Originally Posted by v3.2mc
the text below if from Hotrod
Many factors that influence test accuracy are common to all dynos, including engine dynos; these include temperature, airflow, barometric pressure,
These parameters are not so much an influence on the dynometer, but on the poweroutput of the engine
But the rest of the story gives a good picture.
A dynometer value itself says nothing; it's a fictional number.
Tuning your car and doing different dyno runs give an insight in the gains achieved, and when you want to compare, you should have a (verifiable) and documented list of runs from other cars on that same dynometer (under similar circumstances). Only then you can have an insight of your car's performance.