Originally Posted by mhabs
Eurotuner Magazine has tested the following cat-backs....AFE = No gain; BMW M Performance = +2-3 hp avg (+5 hp on best run); and Gruppe M titanium +3 hp avg (+5 hp best run). In addition to hp gains, area under the curve in both hp and tq was slightly better at 2-3.5k RPMs.
In order to prove out such a small gain manufacturers do very extensive dyno work under controlled conditions along with a careful analysis of the very comprehensive data they took simultaneously. Only in this way can they account for ambient temperature, humidity, intake temperature, coolant temperature, engine oil temperature, transmission temperature, differential temperature, air fuel ratio, differing moments of inertia, loading, dyno calibration, and many more factors.
When someone takes a set of cans, makes a few runs on a dyno, and says 2 or 3 hp increase the correct translation is that they were not able to prove out any increase whatsoever as the runs fell within normal run to run dyno variability. It is meaningless.