Originally Posted by mapleleaf
Changed the VE of the motor?
I assume they would.
So, there's more air going through the motor.
So, there's more air going past the MAF sensor.
So, the ECU is adding more fuel accordingly.
So, the question remains: what causes the mixture to lean out?
Bad MAF scaling is my only guess. The motor moves a little more air, so it run a little higher MAF voltage, and the MAF curve is wrong at that voltage, so the mixture leans out. But it's just a guess, I'd love to hear other theories on this.
Anyway, those two charts are not just showing the results of the KB headers, they are showing the results of running a leaner AFRs.
Good comparisons are really hard to do.