There are no performance related tuning changes required when installing long tube headers--all you are doing is helping the engine breath better. The MAF will continue to accurately measure the incoming air and the PCM will continue to calculate the proper amount of fuel to be injected.
However one issue that might occur is that the PCM may throw DTCs P0133
which indicate "slow response" from the front O2
sensors. This is because LT headers relocate the front sensors further downstream causing the time between the PCM's altering the AFR (in closed-loop mode) to be detected by the sensors.
This is corrected in the tune by increasing the values in the O2
Transport Delay table--those values are the amount of time (in seconds) the PCM expects the O2
sensors to respond to changes in make in fueling. Usually bumping the values by 10 to 15% will fix things up.
In the table below the highlighted values are the changes made for long tube headers, the non-highlighted values are the stock numbers:
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq
Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.