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What is Horsepower? (Part 2)

December 11, 2012 - When we left off yesterday, we had just established that 1hp is worth exactly 550ft-lbs/seconds. While 1hp might not actually be equal to the power of 1 horse, it is still an exact measurement of power.

          So what does all this mean for your car?

It is safe to say that the more horsepower that a car has the quicker it is going to be able to get from point A to point B. Also, horsepower is directly related to the top speed of the car. But horsepower of the engine is not the only thing that affects the speed of your car. After all, the power of the engine has to be passed through the different mechanical systems of the car in order to get to the wheels. The real question is:

How do you measure the horsepower of a vehicle?

We are bombarded with all these numbers from automakers about the speed and power of a car. But what do they really mean? Where are these numbers coming from? And is the horsepower that they advertise in car commercials, the actual horsepower of the car when you’re driving it?

The easiest way to answer these questions is to begin with an explanation of how horsepower is measured in an engine. Horsepower is measured using a dynamometer. More specifically, a dynamometer measures torque. Torque is a measurement of turning force.

An easy way to think about torque is to imagine a wrench. If you have a wrench that is 9 inches long and you apply 60lbs of force to that wrench to tighten a bolt then you are generating 45ft-lbs of torque. 9/12 x 60lbs = 45ft-lbs of torque. To convert torque to horsepower you multiply the torque measurement from the dynamometer and multiply that by the rpms of the engine/5,252.

So, if the engine is generating 424ft-lbs of torque at 4600rpms your engine would be putting out about 371hp:

424ft-lbs x 4600rpms/5252 = 371.38hp

This equation is how horsepower is calculated for every car.

So, where do we attach this dynamometer to give us the most accurate reading? To the engine? To the wheels? Here in lies the problem of using horsepower to compare the power of two vehicles. Obviously if you measure the horsepower directly at the flywheel it’s going to be different than if you measure it at the wheels. So, which one is more representative of the true power of the car?

Let’s look at an example:

Let’s pretend we are working with a 1998 Chevrolet Corvette with 5.7 Liter V8 engine. When Chevy advertised this car in commercials, they touted a 345hp engine, but when the car was tested by a third party, the Corvette came in at about 285hp.


It all has to do with the way the two groups tested the car for horsepower. The 345hp that was advertised by Chevrolet was not a fabrication; it was just a measurement of gross horsepower.

Gross horsepower is the measurement of engine output taken at the flywheel, without the engine installed in the car.

Without any load (resistance or frictional loss from the other mechanical parts of the car) the entire engine is used for generating horsepower. You could say that gross horsepower is the ideal horsepower of the engine. Gross horsepower is usually measured in a lab, in perfect conditions, with an engine that has been tuned to perfection. But once all the other mechanical parts of the car have been attached to the engine you are never going to get 345hp when you are driving the 1998 Corvette. For the most part, gross horsepower has been abandoned because it does not accurately reflect the horsepower of a car. After all, a car is much more than just an engine.

The reason that the third party clocked the 1998 Corvette at 285hp instead of 345hp is because they were measuring wheel-driven horsepower. Wheel-driven horsepower is measured (yes, you guessed it) at the wheels. First, the car is placed on a roller. The car is then accelerated into the red in 1st or 2nd gear—usually around 6000 to 7000rpms.

The ability of the car to turn the roller—the amount of torque generated by the wheels—allows you to calculate the wheel-driven horsepower.

Because of frictional loss, wheel-driven horsepower is always going to be less than gross horsepower, but more representative of real world driving conditions.

The difference between the wheel-driven horsepower and the gross horsepower will always depend on the vehicle. In the 1998 Chevy Corvette the power generated by the engine only has to go through the transmission and down two short drive shafts to get to the wheels. That is why the loss between the gross and the wheel-driven horsepower is barely over 10%.

About 10 to 15 percent loss of horsepower between gross and wheel-driven is considered standard.

However, to make matters more complicated for consumers, car manufacturers have started advertising the net horsepower. Because critics claim that gross horsepower is an inaccurate representation of the power of the car, they have put pressure of automakers to test the net horsepower of the engine.

Net horsepower is measured at the engine, but with all the necessary peripherals attached, as if the engine were installed in the car.

This is a more accurate representation of the power of the engine, but it still is always going to be higher than wheel-driven horsepower by about 5% to 10%.

If you are looking for a car and horsepower is something that is important to you, make sure you know whether the advertised horsepower is gross, net, or wheel-driven. You want to be sure when comparing two cars you are comparing apples to apples. Automakers don’t exactly make it easy to figure out which one it is, hence all the confusion across the internet about what these numbers really mean. If you have any doubt about how the horsepower was measured in a particular vehicle, just call the dealership. As far as I know, gross horsepower is supposed to have been abandoned on all cars produced after 1972, but there are rumors circulating around the internet that US car companies don’t always abide by these. My best advice is to not trust what you see in car advertisements—there is usually some type of fine print.

If this all seems confusing, it is because the auto marketplace is not designed with clarity in mind. Information is your best weapon for saving money.

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