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Discussion Starter · #1 ·
In conversations with fcalmes,he asked me to put together some comments on dynos,hopefully to shed some light on them. My background is not in automotive dynos, but rather in 35 years of designing and testing industrial brakes and torque and force sensing strain gauge sensors. In addition we supplied brakes for dynos up to 50 hp. When testing a brake, you are essentially building a dyno, with an electric motor driving, and the brake absorbing the horsepower, and a load cell measuring that torque.

Comments on Dynos:

There are two ways of measuring horsepower at the rear wheels of a car. 1) an inertial dynamometer, or a 2) brake dynamometer.

1) Inertial type:
The inertial type has the rear wheels sitting on large (about four foot diameter) drums. The car accelerates these drums up to the car's maximum speed. The only two things measured are the rpm of the engine, and the rpm of the drums. (the rpm of the engine is only needed to produce a graph of the hp vs. rpm curve for the customer). The actual horsepower is all calculated from knowing the inertia of the drums, and the rate of acceleration of the drums. This can be done, using Newton's law, F=MA, to calculate torque, and the hp equation, hp = T x RPM/ 5252.

For a good article on this by Hib Halverson, see

2) Brake type:

This dyno generally has smaller, lightweight, drums that are connected to a brake. The brake puts a torque load on the drums that the motor must match. The rpm of the drums is measured, and the horsepower is calculated by the hp equation mentioned above. One sequence used in testing is having the car run up to max speed, and the load is increased until it slows the car wheels down. The hp is then plotted vs. rpm.

Advantages and disadvantages of each.

The inertial dyno can only measure and readout hp during acceleration, while a braking type dyno can measure hp continuously.

Well then, let's always measure continuously! Not so fast. The brake on a braking type dyno has to absorb all that horsepower, which means it gets HOT! A typical 100 Watt light bulb gets pretty hot. Image a brake that is absorbing 400 hp. That's 299,200 watts, or nearly 3000 of those one hundred watt bulbs in one spot. The brake dyno has to get rid of that heat, and generally can't do that for a long time. For continuous hp measurements, typically the engine is out of the car and mounted to a dyno that can dissipate the heat continuously. So that gets us back to fairly short runs with a brake dyno. You can however run for some short time at a fixed rpm (or horsepower) to analyze some facet of the engine's operation. If you go to the Cruise-In be sure to visit the Holley plant and see the engine dyno in action.


This is where the most questions arise. What sets the accuracy of the dyno?

In both types of dynos, rpm is very accurately measured, to fractions of a rpm. No problems here.

In the inertia dyno, the inertia of the drum must be measured or calculated. This can be done to great accuracy, and the reference article cites the inertia is measured to 4 digits. For a pair of 4 foot drums that weigh 2700 lbs each, that is about one part in 5000. Very accurate.

In the brake dyno, the measured item is torque. You either have to measure the actual torque in the shaft of the brake with a strain gauge torque cell, or measure the force on the brake mountings with a strain gauge load cell. This can be done very accurately, but not to the precision level of the inertia calculation. I found load cell accuracies on brake dynos to be in the range of ½ to 1% stated in their websites, but no overall dyno accuracies were stated.(Remember that's 1% of full scale, so if it's a 1000 hp dyno, it can't tell the difference of 10 hp)

If the brake type dyno has a load or torque cell to feed back the torque settings, it can be as accurate as the accuracy of all its parts. If there is no feedback of torque, i.e., an air brake or an electric brake is just set to some value, then accuracy is simply lousy. Good dynos always have feedback.

I haven't mentioned the effects of air temp, density and humidity. These all have to be calculated in to produce a number with a reference to a universally accepted standard. These effects are large, and the dyno manufacturers have them worked into the computer controls that plot the hp data. I also haven't mentioned the possibility of slippage between the tire and the drums. The dyno will measure what gets to the drum, but errors can be incurred.

The dyno numbers are rear wheel horsepower, but are typically referenced back to engine hp, by assuming a drivetrain loss of about 15% (for a manual transmission). Why is this done? Simply to get "bigger" numbers. (Mine is bigger than yours) The 15% is a GUESS. (Guessing should not be done when Measuring) It can be measured by "coast down" torque measurements, or by knowing the coast down of the inertial drum and subtracting that out of the numbers, but it's really not important for most people. It's what is at the rear wheels that's important.

