The Science of Dyno Testing

[volvotuning]

I have written an article about dyno testing. For anyone who wonders why people argue about accuracy of figures etc, have a read of this. It's quite long, and a bit complex in places but I've tried to keep it as simple as possible.

Here is the link -

www.dyno-tuning.com\dyno-tuning\dynoscience.pdf

Adam.

[JUDGENINJA]

Nice ...not overly complicated article...now it's late so go to bed...!!!!

[Andrew]

and the correct URL is:

http://www.dyno-tuning.com/dyno-tuning/dynoscience.pdf

Well done Adam.

[Vikingxl]

Cheers Adam!

[volvotuning]

If anyone does read it (without falling asleep) and would like any clarification about any point discussed, please let me know so I can include it in the document.

Adam.

[Mrsmopp]

Blimey - beauty and brains LMSO!!

Useful info, makes for interesting reading (and i am still awake!!)

Thanks

x

[craig]

I have written an article about dyno testing. For anyone who wonders why people argue about accuracy of figures etc, have a read of this. It's quite long, and a bit complex in places but I've tried to keep it as simple as possible.

Here is the link -

www.dyno-tuning.com\dyno-tuning\dynoscience.pdf

Adam.

ya i no !!!!

[TwistedSanity]

interesting read Adam, one point i must have missed in the text(which i feel is the most important) is dyno calibration, how and how often do you calibrate?, ive seen some muppets put there mums saxo on rollers before and calibrate from the factory specs :-), how do you do yours?
surely none of the text is valid if the calibration is incorrect

[volvotuning]

interesting read Adam, one point i must have missed in the text(which i feel is the most important) is dyno calibration, how and how often do you calibrate?, ive seen some muppets put there mums saxo on rollers before and calibrate from the factory specs :-), how do you do yours?
surely none of the text is valid if the calibration is incorrect

Yes you are right in that the accuracy of the "science" of dyno testing hinges on the correct calibration of the dyno itself, just like any other scientific instrument. Of course, my text assumes that all dynos are correctly calibrated, but since assumption is the mother of all f**kups (as Hamish keeps saying), I do really need to explain calibration. I'll briefly explain it here, but will add a more detailed explanation in the article. Certain uses for dynos don't need calibration, and I will explain that too.

Here goes...

Every dyno needs to be calibrated so that the power and torque measurements that it displays are indeed true, precise and consistent, and not under-reading/over-reading. Let's not confuse incorrect calibration with apparent under-reading and overreading of power because all the factors mentioned in the article can affect the power readings. Therefore, calibration cannot be determined by testing a car and comparing to factory specs!!! Nor can calibration be confirmed by putting a car on a different dyno and seeing if the results are the same. That would be like comparing two watches to see if they are both telling the same time, when they both may be 3 minutes slow. Calibration itself therefore needs to be precise and correct.

The method of calibration of the dyno depends on the method of testing with the dyno. For example, our dyno can use one of two modes for measuring power and torque - 1) Inertia and 2) Torque Cell. This means that our dyno needs to be calibrated seperately for each method.

Inertia calibration - this subject is indirectly mentioned in the article. Remember the equation Force = Mass x Acceleration, where the mass of the roller is constant, so by measuring the acceleration you can calculate the force and therefore the power. (The formula is a bit more complex when applied to a rotating mass, but the principle is the same). This is a fundamental law of physics which cannot be changed. But the best bit is that the same formula automatically calibrates the dyno! The mass of the dyno (including retarder) is precisely measured at the factory. These values are stored in the dyno software and cannot be changed. So the dyno is calibrated from the factory for Inertia testing and this will always remain correct because the mass of the roller does not change. Ie, for inertia testing, the Dynojet does not require any calibration. I must add that for this calibration to be correct, the weight of the entire rotating mass must be measured extremely accurately, and even the bearing drag must be accurately measured. For this reason, a dyno which was not designed to be an inertia dyno from the factory will probably never ever give accurate results.

Torque Cell Calibration - This is the method which most dynos equipped with a retarder will use. Torque is measured by a torque cell. If you apply a precisely known amount of torque, then you can calibrate the torque cell to give you the exact same torque reading as the true torque you are applying. This is commonly done by applying a known mass to the end of a known length bar (calibration arm), which is attached to the retarder. This rotational for exerts a force on the torque cell which enables the calibration to be made. Unfortunately, this is where all such dynos can be incorrectly calibrated, be it on purpose or by accident. By applying slightly more or less weight on the calibration arm, or by using a slightly longer or shorter calibration arm, then the calibration will not be correct.

This is where our Dynojet has a big advantage - we can "confirm" the torque cell calibration by comparing the dyno test results from a torque cell run with the results from an inertia run. The power and torque curves will be exactly the same in inertia mode as in the torque cell mode if the torque cell is calibrated correctly. This is how we can prove that our torque cell is calibrated correctly, since the inertia calibration is always 100% correct. We use a normally aspirated car for this test, because a turbo car can often behave differently with repeated runs due to rising charge temps.

So what does this mean in terms of the tuner and the customer? Well, a good tuner has no interest in having a badly calibrated dyno for the simple reason that this would prevent them from making accurate measurments themselves. We tune cars for people who taking racing very seriously, where 1 or 2 bhp extra can be enough of a difference to win or lose a race. If our dyno was not accurate, then we would simply not be able to do our job properly. In terms of what you the customer sees, well that really does depend on the dyno and tuner you are using. How do you know if a torque cell dyno is calibrated correctly? The honest answer is that you don't! That is something that you learn about with the reputation and credentials of the tuning shop you are using. Does it matter? That depends on how accurate you need the results to be.

Adam.