Horsepower and Torque… what is it exactly?

When I was a kid I learned about horsepower watching Mustangs on western films. I also saw those magnificent Budweiser team draught horses pulling around massive beer wagons . Even being so young it was clear that there was two kinds of horses, one built for speed and the other was built for brute strength.  It was like watching a gazelle vs. an oxen.
Then of course, growing up in San Diego drawing cars and having friends introducing me to the mega strengths of drag racing versus the nimble cornering of autocross. I distinctly recall funny looking notebook sketches of Detroit Mopar cars with fat rear tires and barely fitting V-8 engines driven by pimpled, helmeted, bloodshot eyed, big nose, fanged tooth, glove wearing and flapping tongue characters screaming past red lights doing wheelies.
These drawings, and I made a few of them, was about horsepower and torque. At that time, it was even more extraordinary to see anything related to Porsche.


Energy output can be measured in light, electricity and mechanics.
All mechanical cars, trains, planes, motorcycles, and more can have  a measured energy output. These measurements are calculated by using a formula which makes a relationship resulting in watts.




In order to understand how the formula works you need to clearly understand “what is a watt?”. Electrically, a Watt is a measure of electric power that depends on amps and volts. The bulb in the middle (yellow / hotter glow)  makes the most light because it uses more watts than the other two (reddish / cooler glow) . But notice that the bulb on the right is using the same amount of power as the bulb on the left, even though it’s using only half of the current. Watts = Volts x Amps

In terms of newton physics, if you can imagine two forces facing opposite directions pulling away from each other.

To set a constant in measurement, Issac Newton rigged up two horses facing away from each other. Both horses were the same size, and weight. The rope also was attached to the horses in same ways. Issac knew something about a principal known as reaction forces. For example… if you placed a stone on a surface not able to resist its weight (or better) it’s going to sink. The issue he had tried to come up with a constant for motion. So he came up with an arbitrary unit distance of the day the meter.

What Issac did was to paint a red stripe directly on the rope. Having meter measurements chalked off on the grass, and able to measure the energy exerted from the horses; when it was that yank happened at a constant rate of “one meter per second” that energy release would be called a one newtons.

In terms of classical mechanics, one watt is the rate at which work is done when an object’s speed is held constant at one meter per second against constant opposing force of one newton.

If you are still confused; think of how fast an English explorer would sink into quicksand. That constant rate of energy, would be similar to a newton.

Horsepower (HP) is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.

Horsepower was originally defined to compare the output of steam engines with the power of draught horses. The unit was widely adopted to measure the output of piston engines, turbines, electric motors, and other machinery.

The definition of the horsepower also has varied between different applications:

The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per second is about equal to 745.7 watts. It is the foot pounds per second that makes the horse power, not just speed without mass.


Torque is a measure of how much a force acting on an object causes that object to rotate. The object rotates about an axis, which we will call the pivot point or Fulcrum. We will call the force ‘F’ the cause of torque. The distance ‘d’ from the Fulcrum to the point where the force acts is called the moment arm. The direction of rotation, about the Fulcrum pulled by the Force is torque.

Figure 1 Definitions Torque is defined as = d x F = d F sin().

Easy calculus lets you plug-in the sin-x angle of the expected spin (such as .1 degrees, 20 degrees, 300 degrees, or even 1080 degrees) to give you torque.

In other words, torque is the cross product between the distance vector (the distance from the pivot point to the point where force is applied) and the force vector, ‘a’ being the angle between r and F. (Imagine a small triangle)

Using the right hand rule, we can find the direction of the torque vector. If we put our fingers in the direction of r, and curl them to the direction of F, then the thumb points in the direction of the torque vector.

Imagine a tire spinning before it catches to move the car forward. The force of your gas pedal pushes (F) which causes the tire to rotate about its axle (the pivot point, Fulcrum. How hard you give it gas depends on the spin of the tire before it catches to the pavement, amount of stretch the rubber might have, amount of energy stored in the wheel, and several other factors. The closer your tire/wheel the axis (i.e. the smaller r is), the more gas you are going to need to give the engine to get it to push forward. The torque you created on the engine is smaller than it would have had larger tires, away from its axle.

Note that the force applied, F, and the moment arm, r, are independent of the object. Furthermore, a force applied at the pivot point will cause no torque since the moment arm would be zero (r = 0).
Another way of expressing the above equation is that torque is the product of the size of the force and the perpendicular distance from the force to the axis of rotation (i.e. the pivot point).


Taking two cars and try to analyze the horsepower and torque can be pretty easy to then measure which one would cross a finish line first. The first two group of cars is a 1975 911 and 1978 911 Turbo.

Try guessing which would come in first from the specification numbers. Both cars weigh the same, both cars have same size engines at 2994 cc, both drivers leave the starting line at exact the same reflexed time. The car in LANE (1) is older and has a turbo.


LANE (1) – 1975 911 Turbo 3.0L produces: 260 hp @5500 rpm and 254 lb ft

LANE (2) – 1978 911 SC 3.0L produces: 180 hp @ 5500 rpm and 190 lb ft

HEAT 1 – WINNER: The Turbo in LANE (1). My guess is that if you 100 feet of track… The LANE (2) car would be 30 feet behind LANE (1) car. Easy? In then next heat, try three different cars.


The second group will be Alfa Romeo. Try again to guess which will win.

LANE (1) – 1979 Alfa Romeo GTV-4 Turbo I4 – 1,962 cc – 150 hp (110 kW) @ 5500 rpm and 231 Nm (170 lb.ft) at 3500 rpm

LANE (2) – 1979 Alfa Romeo GTV-6 V6 – 2,492 cc 0 160 hp (118 kW) @ 5,600 rpm and 213 Nm (157 lb.ft) @ 4000 rpm

LANE (3) – 1979 V6 Callaway Twin Turbo 2,492 (171 kW) @ 5,600 rpm and 332 Nm (245 lb.ft) at 2500 rpm.

HEAT 2 – WINNER: Will be the Callaway in Lane (3). If you can understand why … notice that torque is WAY higher at a much lower RPM than the other two cars. The Callaway would be expected to smoke the competition. The other two cars will come in almost split seconds from each other. If you wanted to do the math, you could divide out the torque and the horsepower to make a pretty good guess. (Lane 1)

Can you calculate who would win between winners of heat 1 and heat 2?

So the next time you want to get a better idea of just how much more torque and horsepower some of these very intense automobiles out there, get the specs on your own car and make a comparison.


19 responses to “Horsepower and Torque… what is it exactly?

  1. Pingback: Electro Drive v.8 (2011)

    • Quite possibly one of the most written about topics regarding how physics can move hunks of steel around, but also one of the most difficult to fully understand. I hope I put on some light regarding it.
      Maybe I need to write something when or how too much is too much?

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  4. Your horsepower blog helps me really understand this concept in a nice way . Excellent information it helped alot figure out new and exciting ways to understand my car. I will definatly come back and read other blogs abauerporsche writes. Now I want to get my Porsche serviced here.

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