Car in Sig. Just put on Pacesetter LT Headers with catted Y. With Our cars, Do they usually run rich or lean after these Mods?I also have a ported Maf which tends to run lean.I have an adjustable handheld tuner which can adjust these perameters. Thanks for Info.

 

Any time you allow your engine to breath better (let more air in) it is going to run lean.
The PCM will adjust the ltft to add more fuel

 

Not at WOT, though a LTFT may carry over from time to time...

 

Headers increase scavenging in the midband and reduce
stack-up at higher RPM. If you were depending on speed
density this would give you more air than before hence
lean. On a carbed motor too. But a mass-air setup sees
the real air (until you make with the lies ala aftermarket
or porting etc.) and keeps you neutral. However you also
pick up more cylinder air charge and charge quality so the
motor may ping even at the same mixture; not lean per
se but just needing adjustment of timing.

Down low you have the issue of oxygen sensor thermals
and degraded (or even narcoleptic) response which can
pull your fuel trims to bogus rich fueling that hits both
idle / low cruise and carries over to WOT if you mat it
from a bad place. Also fuel trimming details can lump most
of your driving envelope into one or two cells and make
these drift back and forth, applying recent mid-cruise
trimming to idle and whacked idle trims to mild tip-in, etc.
Coated headers are better than not, but still tend to give
some degradation. If you can reduce proportional fuel
amounts, transport delays etc. for low exhaust gas flows
this can tighten it up some and minimize code setting
and mixture over-travel (which can stimulate surging at
warm idle).

 

damn..I knew a lot of that but reading it the way JimmyBlue wrote it made me feel like a little kid. lol GREAT advice