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Torque vs. Horse Power: What’s the difference and why is “rear wheel” HP different from what the manufacturer quotes?

This article discusses some basic measurements and terminology and how they apply to us performance enthusiasts. There are some basic questions that we need to answer and to make clear what specific terms mean and when they apply. We will address two fundamental questions in this installment:

  1. What is the difference between Horse Power and Torque?
  2. What’s the difference between “flywheel” horsepower (or torque) as quoted by the car manufacturer and “rear wheel” horsepower (or torque) which is what a dynamometer (dyno) measures and is what we use in the Dynoperformance data base?

To answer question 1 we will need some basic physics, but don’t worry, we will only discuss things conceptually and keep it simple. Question 2 will then be answered very quickly once we get #1 out of the way.

When most people think about a car’s “power” they think of horse power. If your cousin were getting a new Mustang Cobra he would call you up and say “this thing has 390 horse power” at which point you would say, wow. So, what is “power”? Power is defined as the rate of doing work, which may seem too technical but is really quite simple if you re-phrase it to mean the rate of using energy. Energy is easier to think about. Take the following example. Let’s say I run the 100 yard dash. I am going to be sprinting and using up energy at a high rate. At the end of the sprint I will be panting. However, if I were jogging the 100 yard distance, I might only be a little winded at the end and that’s because my rate of using energy was much lower.

Now, the actual unit that has stuck in our society for power is horse power and that is because we were used to it. Back in the day when everyone had horses you knew how heavy of a load a horse could carry over some distance at a particular speed. When we switched over to engines it was “logical” to use horse power to describe the power output of the engine. The average person had no conception of physics or the actual energy units involved so it was easier to convert it to horse power since everyone had a horse. The definition of one horse power is the ability to lift 550 pounds one foot in one second or 33,000 lb-ft/min (550 lbs x 60 seconds).

It turns out, however, that we cannot measure horse power directly, but we can measure its affects, in this case torque. Torque is defined as the amount of force applied over a radius to turn something. For example, let’s say I had a 16” rim and I applied X amount of force to it at the edge, I would cause a torque on it that would cause it to roll. You can also think of trying to tighten a nut with a wrench. If you hold the wrench at the very end it is easier to move the nut versus holding it one inch from the nut. This is because you generate more torque at the end with the same amount of force than you do at the base. Some of you have brought your cars to the dyno to measure its power. The dyno has these massive drums that have a fixed resistance to the turning of your tire on top of it. From this resistance, the other parameters of the dyno, and the speed at which your car can turn it (there is some other stuff involved) the dyno measures the torque of your car. This torque can then be converted to horse power by multiplying by the RPM and dividing by 5252. Why 5252? That is the number that results when you divide 33,000 lb-ft/min by 2π which gets us a rotational value (remember that the engine is turning something). This is a universal equation and can be used to go back and forth between horse power and torque.

Horse Power = Torque x RPM / 5252.

This is the equation that we use in the Dynoperformance database to get horse power numbers and the equation that the dyno software uses to convert the torque to horse power. Once you know the torque, you simply apply the above equation and you have horse power. Now, some of you may say that 5252 sounds familiar. That is because it is the RPM at which horse power equals torque. Check out the graph below. This is always a good thing to remember, because if you ever see a horse power and torque curve that do not intersect at 5252, something is wrong with the data.


So, how does rear wheel horse power and torque differ from the above? It turns out that the power that is actually propelling your car goes through the drive train to your tires (rear tires for rear wheel drive etc.). Along the way from the engine to the tires, since nothing mechanical is perfect, you lose some energy which reduces the power that is actually leaving your tires to move your car. This is why you usually want to know the power to the rear wheels, it gives you a direct comparison point for performance because it factors out the efficiency of the drive train or any other car specific system. For example, one car might have 300 horse power leaving the engine – or flywheel -- but lose 15% of it to the drive train. Another car might have the same power at the engine but lose 18%. So, if both cars were otherwise identical (weight, tires, etc…) the car with the 15% loss would have more rear wheel horse power and torque and therefore better performance. See article #5 for a discussion about dynos and why we usually use rear wheel measurements instead of flywheel.

To sum up and review we have the following points:

  • Power is the rate of using energy
  • One horse power is the ability to lift 550 lbs one foot in one second
  • We can’t measure power directly, so we measure torque instead we convert torque to horse power using the equation HP = TRQ * RPM / 5252
  • From the equation we know that horse power equals torque at 5252 RPM
  • No car has perfect efficiency so we typically want to know the power at the wheels which is what is actually moving us.

I have simplified some things in this discussion for the sake of getting the point across. If you have a question about a particular aspect just send me an email at pantazis@dynoperformance.com. In future articles we will discuss how to interpret horse power and torque curves.






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