So I put the DC 4-1 on last weekend. I had a couple of issues rewiring the o2 sensor because I had to make it longer but, to make a long story short- G-Tech says: -2 Hp and - 0.1 in the 1/4 mile. Wtf?? I did actually gain 0.7mph but that could have just been the run. the other numbers were bests. The following is a list of possible causes in no particular order, feel free to add or strike as you see fit.

-Added airflow, no compensation with added fuel (lean)
-Cooler exhaust gases due to ceramic causing a decrease in velocity
-Powerband moved out of useable rpm range
-Stock catalytic converter negating increased exhaust flow and/or velocity
-Shut up and quit whining

Suggestions?

 

Have you tried cutting the power to your ECU to reset it after installing the header?

 

Yep. I did that already because when I changed the o2 sensor I got a CEL, code "43".