Spring 2002 Forward to a Friend

WHEN YOU purchased your Subaru vehicle, you undoubtedly collected information to help with your selection. Maybe you read road-test reports in automotive magazines, went online, or visited a Subaru showroom to read window stickers and brochures.

Each of these sources had a section devoted to the vehicle’s engine and performance specifications; usually a table filled with numbers and technical phrases. You probably saw two terms, horsepower and torque, again and again in these listings. They’re both important factors in how a vehicle will perform, but sometimes they’re misunderstood. Let’s take a look at the basics of what horsepower and torque are, and how they work together to define how a vehicle performs.

Defining Torque and Horsepower

Let’s start with some definitions. Torque is the twisting force produced at the heart of the engine, its crankshaft. Torque is measured in pounds-feet. One pound-foot is equal to the twisting force produced when a one-pound force is applied to the end of a one-foot-long lever.

For example, if you placed a wrench on a horizontal bolt and exerted one pound of pressure on the handle one foot from the center of the bolt, you would produce one pound-foot of torque, or twisting force.

The key word here is force; torque is a measure of an engine’s ability to apply twisting force to the vehicle’s crankshaft.

Horsepower (HP) is a general expression of an engine’s ability to do work, expressed as a rate. Horsepower is calculated using factors of force, time and distance. The key word here is power. One horsepower is equal to the amount of power it would take to lift 550 pounds one foot in one second.
The phrase horsepower came into use in the late 18th century, when steam engines started doing work previously performed by horses, and potential steam engine buyers needed an easy comparison to understand a machine’s capabilities. Although we’re a long way from comparing horses to automobiles, horsepower’s scientific definition continues to be the standard for defining an engine’s power.

In vehicle specifications, the torque and horsepower ratings are usually followed by the @ symbol and a number indicating the engine’s revolutions per minute (RPM). For example, in its specifications, the 2002 Subaru Impreza WRX is listed as having 217 pounds-feet of torque @ 4000 RPM, and 227 horsepower @ 6,000 RPM.

What’s the RPM?

Why factor in the RPM? Because an engine produces its maximum power at a particular rate (or range) of revolutions per minute, and that rate in RPM varies among different engine sizes and designs. Two different engines could have the same rated torque or horsepower, but could reach those ratings at different RPM.

If Engine A produces 200 horsepower at 2,000 RPM, and Engine B produces 200 horsepower at 5,000 RPM, Engine A will accelerate better at low speeds, because it reaches its maximum horsepower at the lower 2,000 RPM. Conversely, Engine B will accelerate better at high speeds, because it reaches its maximum horsepower at the higher 5,000 RPM.

The same rules apply to torque. If Engine A produces 200 pounds-feet of torque at 2,000 RPM, and Engine B produces 200 pounds-feet of torque at 5,000 RPM, Engine A will have more towing or hauling capability at low speeds, because it reaches its maximum torque at the lower RPM. Engine B will have more towing or hauling capability at high speeds, because it reaches its maximum torque at the higher RPM.

A vehicle’s horsepower and torque specifications are often illustrated in a graph, like this one for the 2002 Subaru WRX. The curved lines are plotted using a dynamometer, which measures the engine’s horsepower and torque at various points in the engine’s RPM range. Connecting the points produces the curved lines. The green line indicates torque, and the red line indicates horsepower. The graph’s horizontal scale represents engine speed (in RPM) and the vertical scale represents pounds-feet of torque and horsepower. This chart shows the engine reaching peak torque between 3,000 and 6,000 RPM, and peak horsepower at 6,000 RPM. It also shows a smooth increase in torque and horsepower as the rpm increase.


Different Vehicles … Different Feel

Horsepower and torque work together (along with many other factors of a vehicle’s engineering) to define how a vehicle will perform and “feel” on the road. Subaru has carefully engineered each vehicle in its model line to best reflect buyers’ needs and desires.

The 2.5-liter engine used in the Forester and Outback is designed to provide good all-around performance (including hauling capability) as well as fuel economy. The Impreza WRX model’s 2-liter intercooled and turbocharged engine occupies the high end of the spectrum, engineered to provide more exhilarating off-the-line acceleration and top-end performance.

The full line of Subaru vehicles offers plenty of options. The next time you’re researching a new vehicle, discuss your needs with an authorized Subaru dealer. You’ll be ready to make an informed choice, whether you’re interested in kick-in-the-back acceleration, low-end “grunt,” or a balance of the two.