Glad I could help

-- Ed

 

Glad you help too

 

[quote]I think what the op needs to know is that there is no international calibration standard for dynos [quote] Kinda says it all really - there have been quite a few guys over here in UK who have spent seriously good money on engines that have been dyno'd by USA engine builders who are far from being 'shade tree' merchants only to find that when tested over here on same makes of dyno, same fuel, same ambient temperatures etc etc can never replicate the makers stated figures - they always seem short by 10% or so. The comment of a dyno being a reference guide is probably the best explanation - purely another tool to use, albeit a rather expensive one

 

[QUOTE=roscobbc;1581672181][quote]I think what the op needs to know is that there is no international calibration standard for dynos If you read about the history of dyno's there is a set standard, that being SAE which started in Detroit many years ago. This problem about different numbers coming up was addressed and a correction factor was established to standardize the tests from dyno to dyno that being the dyno is corrected to the same altitude, air density and so on as the first dyno in Detroit. Where things go wrong is the dyno operator not plugging in the correct data, or simply fudging the numbers with the software involved.

Read this as it explains the whole procedure:
http://wahiduddin.net/calc/cf.htm

 

The SAE atmospheric correction factor and calibrating a dyno are 2 completely different things....

 

Well, when dyno to dyno results can vary by 10% to 20% that correction factor really doesn't matter very much.

SAE might have a standard saying how to to measure power but building something and saying it meets that standard does not mean it's calibrated.

On a similar vein, you can say you built something to meet a CSA standard, but you can't put the CSA mark on the product unless you subject it to tests and prove it meets the standard.

 

It really doesn't say anything useful about calibrations.

It does say that the same dyno needs to be used for tuning and that the atmospheric corrections are needed at that time. The same as what has been posted in the thread already. About the only interesting part is the mention of 2 standards for atmospheric correction and the statement the difference between the standards is about 4%.

 

Sorry I'll take it down, no better still I will post it again as there is some good info into why dynos do not produce the same output. There are some ideas that faced with overwhelming fact from experts that work with this type equipment on a daily basis refuse to acknowledge the ideas are wrong and continue on to pursue the matter which only makes the ideas look foolish. ( I am not attacking anyone, just ideas in this thread )

From the second paragraph of the PDF file, the rest is very interesting and should be read.

CHASSIS DYNAMOMETER TESTING
There are two main types of chassis dynamometers (dynos) in widespread use today.
· An inertia dyno uses a large spinning drum that is accelerated by the drive wheels of the test vehicle. Power is then computed by
knowing the inertia of the drum and how quickly it was accelerated. Torque can then be calculated by knowing the speed of the drum. -
· An eddy-current dyno absorbs and measures power by rotating a metallic disc through a magnetic field

Here is the link:http://www.google.ca/url?sa=t&rct=j&..._1hKgQ&cad=rja

 

So if it's not attacking then what does posting that some people are looking foolish mean exactly?

The link does have some good info. It just does not address the dyno calibration posts.

The quote isn't very correct though. Ed and I have both described how an eddy brake dyno has to add the torque in the brake and the torque accelerating the components. You can say power if you want. With how closely torque and HP are related the exact calculation details used to calculate the curves really don't matter.

 

I thought it was a very good write up.... Thanks

Ralph

 

Glad I could help, instead of argue, it is a rather in depth article that I hope in your case explains some of the reasons why you will get different readings when you dyno your car. I guess I have been lucky as I had my 350 and 406 dynoed at least 4 different shops and the results were all very close to each other

 

OK, you're still looking for info on why different dyno's read differently. The thread got off track but I was trying to get to the fact that the power and torque are calculated, and any time you calculate something you can introduce errors. I'll try to explain some places where these errors could occur.

The idea behind an inertia dyno starts with Newtons second law of motion, F=ma. This gets converted to a rotating version which is torque = inertia x angular acceleration. In simpler words, the torque is the force needed to change the rotational speed of a rotating objection.

The next step is to thow in the fact you want to know the rate work is being done or the power, which is torque x angular velocity.

These are the basic formula I used to give the calculations and the basic formula for the inertia dyno. Given these formula it would seem you plug in some numbers and get the exact result you expect. Seems simple, right?

Here's the main problem. The above torque formula that everything else follows is an ideal formula. It applies in a world where no other forces act on the object. Too bad that living on the Earth, we can't find those conditions. The roller drums are subject to air resistance and the bearings have friction. There are probably other factors I'm missing.

Here's another more basic place an error can be introduced. The inertia of that drum must be figured out in some manner. Everything else hinges on knowing the inertia of that drum.

So, the above basically comes down to the manufacturer and how they decide to address these issues. They can decide what issues to address or not address, and how accurately they want to address them. They might throw in a simple fixed correction factor for the bearing friction, a complex one that varies with drum rpm or they might not use any correction factor. They might just calculate the drum inertia or they might measure it. Either way, the inertia number will have an error in it and there will be some error in the way they compensate for the no-ideal conditions affecting the torque calculation.

Besides the above, there is the SAE enviromental correction factor. The problem here is that it is a correction factor based on expecting your engine power to vary according to the enviromental conditions. Unfortunately, there's no guarantee your engine power will vary exactly as the formula predicts. So, different days can see different results just because your engine doesn't respond to the changes in the weather the same way the formula predicts it should.

Then finally, there is the operator effect. Basically, the operator can be responsible for ensuring the correct data is entered. It could range from all the info, including the operator having access to the drum inertia and any correction factors entered, to simply being able to enter the enviromental conditions at the time of operation. They might be using some cheap *** weather station that isn't very accurate. The operator might also give bad results or simply operate the dyno incorrectly (you end up with your jagged curves which are mostly garbage).

Overall, I hope this helps you understand where errors can occur. Dyno's do not measure to some standard. There really isn't one, the manufacturer decides how they make the measurement.

Having posted this, is there anything else about your particular curves you're still unclear about?

 

I am clear that from the experts that use this equipment every day that an inertia dyno can in fact measure/caluculate Power in the absence of any Tq measurement/ calculations. If one were to read what was posted from them they would understand. This was the only point I wanted to make the rest was to help the original poster understand why there might be differences from dyno to dyno

Anyway the fact that those who actually own these rather expensive chassis dyno have in fact backed up what I had to say so I consider this matter closed.