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Dynos torque and power

A dynamometer (dyno) is a device that measures force and power.  There are lots of different kinds of dynos, We have an inertia/eddy current chassis dyno.  It measures the force and power that the spinning wheels of a vehicle produce.  It consists of two great big heavy drums and a very large eddy current brake connected to the rollers, which is connected via a few sensors to our dyno computer.  The wheels of a vehicle spin the dyno drums, and the computer measures the speed, acceleration and torque applied to the torque arm which is suspended on the end of the eddy brake.

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.

Force = Mass x Acceleration

That's one of Newton's laws.  Force is one of the things that we're looking for.  Force in the automotive world is called 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 250kg.  It would take a pretty impressive force to push a 250kg object across the floor.  Now imagine just spinning those drums.  The shaft going through the center of the drums rests on two bearings. 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 2kg.  That means that spinning the 250kg drum is like pushing a 2kg weight across the floor.

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.

Torque = 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.

Power is then calculated from the equation

Torque x RPM / 5252 = power

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.  However since the rpm of the engine is factored in on most good dynos, the different speeds developed by the different gears are negated.

Some dynos don't use engine rpm for their calculations, only roller rpm and due to changes in ratio caused by the gear box and diff they calculate higher torque readings. That's why you can see on some dyno charts for Supercharged Commodores 1200nm of torque, that's about the same stated torque as a Bugatti Veyron!!! Which of cause is ludicrous and not at all possible. Those of you that aren't lost buy all of this will realize it all comes back to the RPM source, if a Bugatti was tested on one of these dynos in forth gear it would produce well over '2000nm' of torque.

So for those of you that all this makes sense, you will be able to see that changes in gear ratio will not affect power it will only affect torque due to the rpm component of the equation and where it is sourced, engine or rollers.



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, let's compare a human weightlifter to an automobile engine. An engine in a typical 4 cylinder car might have 100 hp and 100 ft-lb of torque. How about the weightlifter? Let's hook him up to a hand-wheel 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 hand-wheel and tie that to a 550 lb. weight.  How is our weightlifter going to do now? You can imagine - not very well. Attach the hand-wheel 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.

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?