Understanding the relationship of torque and power


 

Torque and power

Sadly this is one of the most overcomplicated and misunderstood concepts in the automotive world. Sad because they are both very simple concepts that anyone capable of middle school level math can understand if they step back and look at simple facts instead of this convoluted mess the automotive world has turned it into.


 

Car commercials brag about cars or trucks and how much torque they make. People still get in arguments about which is better for racing or for towing or whatever else. The irony here is people arey trying to compare two things that cannot be compared. They are two completely separate and different things.


 


 

So where did this all start?
I would say it most likely started when car manufacturers started providing torque and HP figures to motors to market them. Since it was likely not practical to provide dyno graphs or expect the public to understand or care about them they needed a simple way of conveying how much work a motor could do. They started using peak power to show it's peak potential. This doesn't however give a very good idea of how the motor performs over the entire powerband so the car companies would add it's peak torque figure and if they were smart the RPM at which both peak HP and peak TQ were made. If you understand the relationship of torque and power this gives you the ability to graph the powerband and have a rough idea of how the motor should perform across that range. People who didn't understand the relationship of torque and power started to assume that the torque figure actually referred to how much work the motor could do in the lower RPM since the torque figure was always in lower RPM and the HP was always in higher RPM. Over time the automotive world built this up to mean something it doesn't and eventually it got built up to the point where car manufacturers car magazines, car people and the greater majority of everyone all use torque in reference to low end power. The truth is car manufacturers and magazines should be too embarrassed to use this bad math. Real car people should know better and laugh at less educated people who believe this to be true and then make every effort to educate them on the proper physics. Someone who understands cars and motors should be proud to know the good math and when they hear someone using torque or power incorrectly it should be a sign of lack of education and or experience. Instead this bad math has been embraced and accepted by the automotive world.


 

I am writing this today because I am embarrassed for us . We should all be embarrassed that if a 7th grader actually took a second to step back and look at the math they would think the whole automotive world consisted of a bunch of cave men who cannot even use proper terminology or understand the insanely simple equation that ties this all together.


 

Let's start with the basics.

Torque is a twisting force around an axis. When you torque your lug nuts you are applying torque to the lug nut by twisting the tire iron or torque wrench around the axis of the nut.


 

Power is a measurement of work. It represents how much work can be done in a given amount of time. When it comes to internal combustion engines that work is a factor of torque and RPM. Let me stress the importance of RPM.


 


 

If you weigh 200 lbs and stand on a 2 foot breaker bar you will apply 400 lb feet of torque to a bolt. If that bolt is already tighter than that you will stand on the breaker bar and it will not move. Right now you are applying torque but no work is being done. You are making zero horsepower. Now if you get a four foot breaker bar and stand on the end of it you will apply 800 lb feet of torque. Now the bolt will start to move but remember that for every inch the four foot breaker bar moves the two foot breaker bar would only need to move a half inch. Now that your leverage is doubled so is the distance it has to travel.


 

Let's look at how this applies to a motor that is doing the work.

If a motor can apply 100 lb ft of torque at 1000 RPM it will do half as much work as a motor that can apply 100 lb ft of torque at 2000 RPM. Here we have a motor that produces the same amount of torque but will do twice the amount of work. If that work is lifting something it will either lift the same amount of weight twice as fast as the motor that makes 100 lb ft at 1000 RPM. Alternately you could run it through a 50% reduction gear box. Now it will be able to lift twice as much weight at the same rate as the 1000 RPM motor.

Despite producing the same amount of torque one produces twice as much power and can therefore do twice as much work.

This is where the unit of torque gets abused. How much torque something applies is useless until you give it an RPM at which it does it. At that point you have power such as HP or KW or any other measurement of work. Without that it is meaningless.
Here is an example. Two big truck company commercials are bragging about their heavy duty motors. One company claims their truck makes 450 lb ft of torque. The other company claims their motor makes 600 lb ft of torque. This doesn't tell you anything about how they will perform.


 

What if the truck that makes 450 lb feet makes that at 3000 RPM?
What if the truck that makes 600 LB feet makes it at 1500 RPM?


 

Well the first truck making 450 lb ft makes 257 horsepower at the same 3000 RPM.


The second truck making 600 lb ft only makes 171 HP at 1500 RPM.


 

I know this is hard for those people stuck on the importance of torque but figuring out which motor will do more work at that RPM should be quite simple.


 


 

So why do people stress the importance of torque when towing.
Again it's a misnomer for low end power and in the end it really has nothing to do with how well it will tow.

Let's take the motor that makes 450 LB ft at 3000 RPM. Now remember that is 257 HP.

Now let's build ourselves a little 1.6 liter motor designed to make 257 HP at 12000 RPM.


 

Assuming both motors are attached to transmissions with the right gearing they will both haul a large load equally well. It's not actually hauling the load that makes the engine with more low end power superior. There are two primary factors that do.
The first is that with the low RPM motor you don't have to rev it as much to get going from a stop. A clutch will last insanely longer if you let it out at 1000 RPM than it will slipping it out at 7000 RPM. The input side of the transmission will also be spinning much slower. The clutch and drive train will last much longer on the slower motor.

Just as importantly the slower motor will also last much longer. Look at many of the big truck engines and Semi engines that can happily go 200k to 400k miles without a rebuild and much of that time spent working hard. On the other hand our little 1.6 liter engine although it would haul just as well would need to get rebuilt every few thousand miles if you were lucky.

So to recap

1. Low end power has nothing to do with how well a motor will haul a big load but has a lot to do with how long components will last.

  1. Torque has nothing to do with anything until you apply RPM at which point you know how much work it can do. That is called power.


 

The same theory applies to race cars.

Torque is irrelevant.


 

People will say things like if two motors make the same amount of power the one with more torque will be faster. This is an uneducated way of saying the motor with a broader power curve will be faster. A motor that tends to have a higher torque peak will tend to have a broader power curve but it is not the amount of torque that is important, it's the RPM that it can do it at.


 

The other major downside is that using this terminology breeds further ignorance and propagates people using the wrong terminology and well as making it seem much more complicated than it is. People should try to use the most simple, most accurate and most educated terminology possible and should also try to correct those who don't.


 


 

I tried to use some quick examples to draw attention to this issue but there are others who can explain the math and science much better than I can.
One good place to start is wikipedia under horsepower.


 

Another good place is howstuffworks.com. You can look up the following pages.

How Force, Power, Torque and Energy Work

How Horsepower Works


 

And this one is my personal favorite.

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

http://www.epi-eng.com/mechanical_engineering_basics/energy_and_work.htm


 

If by the end of this you still think you can compare torque and epower or that one is better than the other or if you think you are right and I am wrong then please drop the wrench and step away from the vehicle.


 


 


 

 

 

courtesy of webmatter.de