Driving Impressions and Tech Talk

What does HP have to do with horses?
David Cipolloni
January 29, 2001

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This strange term, horsepower, has caused many people to wonder exactly what the meaning of the term is. We all know that more horsepower is a good thing and the term is commonly assigned to car engines, bench grinders, vacuum cleaners, lawnmowers, etc. But how we ascertain exactly what horsepower is and how we measure it remains a mystery to many. 

Let's start by placing the credit/blame on a fellow by the name of James Watt. This Scottish engineer/inventor was credited with many inventions during the mid to late 1700's, especially with his work relating to improvements of steam engines. Watt was determined to provide a way of measuring the rate at which an engine could produce mechanical work. While observing horses being used to haul coal from mines, Watt had the idea of using them as a measurement of work performed. The term Horsepower was coined and is usually abbreviated hp. One Horsepower is the electrical equivalent is 746 watts, and the heat equivalent is 2545 British thermal units per hour. 

One horsepower was originally defined as the amount of power required to lift 33,000 pounds 1 foot in 1 minute, or 550 foot-pounds per second. This strange way of measuring work enabled Watt to explain to buyers of steam engines how much work the engine could do for them. In Watt's judgment, one horse can do 33,000 foot-pounds of work every minute. So imagine a horse raising coal out of a coal mine. A horse exerting one horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1000 feet in one minute, or 1,000 pounds 33 feet in one minute, etc. It was this original relationship between horses and mechanical devices that led to a system of measurement that continues to be used today. Three different horsepower values are used to quote the power output of an engine: (1) Indicated horsepower is the theoretical efficiency of a reciprocating engine, which is determined from the pressure developed by the cylinder(s) of the engine; (2) brake or shaft horsepower is more commonly used to indicate the practical ability of the engine, or the maximum power, which is the indicated horsepower minus the power lost through heat, friction, and compression of the internal workings of the engine; (3) rated horsepower is the power that an engine or motor can produce efficiently for sustained periods of time.

Typical HP vs. Torque curve

The problem with measuring horsepower these days comes when trying to translate that linear motion of the work performed by the horse, to rotating motion performed by the internal combustion engine. Since the crankshaft of an engine is spinning, there needs to be a way of measuring the force of this rotation. To do this we apply a load to the crankshaft in an attempt to slow it down. This force against the load produces a measurement called Torque. Torque, or turning force, can be understood easily by thinking of a socket and handle (breaker bar) at work. Imagine a socket with a 2 ft long handle attached and a force of 50 lbs applied to the end of it. The torque at the nut or bolt would be 100 ft lbs. Likewise, if you applied 1 lb of force at the end of a 100 ft long handle you would achieve the same 100 ft lbs of Torque at the nut. 

To measure the amount of torque (twisting force) an engine will produce we use a dynamometer. The dynamometer utilizes a water brake to apply a load to the crankshaft while the engine is running. With the engine running at full speed the load at the crankshaft can be increased to slow the engine and measure the amount of torque the engine is capable of producing. By doing this a technician can create a computer generated graph which shows a curve indicating the Torque output throughout the engines operating range. So far we have measured the speed of the engine (RPM) at the crankshaft and the Torque (lb ft) output at those various speeds. So how do we end up with horsepower? 

To determine a horsepower rating we do a simple calculation. We simply multiply the torque by RPM / 5252. For example: If a given engine produces 300 lb ft of torque at 5,700 RPM we simply multiply 300 X 5,700 and then divide by 5252, we end up with 325.59 horsepower at that speed. Here's an interesting bit of trivia; below 5252 rpm any engine's torque will always be higher than its horsepower, and above 5252 rpm any engine's horsepower will always be higher than its torque.

Using a data acquisition computer connected to a dynamometer we can plot a curve indicating the horsepower output at all engine speeds. We now have a graph showing engine speed, torque curve and horsepower curve. Another thing you can see from a car's horsepower curve is the place where the engine has maximum power. When you are trying to accelerate fast, you want to try to keep the engine close to its maximum horsepower point on the curve. That is why you often downshift to accelerate, by downshifting you increase engine RPM, which typically moves you closer to the peak horsepower point on the curve. 

How this all relates to the average driver can be observed in the torque output of an engine and less in the horsepower output. At slower engine speeds the torque output of the engine will be higher than the horsepower output and you will feel this when the car accelerates. For those of you that remember the big displacement engines so commonly found in some of the Detroit cars of the 60's will recall how easy it was to turn the tires into burning rubber at the stoplight. These engines, in some cases, produced in excess of 400 lb ft of torque at very low RPM levels. Since an engines horsepower output typically increases as engine speed increases we need to get the engine turning faster to utilize the horsepower (work) output. 

As engine speed continues to increase, the torque output will start to fall off and the horsepower output will continue to rise. However, a vehicle's acceleration rate will closely follow it's torque curve. From this we can understand why we would want an engine to produce the maximum amount of torque at the highest possible RPM level. Here is a simple way to understand this relationship: If you take your 10 speed bike and try to start up a steep hill in 10th gear you will barely be able to start peddling. On the other hand, in first gear you can easily get started. The problem is that in first gear you are peddling like mad and not going very fast, so you start switching gears at the appropriate times to optimize your power/speed output. If you shift up too soon you will begin to slow and if you shift too late your speed will stabilize and your friends will start to pull away. This is why automotive engineers will try to match a vehicles gearing to it's torque curve in order to optimize the acceleration rate of the vehicle. Remember 3 on the tree? then came 4 on the floor, followed now by 5 speed and 6 speed transmissions. 

Just remember, we don't measure horsepower, we measure torque and calculate horsepower from it. So remember to look at those torque figures and gear ratios when you consider your next vehicle and what you expect from it. Maybe we can do a little comparison shopping for you in the future. We hear there could be a new S2000 in our future, maybe we will take a look at a few of the Honda tricks of the trade.

Maximum acceleration at any speed occurs at the HP peak.
 Maximum acceleration in any gear occurs at the torque peak

Comments can be sent to the author at contacts@autoracing1.com.



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