Originally Posted by Fabrik8

Actually I'm retarded. And I have to disagree.

Corrections for air temp, altitude, etc., on a quality dyno will give you consistant results. If you're not getting repeatable readings on a dyno, the dyno is garbage and probably isn't being used correctly.
My point is that if you use different dynos, and get different numbers, how do you know which one is correct? How do you know if any of them are correct?
If you weighed yourself on 5 different bathroom scales, and got 5 different readings, which scale (if any) is accurate?

How can you possibly argue that taking a car to a track is a more consistent measurement of performance change than a dyno in a controlled environment? Are you going to graph your speed every foot along the track, and apply a transform to correct for track temp, humidity, air temp, barometric pressure, etc? And are you a professional driver who has the consistency to judge improvements of the order that you imply a dyno couldn't measure?