an opposing force. As an example, with gravity as the opposing force, if you lift a 20-pound manifold off the floor and put it on a shelf 5 feet off the ground, you've done 100 ft-lb of work (20 x 5 = 100). Now factor in time and you've got horsepower, which is work done over a period of time.

James Watt, he of steam engine fame, came up with the notion of horsepower because he needed a way to sell his new invention as replacements for horses that were the power source of the 18th century. There are a number of versions concerning Watt's method of determining what constituted horsepower. One involved a typical horse walking in a circle while hitched to capstan that turned a mill, pump, or whatever. Somehow, Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with that number is suspect (which we'll get to). Watt observed that the horse traveled at approximately 181 feet per minute; he multiplied that by the 180 pounds of force the horse produced (181 x 180) and came up with 32,580 ft-lb/minute, which he rounded to 33,000. Another study involved ponies used for bringing coal up from underground mines. Through a somewhat convoluted process, Watt determined the average horse could raise 200 pounds 165 feet in one minute, which equals 33,000 ft-lb/minute.

With all that said, we have to point out that Watt was selling steam engines with a money-back guarantee and he was not about to take a steam engine back or return any money. So it doesn't come as a surprise to find there were those who claimed that Watt's definition of one horsepower was a far greater amount of work than a horse was capable of and that his steam engines were vastly overrated. In fact, in 1843 Frederick Simms prepared a paper stating that a horse that was worked at 33,000 ft-lb/min was likely to drop dead in short order. Nonetheless, Watt's numbers of 33,000 ft-lb/minute are what we work with today when measuring horsepower.

Compared to horsepower, torque is a fairly simple concept-it's twisting or turning effort. However, it may or may not result in motion. At this point, we need to make an important distinction between work and torque. As we said, work implies movement and is measured in ft-lb, while torque is the ability to do work and is measured in lb-ft. Think of torque this way: If you pushed on a crank handle that was 5 feet long with 20 pounds of pressure, you would be applying 100 lb-ft of torque whether or not the crank was turning.

Which Is More Important?
From a driver's standpoint, torque is what you feel when you get shoved back in the seat, and a car will accelerate the hardest when the engine is at its torque peak. In most cases, engines develop maximum torque at low to intermediate speeds. This is primarily due to the fact that the cylinders have enough time to fill with a combustible mixture of air and fuel. The more air/fuel mixture there is in the cylinders, the higher the pressure is from combustion. But as the engine speeds up, volumetric efficiency-or how well the cylinders fill on the intake stroke-decreases and consequently torque declines.

So, if torque is what makes things move, why is horsepower important? Think of this the next time you see a big truck take off from a standstill. A large diesel engine may make well over 1,500 lb-ft of torque while only producing 400 or so horsepower; that's because its rpm range is very limited. So, while a big truck engine may be capable of doing a tremendous amount of work, it does it slowly. In simple terms, torque is the ability to do work-the more torque, the more work can potentially be done-but horsepower determines how quickly that work is accomplished.

As most SRM readers know, the device used to determine horsepower and torque is a dynamometer. But what some may not realize is that a dyno only measures torque and speed; horsepower is determined by the formula horsepower = torque x rpm / 5,252. Because of this, you'll see that horsepower and torque lines on a dyno graph always cross at 5,250 rpm (or at least they should).