did you guys get a pcm tune after the header install??? If not that could be the problem right there. My buddy I just mentioned had that same problem, he said the car actually ran worse due to false knock and what not, but otherwise didn't get any gain at all. he got his knock sensor desentisized and had the pcm tuned for the headers and ended up running 8 tenths quicker afterward. So I'd guess you are both getting false knock from the headers and had timing pulled. Get headers, get a tune and get movin....

 

On my car, which was basically stock (only had CAI), headers and catback gave me 30 rwhp and 30 rwtq I gained 4 mph and 0.6 seconds in the 1/4.

Dan