What Is Wheel Horsepower & How Is It Different From An Engine's Standard Horsepower?

An engine's power output is a vital piece of knowledge, and a great marketing tool. Since the dawn of the steam engine, the power of internal combustion engines has been calculated in horsepower. It still is in many parts of the world, though some use metric measurements now. Obtaining that measurement might seem tricky, especially if your vision is 455 horses yoked together to try and win a tug of war with a Buick. However, there are mechanical means for obtaining this measurement, and even a way to estimate it with an app!

Actually, horsepower is calculated from torque, in pound-feet or Newton-meters, at a certain number of rotations per minute. That's because the measuring device is connected to something that is rotating — such as a tire or the crankshaft — and the resulting rotational force is what is measured. The amount of torque, over time, produces power measured in watts, and one horsepower equals approximately 746 watts. Whether the device is connected directly to the crankshaft or not is important either, because every element between them introduces power losses.  

Crank horsepower is easy to understand, but difficult to measure. First, the easy to understand part. Crank horsepower is the amount of power coming out of the engine right at the end of the engine itself. Typically, anything that would reduce that power, such as air conditioning, is disconnected or removed. 

The engine is bolted to a piece of test equipment called an engine dynamometer. Most of these dynamometers use a fluid, friction or electromagnetic field creation to measure the torque put out by the engine at specific RPMs. Once these are known, horsepower can be calculated, with the horsepower equaling torque multiplied by RPMs, then divided by 5,252.

Measuring crank horsepower is easy, but only if the engine is not installed in a vehicle and if an engine dynamometer is available. However, if it's in a car, or you're out on a track in the middle of England, the engine would have to come out and be prepped before being put on the test stand. For this reason, other ways of measuring horsepower are needed.

For those occasions where it is impractical to put the engine on a stand by itself, there are other ways to measure horsepower. It can be measured at the wheels with a chassis dynamometer. These devices measure the speed of the drive wheels as well as the engine RPMs. This is a less accurate way of measuring an engine's power compared to an engine dynamometer, because the drivetrain between the crankshaft and the wheels introduces system losses. Still, the convenience of driving onto the dynamometer, measuring the result and driving off also saves a lot of time and money. 

Chassis dynamometers can be portable, as the BBC's "Top Gear" showed in the cheap Italian supercars challenge. The dynamometer they use is brought in on a flatbed truck. Does it matter whether horsepower is measured at the crank or at the wheel? You could say that the latter better shows what is happening where the rubber meets the road. If you're adding upgrades to a HEMI engine and it's out of the car anyway, though, then an engine dynamometer run can be worthwhile.