So what do we do? We put our car on a dyno, and get a reading. We then believe that number like it was etched in stone (if it was a good number), or we don't like it we do another run, or badmouth the dyno or the operator. Look at the actual variables. Using Dynojet's numbers, the accuracies for inertia is about .02%, and rpm is about .01%. Those are insignificant numbers.

With a load cell feedback, the loadcell accuracy I'll estimate at 1%. (I assumed a .5% load cell that hasn't been recalibated, which is typical). Then I would assume the mechanical friction and windage losses are in the 1 to 2 % range. So we have from a ½ % up to a 3% accuracy, and these are conservative numbers. Let's say it's 2%.

Then the correction factors for temp and humidity are put in. If you look at Halltech's website, he lists numbers on runs from .99 to 1.01. That's another 2%, and he always tries to test at the same atmospheric readings. He also claims to be careful to have the oil and water temps at the same readings each time. Why? So the performance of the engine and the frictional losses in the oil and drivetrain are the same each time. (And to do this we read the numbers on the DIC. Anybody want to guess how accurate the temp readouts in a car are?)

So if we have up to a 2% accuracy, and a 2% atmospheric adjustment, on a 325 rwhp reading, the delivered hp could be actually 13 hp higher or lower than the reading. So depending on the dyno, the air temp, the oil and water temps, etc, it's a 312 to a 338 hp car.

There has been discussion of the ram air effect on horsepower in other forums. Some engineer did do some calculations, and concluded the ram air effect was negligible, and the real hp gains were due to the lower temperature of the air, which is taken into account by the correction factor.

Just to confuse the issue a little more, the repeatability of the dyno must also be considered. It is likely however, the repeatability (of the dyno alone) is much better than the absolute accuracy. It cannot be assumed that the repeatability of the car is better than the dyno. You can't test dynos by running the same car on different dynos.

To further confuse, a person from B&B stated in a post on the Corvette Forum, that he could "change the valve caps on the tires, and make the reading change by 5 hp". Also see Corvette Fever, May 2001 issue, page 89, where Mr. E. J. Poss states, " The Dynojet seems to read higher than many other brands, like 11% higher than our Mustang dyno." I don't know him or anything about him, but he certainly believes there is a difference in dynos.

Nobody can say what the absolute accuracy of a reading really is. So, what to believe? It is reasonable to believe the dyno numbers for your car. If you test on the same dyno, and the car temps are the same, you should get fairly repeatable results. But understand that the small hp increases of something like a throttle body coolant by-pass or a smooth air bridge coupler can get totally lost in the accuracy and repeatability of a dyno that cam measure 350 hp. And remember, your car's numbers will probably be different on the other guy's dyno.

Reference: Hib's article See lab accuracy numbers See torque transducer accuracy

2,521 Posts
Good discussion. Feel free to cross-post at in the Tech/Perf. section.

As an aside, I personally prefer to use the dyno for relative comparisons from my car to my car, not to other cars. I use the track to compare my car to other cars. But dynos have their uses just the same.

Thanks for posting. /phpBB/images/smiles/icon_smile.gif

26 Posts
Discussion Starter · #8 ·
Thanks for the positive comments, guys.

The dynos are actually pretty accurate, it's just that if you are not in the measuring and testing games, it's difficult to get a handle on absolute accuracy. Most of us don't care too much if the butcher is off by 3 or 4 ounces on a 20 pound turkey, but that's equivalent to 3 or 4 hp on a 325 hp dyno reading. By the way, butcher scales are good to .2%. That's a half ounce on a 20 lb turkey.( and they require yearly certification)

I'm not trying to undermine anyones dyno results, or the dyno manufacturers. Heck, the chassis dyno is a heck of a lot better than relying on my ability to launch correctly at the strip, to evaluate horsepower. They do it the same way each time. It is just that the dyno manufacturers know that absolute accuracy of 4 or 5 hp on a dyno that can measure up to 500 or 1000 hp is really not important. What is important is consistant results on the same dyno. An that thay do provide.

I have a lot more faith in the dynos than someone's SOTP horsepower readings.

629 Posts
I have thought of purchasing a chassis dyno for my shop.

Can the people that have used different types of dynos (Dynojet, Superflow etc.) tell me what they thought about them?
And good/bad points about the shops that had them?
Why did you use that shop?
What would you change for better service?

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