Well there is no % Calculation its the fact that the Dyno thinks the car is at a different RPM then it really is. So the TQ # will always be way off.

Here is a DynoDynamics that I was told was not in shoot out mode and it makes 40+ more horse the it does on a Dyno Jet. This car was also winding 9200rpm so you can see its off. On this car to.
These are both the same car on the same tune.

 

Let me be a bit more explicit. The RPM is being used as an input to the calculation that the dyno is using to give you a result right? Now if you know the actual rpm is off by some set amount or even a set percentage then you can calculate what the % error of the output the computer gives you is going to be right?

So using your example:

3000 (assumed RPM) - 2850 (actual RPM) = 150

((150/2850) * (100)) = 5.2% <--- this is roughly the amount of error you are going to have in any calculation that the dyno is using RPM in.

If you didn't know the difference between assumed and actual then you'd have a case point . Computers aren't as smart as people would like to believe them to be...garbage in, garbage out.

Even so, you know quite well that comparing numbers across dynos is a fruitless exercise. It's why for better or for worse once you get numbers with one dyno you have to keep going there to see your true progress.

 

I see what you are saying but alot of cars you would never know the Exact amount the Tach is off.

I know you should keep going back to the same dyno but i know around here the MD # are changing so much that the same dyno will never read the same since the owners mess with them.

My point here is how lucky we are to all be able to run on a Dyno Jet that cant be played with and if we get a good tach signal the TQ # always will be right.

 

Agreed. In fact the dyno from the first plot that03Evo8DTC continues to change. One minute tuned mildy modified cars are only getting those figures on it, next minute then untuned mildly modified cars are getting those same figures...