SHELBY GT 350 Member
Joined: Jan 2011
Location: Central Massachusetts
I'm pretty sure you always need to tune for headers to get the best performance.
As I understand it, the issue with the CEL / emissions / O2's is that most "high flow" cats don't work very well, so the rear O2's register that they are not working and then the rear O2 has to be turned off to avoid that code / CEL. But that's a catch-22 because you cannot pass emissions with the O2's turned off, all sensors must be reading "ready"
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day