Ask Away with Jeff Smith: What is Horsepower (Really) and How is it Quantified?

We measure speed in miles per hour and torque in pounds for every foot of leverage, but what the heck is a “horsepower” and how is it quantified? Is it really the power of a horse? What would that horse be doing and for how long? I’m having a problem with this for years and I finally decided I need to know.

J. A.

Jeff Smith:  We all tend to take references for granted, so let’s crawl down the rabbit hole, shall we? We promise we won’t go so deep that we can’t find our way out! The usage of the term horsepower started with a guy by the name James Watt and yes, the electrical term watt is named after him. Way back in 1775, Watt was a Scottish engineer who figured out a way to radically improve the performance of the steam engines of the day. As he went in search of customers, he realized he needed a way to equate his engine’s power to something that people could easily relate. Large draft horses were the power source of choice for most farming and industrial applications, so Watt took it upon himself to equate his steam engine’s power to a draft horse. Our good Mr. Watt determined that a draft horse could turn a 12-foot radius mill wheel at the rate of 2.4 times per minute. Doing a few calculations (we’ll spare you the boredom of review), Watt came up with a rounded-off figure that the draft horse could produce 33,000 lb.-ft. of torque per minute.

Once we convert this to pound-feet of torque per rpm, this equates to 5,252. So the classic definition of horsepower became:

Horsepower = Torque * RPM / 5,252

As an example, if our 540 cubic inch big-block Chevy engine makes an impressive 675 lb.-ft. of torque at 5,000 rpm and we plug that into our formula we get:

HP = 675 * 5,000 / 5,252

HP = 3,375,000 / 5,252

HP = 642.6

So our Rat makes 642 horsepower at 5,000 rpm. But let’s look a little deeper. All engines produce a twisting motion that we call torque. But this twisting motion does not take into account the time it takes to produce that effort. Horsepower is, by definition, a measurement of the amount of force (torque) over a period of time, which in our equation is rpm. As defined by the equation given to us by Mr. Watt, it appears that even when we make less torque, if we do so over a shorter period of time (higher rpm), then we can make more horsepower. This is all true.

You should also be able to see by the equation horsepower and torque will be always be exactly the same at 5,252 rpm. So if our engine made 650 lb.-ft .of torque at 5,252 rpm, then it will also make 650 horsepower. That’s why the torque and horsepower curves always cross on a graph at 5,252 rpm.

All internal combustion engines have one particular point where they are the most efficient and that also happens to be peak torque. This is the point when the induction and exhaust systems are operating at their peak efficiency. This is almost always where the engine experiences peak volumetric efficiency (VE). This means that the engine is able to contain the most amount of air to produce a maximum torque number. Sometimes, highly refined race engines can actually broaden this peak torque value over a span of several hundred rpm. Below this rpm, the combination of low intake port velocity and cam timing are such that the engine cannot fill the cylinders with enough air to make the same torque. Conversely, at engine speeds above peak torque, while there is sufficient port velocity and perhaps sufficient valve opening duration, there is increasingly less time because of the higher engine speed to allow the cylinders to achieve proper cylinder filling. This means that the VE begins to fall off, which means the torque also diminishes.

An engine’s power is often defined by its power curve, which is defined as the rpm spread between peak torque and peak horsepower. Generally for street V8 engines, the power curve will often be 1,500 to sometimes 2,000 rpm. That means that if our normally aspirated engine made peak torque at 4,500 rpm, we can safely assume that peak horsepower could occur somewhere around 6,000 rpm to perhaps as high as 6,500 rpm. A wider power curve is always advantageous.

You can probably see that beyond peak horsepower, the torque drops off so rapidly that horsepower falls. Continuing with our big block Chevy example, let’s say that our 650 lb.-ft. number occurred at 5,000 rpm and that the torque began to fall after this rpm so that it was only making 600 lb.-ft. at 6,500 rpm. That would produce a peak horsepower number of 742. But let’s say that now we change cylinder heads and camshaft and now peak torque remains the same but the engine now makes peak torque at 5,750 rpm, which would produce 711 horsepower at peak torque. The dyno test also shows then that we produced a power band that’s a little narrower than the previous test and that peak horsepower occurred at 7,000 rpm and that the engine was still making that 600 lb.-ft. of torque. Now our peak horsepower number is 799.69 that we’ll round off to an even 800 horsepower.

The point of this exercise is to show you that one way to make more horsepower is to simply spin the engine at a much higher rpm. We are making the same torque but we make more horsepower because we are doing this work in a much shorter period of time, by means of more revolutions per minute. That means that we are creating more cylinder firing pulses per minute, which does more work in a shorter period of time.

This sounds really easy, right?. Except that simple physics gets in the way. When you decide to spin the engine at a higher rpm, g-forces in the piston and crank assembly grow astronomically as do the forces on the valvetrain. This means you must use extremely high quality parts to be able to withstand these g-forces. Plus, cylinder head, camshaft duration, valve sizes, compression, and a host of other changes are necessary to be able to produce power at these very high engine speeds. Generally speaking, small displacement engines with shorter strokes tend to withstand these higher engine speeds much better. Overhead cam engines are also better suited to extreme engine speeds, which is why the new generation Ford Mod engines with their single and dual overhead cams are very good at making power at higher engine speeds because A) four valves breathe better than two valves and B) these engines have a more stable valvetrain when the cam is placed on top of the valves, eliminating troublesome pushrods. But then, despite the apparent limitations of pushrod engines, those NASCAR boys are able to make amazing power with a 358 cubic-inch engine at nearly 10,000 rpm and make them live for 500 and 600 mile races.

Hopefully this little jaunt down horsepower lane has been enlightening. If nothing else, it’s bound to give you a better appreciation for the kind of power current engine technology can produce.

It’s fascinating.