Perhaps the most used term in describing a vehicle's performance is horsepower. In this article, we'll dive into what exactly this term means, how it compares to its companion term “torque,” and how we can best look at these to glean insight into real usable performance.
The Definition of Horsepower
The term "horsepower" has been around for quite a while. In 1702, Thomas Savery made reference to the potential work that a horse can do as a measurement of power. In his book The Miner's Friend he writes, "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work..."
James Watt later made reference to the potential work that a horse can do (horsepower) as a way to market a new and improved steam engine. He determined that a horse can do 33,000 foot-pounds of work in one minute (the equivalent of pulling one pound of weight 33,000 feet in distance).
The Definition of Torque
Torque is the tendency of a force to move around a point. In other words, torque refers to twisting force. The unit of measure for torque that we all know, the "ft-lb," "lb-ft," or "foot-pound" is the amount of turning force applied to move one pound a distance of one foot around an axis at a radius of one foot.
So, one full rotation around an axis at a radius of one foot with one foot of resistance yields the following amount of work:
work = (2 * pi) * 1 lb-ft = 6.2832 lb-ft
Where Torque and Horsepower Converge
As you can see above, horsepower is a measure of work/time, where torque is a measure of work. So, if we use the 6.2832 lb-ft per revolution that we came up with above, we can now determine the RPM (revolutions per minute => work / time) to find out at what RPM we would have one horsepower with 1 lb-ft of torque. Let's see what we come up with:
33,000 lb-ft/min / 6.2832 lb-ft/revolution = 5252 RPM
Even though we used the case of one lb-ft of torque and one horsepower to find where these two numbers converge, the horsepower and torque numbers will always be the same at this RPM. Test this statement. Look at a number of different horsepower and torque graphs. You'll find that on all of them, the torque and horsepower lines will all cross at this point. See an example below for the ROUSH Performance M90 supercharger.
What This Means
As we proved above, horsepower is simply an extrapolation of torque applied over time. When an engine is measured for its power potential on a dynamometer, horsepower and torque are not measured as separate entities. Rather, torque is measured, and horsepower is then calculated given the torque at the specific RPM level.
Car owners often use "horsepower" as the end-all be-all rating for engine performance. This perspective is flawed. First of all, when you hear of a car having X horsepower, it only refers to the peak horsepower on the dyno graph. Secondly, it doesn't indicate what the shape of the torque curve is. You can feel the torque that an engine generates as you're pushed back into your seat.
The Case of the 2011 Ford F-350 6.7L
One example of how misleading the measure of horsepower alone can be is the case of a diesel truck engine. Below is a dyno chart for a 2011 Ford F-350 6.7L V8 engine:
As you can see, this engine is a monster with over 640 lb-ft of torque at low RPMs. However, because the torque curve falls off before the torque and horsepower convergence point that we found above (at 5252 RPM), the horsepower remains well below the torque level for the entirety of the power band. Does the relatively low peak horsepower of 307 indicate that this engine has little power? Absolutely not. The truck engine above isn't intended for running at high-RPMs. It's running RPMs are right in its appropriate power band, and it has a high level of pulling power off the line, which is ideal for towing large amounts of weight.
When you hear someone refer to a high torque engine, what they really mean is that the torque band starts high at low RPM (and probably drops off early in the RPM continuum relative to engines not considered high in torque).
The Case of the Horsepower Bastard
On the other end of the spectrum, we see horsepower numbers that are unreal, but because the torque curve only favors the high RPM band, the usable power generated by the given engine is much less than the horsepower number would lead you to believe. You would see this where there isn't a steady torque curve across the usable RPM but rather an upwardly slanted torque curve the strongly favors higher RPMs. You can see this with forced induction systems that take an inordinately long time to spool up.
A Better Perspective on Power
A good rule of thumb to use when trying to get a sense for real usable power is to look at the shape of the torque curve. For performance applications, like a modified Ford Mustang, the shape of the curve should be fairly constant over the RPM band that is intended for use. You want to maximize the area underneath this curve across the RPM range. There really needs to be a better unit of measure for determining usable engine power. One possibility would be to come up with an average torque rating across this band. Another would be the area under the torque curve. Of course there are many other possibilities, but you get the idea.