Originally Posted by Ziggyrama

Stock ECU constantly compensates for various conditions like temperature, atmospheric pressure, quality of gas, what it has learned in the past itself, etc, etc. On any given day, the results will fluctuate and produce variations in outcome if you were to dyno the car every day. Add to that the fact that dynos register behavior induced by a driver, which in itself can also vary from run to run. On top of that, dynos themselves have margins of error and will vary in readings. Results will vary and it's important to remember that.

As far as ECU 'learning' things, the ECU does not learn around increased flow of air. More oxygen means more fuel which means more power assuming other factors remain fairly constant. If you add a well flowing intake and exhaust that improves flow, you will make more power. The hard part is to quantify when something is actually working better then the OEM component. If I were evaluating an intake or exhaust, I would start by logging my MAF output to see if my flow got better, not go to a dyno and do 10 pulls to stroke my own ego. If I see consistent improvement in the rate of flow (g/s) at the same RPM range, I know I am making more power. That's me and I haven't really seen people talk about this at all. Seems like everybody gets hung up on the bragging rights of how much horsepower their car makes off mod X.