What is a dynamometer?


A dynamometer is a device that measures force and power.  There are lots of different kinds of dynamometers, including the kind that test springs and shocks, but we don't care about them because I don't have any.  I have an inertia-type chassis dynamometer.  It measures the force and power that the spinning wheels of an automobile produce.  It is not a "brake-type" dyno that measures the power that is actively absorbed by a water, oil, or eddy-current brake or by a generator.  An inertia-type chassis dyno consists of two great big heavy drums hooked up to a computer.  The wheels of an automobile spin the dyno drums, and the computer measures the speed.

Simple?  If you remember the stuff you were supposed to learn in high school, it is.  The computer calculates the acceleration of the dyno drums by continuous measurements of their speed and the time.  If the surface of the drums spin from a speed of zero to a speed of 10 feet per second in one second, then their surface acceleration is 10 feet per second per second, or 10 ft/s2.  Sound familiar?


Force = Mass x Acceleration

That's one of Newton's laws.  It's not important which one it is, because I don't remember.  Force is one of the things that we're looking for.  Force in the automotive world is called - you're supposed to shout it out..... torque.  Torque is rotational force, and its most common unit for us is foot-pounds.  Plain and simple, if you have a 12-inch wrench and you lean on the very end of the handle with 10 pounds, you're applying a force of 10 ft-lb. to the nut you're trying to turn.

The mass in our case gets a bit complicated.  Mass in most cases is easy - how much does the object weigh that you're accelerating.  In the case of the dyno drums, however, it's not that simple because we are not "moving" the drum, we're spinning it.  We are not creating a "translational" motion on the drum, we are creating a "rotational" motion.  To understand the difference, think of the actual dyno drums.  Each one weighs 2700 pounds.  It would take a pretty impressive force to push a 2700 lb. object across the floor.  Now imagine just spinning those drums.  The shaft going through the center of the drums rests on two hugely expensive bearings that I'm probably going to have to replace a lot more often than I want to.  The drums spin with the slightest touch.  To calculate away this difference, physicists came up with the "mass equivalent" of a rotating body, which is very similar to the "moment of inertia."  I don't know the actual numbers, but let's just pretend the mass equivalent of the dyno drum is 50 pounds.  That means that spinning the 2700 lb. drum is like pushing a 50 lb. weight across the floor (forget about the friction - you nerd.)  Now I've heard some people say that this is not the correct way for calculating horsepower, but in this case - it is.  Many of you are familiar with the hp = rpm x torque / 5252 formula.  In many situations, that formula is the way horsepower is calculated.  In the case of the Dynojet dynamometer, it is not.  The Dynojet calculates horsepower even if there is no torque reading, and it does this in the manner that I am in the process of describing.  Those of you that have had your vehicles tested at my shop and have watched the computer screen closely have seen that the computer plots out only the horsepower after each run, and I have to click the mouse a few times to get your torque.  It does this because sometimes there is no torque reading, or the torque reading is faulty.  This can happen because inductive and optical rpm pickups may not be perfect, and/or your ignition system may make things go a little haywire.  If the rpm readings are not perfect, and the computer used the rpm x torque / 5252 formula, the hp reading would be inaccurate.


Work = Force x Distance

Now we get to the good stuff.  Calculating the work is pretty simple for the computer.  It just figured out the force, and it can easily figure out the distance because it knows the circumference of the drum and how many times it has rotated.


Power = Work / Time

We have an answer.  Somewhere the computer factored in the bearing drag, and it throws some constants into those formulas to get the numbers to come out into the right units, but you get the idea.

The computer then back-calculates the torque using the formula mentioned previously.  Those of you with a knack for physics will realize that the torque produced in first gear at the tire-drum interface will be significantly greater than that produced in fifth gear.  Since the rpm of the engine is factored in, the different speeds developed by the different gears are negated - therefore, one can say that the computer reports "engine torque as measured at the wheels."


In the real world - what's the difference between horsepower and torque?

In English, horsepower is the ability to do work in a given amount of time, and torque is strength.  To make an analogy, (and a really good one, I might add) let's compare a human weightlifter to an automobile engine.  An engine in a typical 4 cylinder commuter car might have 100 hp and 100 ft-lb. of torque.  How about the weightlifter?  Let's hook him up to a handwheel that has a 1 ft. radius.  A big strong guy should be able to push and pull on that wheel with 100 lb. of force, therefore generating 100 ft-lb. of torque, easily equaling the engine.  When it comes to horsepower however, our guy is going to fall short.  One horsepower is defined as lifting 550 lb. up one foot in one second.  Two horsepower could be 1100 lb. up one foot in one second, or 550 lb. up two feet in one second or 550 lb. up one foot in half a second, (you get the idea.)  Let's attach a rope to the handwheel and tie that to a 550 lb. weight.  How is our weightlifter going to do now?  You can imagine - not very well.  Attach the handwheel to a gearbox and then to a wheel with the rope and he might be able to perform some work. Unfortunately, he'll be lucky to generate one horsepower before petering out.  Bottom line - both the engine and the weightlifter can be strong, but the engine can perform a lot more work.  The beauty of the engine is that it can maintain a relatively high torque over a broad range of speed (rpm,) and it can continue doing that until it runs out of fuel or breaks.  Our poor weightlifter can't spin that wheel very fast, and the torque he generates will drop dramatically as he speeds up.


Which one do I want for my vehicle?

There are a few common misconceptions concerning horsepower, torque, and the role they play in your engine and in your vehicle.  For starters, they are not independent factors – the horsepower and torque numbers are mathematically linked with a formula:  horsepower  =  torque  X  rpm  /  5252.  Therefore at any given rpm, if one knows the torque, one can calculate the horsepower, and vice-versa.  In the automotive world, torque is strength and horsepower is the ability to perform work in a given amount of time.  So, regardless of how badly one wants that high torque number, horsepower is what actually moves your car down the street or around the track, and horsepower is what tows trailers.

Of course, this does not mean that torque is meaningless.  An engine’s torque curve is its fingerprint.  It shows how strong the engine is at every rpm.  The horsepower curve is merely a function of that torque curve and the rpm.   Therefore, it’s not necessarily the peak torque number that matters, but where in the rpm range that peak is, and over what rpm range one can find a relatively high torque, as that will determine where and what the highest horsepower is and dictate how the vehicle accelerates at a given rpm.  All engines are designed to be the strongest at one particular rpm range.  Heavy cars and trucks have engines with torque peaks low in the rpm range.  This results in relatively high horsepower numbers in that range, giving the engines the ability to accelerate those vehicles without the drivers having to rev them up.  The successful racecar engine has a torque peak high in the rpm range, or at least a torque curve that doesn’t fall too sharply at the high rpm range where the engine is typically operated.  This allows for the horsepower to be at a very high level in this high rpm range.  The typical street car is usually somewhere in the middle.

To summarize – a good analogy is a person on a bicycle.  Someone with high torque at low rpm would be the weightlifter mentioned earlier.  Someone with relatively high horsepower would be Lance Armstrong.  The weightlifter may be able to tow a heavy load slowly, but Lance can maintain a decent torque at a high rpm.  Guess who wins a race?