Why do dyno discrepancies exist?

[grandkodiak]

Why are the numbers often so different between types of dynos, yet all measure the same forces? i.e. Dynojet and Mustang dyno? Put a weight on a scale and it should read the same measurement despite the method used to weigh it or even the units in which it is measured... why is there such a big different between dynos then? Is it that hard to calculate the power output, or is there something inherently difficult in manufacturing a device to measure from the type of input (spinning wheel)? and i'm not talking about testing differences like temperature, altitude or wheel/engine etc. because that can all be acocunted for.

[Chad97z]

most mechanical devices when they get old do not turn or function like they did when they were new unless they are maintained constantly. Bearings may not turn as easily if they are worn and not lubricated and that will cause poor HP and torque measurements in the case of a Dyno. I assume, a dyno needs to be calibrated once a year if it is used often. Just the temperature of the machine could cause a variation in results.

[5.0THIS]

It's all in how the power is measured. Different dynos measure power differently. There are write-ups on the internet about the differences between a mustang dyno and dynojet dyno.

www.google.com

[Chad97z]

i measure it with a yard stick. hehe