well i installed my 4-2-1 header a couple of days ago and to me it didnt feel like it gave me any power at all in the low end department because i had this header before a year ago and i took it off thinking i was gonna sell my car and but never sold my car. But the first time i had it i didnt have my comptech intake just header alone. But now i have the ice box with the header and thats it. What could be the problem?

 

Exhaust leak maybe? Try resetting the ECU.