Horsepower: Gross vs. Real-World

If you've spent any time around cars or car people you're familiar with the term horsepower. And while you may have a general idea of what horsepower refers to, its exact definition and how that definition plays out in the real world can be a bit confusing. Specifically, the terms "net horsepower" and "wheel driven horsepower" are commonly seen in magazine road tests or vehicle spec boxes. But what exactly do they mean and what, if any, difference is there between the two?

We'll get its basic definition out of the way first and then move on to the difference between net horsepower and wheel-driven horsepower.

Horsepower is officially defined as "the amount of energy required to lift 550 pounds, one foot, in one second." This definition dates back to when cars didn't even exist and, despite the worldwide acceptance of the metric system (America not withstanding) in which feet have been replaced by meters, the definition remains in use today.

Generally speaking, horsepower is an indication of how capable a vehicle is at moving itself around and, consequently, the more horsepower a vehicle possesses, the quicker it can move between two points. In addition to determining how quickly a car or truck can move, horsepower has a direct relationship to top speed. As a vehicle's speed rises, additional horsepower must be expended to push it through the atmosphere. Of course, aerodynamics play a major role in top speed as well and most of today's performance cars owe as much to aerodynamics as they do to horsepower for their ability to go 150+ mph.

So if the definition of horsepower is so straightforward (550 pounds moved one foot in one second), how can the same vehicle have different horsepower ratings? For instance, Chevrolet rated the 1998 Corvette's 5.7-liter V8 at 345 horsepower. Yet test in HOT ROD magazine found the car to have 285 horsepower. So what gives?

The disparity between these two figures comes from the fact that one is a gross horsepower rating and the other is a wheel-driven horsepower rating. Gross horsepower is a measurement of engine output, taken at the flywheel, without the engine installed in a vehicle. Since the engine has no load on it, all of its energy can be used for making horsepower.

Wheel-driven horsepower, by comparison, is a measurement taken at the driven wheels of a vehicle on what's called 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 to come up with a figure that represents how much horsepower is actually available to move the vehicle around -- or real-world horsepower. Because a frictional loss between the engine and the driven wheels is unavoidable, wheel-driven horsepower will always be less than gross horsepower.

How much less wheel-driven horsepower will depend on how many mechanical parts exist between a vehicle's engine and its driven wheels. Since we already mentioned a late-model Corvette, we'll use it as our example. The 345 horsepower that initiate in the Corvette's engine compartment have 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 `Vette's horsepower before the rear wheels even begin to turn. Suddenly, the 285 wheel-driven horsepower rating doesn't seem so hard to believe.

On front-wheel drive cars with transverse-mounted engines, the frictional loss is often less severe because horsepower only has to travel from the engine, through the transmission and down two short driveshafts before reaching the wheels. It should be noted, however, that in the same HOT ROD article, a front-wheel drive Dodge Neon R/T, rated at 150 net horsepower, could only muster 117.1 wheel-driven horsepower when placed on a dynamometer.

Obviously, another factor to consider in this discussion is how accurately the manufacturers rate their own engines. It's hard to believe that the Corvette and Neon could lose 17 and 22 percent of their engine's horsepower in just frictional losses. Most experts agree that a 10 to 15 percent loss is normal. This does not necessarily mean that the manufacturers are lying. The additional drop could come from a number of causes including variations in engine performance and dynamometer readings. It could also be that the manufacturers are dealing with an expertly tuned engine's running under controlled conditions in a laboratory rather than a mass-produced engine that was put together on an assembly line and sold at a dealer's lot.

Whatever the cause, it's important to remember that there will be a loss of horsepower between the engine and the rear wheels. If you really want to know how powerful a vehicle is, test drive it. If you're looking for an accurate horsepower figure, a dynamometer run is the only way to know for sure.