I know this has been covered but search was not helping. is there a big power difference between exhaust and header companies like the apexi n1 or greddy evo, or the dc sports header or greddy header

 

not that much difference at all

 

In the grand scale of things not a whole lot of difference. There are certain setups that will make more power (Bisimoto header, straight through exhaust, test pipe and AEM ITR CAI on a D series for example Vs. a DC 4-2-1 and a stock cat and a Apexi WS and a Short ram intake on the same D series)