Definition of Horsepower

Horsepower is probably the term most used when discussing muscle cars. However, few people can accurately define what exactly horsepower is. Horsepower is officially defined as "the amount of energy required to lift 550 pounds, one foot, in one second." But what does this mean? To answer that question, we need to know a little history.

The term horsepower was created by James Watt, an engineer famous for his work with steam engines and is most associated with his measure of electric power (i.e. 60 watt light bulb). Around 1775, Watt wanted to create a standard measure of work to compare the output of his steam engines with the horses that were used to move large loads in mines. Watt studied the amount of work that horses did and found that on average, a mine horse could perform 22,000 foot-pounds of work in a minute (for example, move 1,000 lbs 22 feet in one minute or 500 pounds 44 feet in one minute). He then increased the number by 50 percent and came up with a measurement of 33,000 foot-pounds of work in one minute. This measurement eventually became known as horsepower.

Using various physics equations, you can come up with different interpretations of horsepower:

  • 1 horsepower is equivalent to 746 watts. So if you took a 1-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts.
  • 1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).
  • One BTU is equal to 1,055 joules, or 252 gram-calories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.

Types of Horsepower

There are three basic types of horsepower, SAE Gross Horsepower, SAE Net Horsepower, and Wheel Horsepower. Each is the result of measuring the same engine in different ways. These standards were established by the Society of Automotive Engineers (SAE). The SAE is a group responsible for setting various standards within the automobile manufacturing industry. Founded in 1905, the SAE publishes many new, revised, and reaffirmed standards each year in three categories: Ground Vehicle Standards, Aerospace Standards, and Aerospace Material Specifications. Standards allow entire countries to talk to each other in a common language.

SAE Gross Horsepower or Brake horsepower (bhp) was the standard horsepower measurement by the automotive industry up until 1971. Brake Horsepower Power is measured at the flywheel with no load from a chassis or any accessories and with fuel and ignition operations under ideal conditions. An accessory is anything attached to the engine, by any means, which is not required for basic engine operation. By this definition, this would include a power steering pump, smog pump, air conditioning compressor and an alternator. Ideal conditions, often called laboratory conditions, are standardized settings for use during horsepower measurement. During the 1960s they consisted of a barometric pressure of 29.92 Hg and a temperature of 60 degrees F.

SAE Net Horsepower became the standard measurement in 1972, and is still used today. SAE Net horsepower is the horsepower generated by the engine at the flywheel with all accessories attached. This change was made to reflect the numerous energy sapping accessories that cars began to have, such as an A/C Compressor and alternator, and thus was a better representation of the actual power generated by the engine. This number is always lower than the SAE Gross horsepower. Therefore, the same engine could have been rated in 1971 as 360 SAE Gross Horsepower and in 1972 as 300 SAE Net horsepower without any reduction in "power."

Wheel horsepower is horsepower measured at the actual drive wheels, taking into account the load from the chassis and all accessories. It is the most accurate measure of the amount of energy that the car actually generates to move it forward. Wheel horsepower is measured using a dynamometer. This is done by placing the vehicle's driven wheels on a large roller and accelerating the wheels up to redline in first or second gear. The vehicle's ability to turn this roller is measured and calculated (formula below) to come up with a figure that represents how much horsepower is actually available to move the vehicle around. Because a frictional loss between the engine and the driven wheels is unavoidable, wheel-driven horsepower will always be less than SAE Net Horsepower. How much less wheel-driven horsepower will depend on how many mechanical parts exist between a vehicle's engine and its driven wheels. This is usually measured as a percentage loss due to the "friction" of the intermediate components between the flywheel and the actual wheel. For a Rear Wheel Drive car, engine power has to travel through a transmission, driveshaft, rear-differential, and two axle shafts (one for each rear wheel). That's four separate mechanical components taking a bite out of the car's horsepower before the rear wheels even begin to turn. Front-wheel drive cars with transverse-mounted engines usually have a lower frictional loss because horsepower only has to travel from the engine, through the transmission and down two short driveshafts before reaching the wheels. Typical "powertrain" losses run between 15-22% but vary greatly between cars.

Definition of Torque

Torque is a force that tends to rotate or turn things. You generate torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. Torque is usually measured in English units such as pound-feet (lb-ft), although the international standard is the Newton-meter (1 lb-ft is equal to 1.356 Nm). Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of the lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a 2-foot wrench, you only need to put 100 pounds of force on it to generate the same torque.

In a car, the engine converts the horsepower it generates into torque by turning the crank shaft. The combustion of gas in the cylinder creates pressure against the piston. That pressure creates a force on the piston, which pushes it down. The force is transmitted from the piston to the connecting rod, and from the connecting rod into the crankshaft. The point where the connecting rod attaches to the crank shaft is some distance from the center of the shaft. The horizontal distance changes as the crankshaft spins, so the torque also changes, since torque equals force multiplied by distance. Only the horizonal distance is used in determining the torque in an engine. When the piston is at the top of its stroke, the connecting rod points straight down at the center of the crankshaft. No torque is generated in this position, because only the force that acts on the lever in a direction perpendicular to the lever generates a torque.

Measuring Torque

Torque is also measured using a dynameter. The torque generated is measured at different RPMs and the result is plotted on a graph. Then, horsepower is calculated by taking the torque at each RPM, and converting it using the following formula:

Horsepower = Torque X (RPM/5,252)

This formula is the result of combining several formulas into one. First, 1 horsepower is defined as 550 foot-pounds per second. The units of torque are pound-feet. So to get from torque to horsepower, you need the "per second" term. You get that by multiplying the torque by the engine speed. But engine speed is normally referred to in revolutions per minute (RPM). Since we want a "per second," we need to convert RPMs to "something per second." The seconds are easy -- just divide by 60 convert minutes to seconds. Now what we need is a dimensionless unit for revolutions: a radian. A radian is actually a ratio of the length of an arc divided by the length of a radius, so the units of length cancel out and you're left with a dimensionless measure. You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252. So if you multiply torque (in pound-feet) by engine speed (in RPM) and divide the product by 5,252, RPM is converted to "radians per second" and you can get from torque to horsepower -- from "pound-feet" to "foot-pounds per second."

Putting It All Together


From the graph, we can determine the peak horsepower and torque ratings - the actual highest value of horsepower and torque, and at what RPMs they are obtained. These values are the most common way of describing the power generated by an engine and is expressed as "320 HP @ 6500 rpm, 290 lb-ft torque @ 5000 rpm." Note that by definition, horsepower has to equal torque at 5,252 rpm and therefore, the lines will cross at this point. Torque will always be higher than the horsepower below 5,252 RPM, equal to horsepower at 5,252 RPM and less than the horsepower above 5,252 RPM. That is why the torque peak occurs at a lower RPM than the horsepower peak.

It is important to realize that the area underneath the curves is just as important (or more so) than the peak value. A fairly flat curve (especially for torque) means that power is available throughout the RPM range and usually occurs with large displacement engines. Smaller engines generally have a drastic peak in output, with dramatic increases and decreases over the RPM range. Two engines may have the same peak horsepower, but vastly different torque ratings and curves.