[ausie]

The actual HP numbers the stock motor pushes out is more close to 420Hp to 425HP. The underrating of the motor by Ford served two purposes: 1. reduction in insurance cost, 2. Ford did not want another 99 issue. To comply with EPA noise pollution, the cat back and intake were restricted to reduce the noise level of the supercharger (that is something I can believe since after changing to an aftermarket exhaust the supercharger sings much louder and can be heard over the unrestricted noise from the tailpipes. ) I have yet to unleash the intake to its fullest potential other than a high flow filter and no air silencer. Going to LT headers may require a different K-member for added clearance. You may loose a little bit of torque since there will be more scavaging from the LT's. From what I remember reading, the difference between the cast iron headers when compared to gains from LT headers on the 03/04 motors is not much considering the cost to do it. However, there is something to gain from the mid-pipe and different cats (unless you go catless). If you change the cats there may be some tuning to do just in case the A/F ratios change since the O2 sensors will effect the fuel curve. I would at least have the fuel ratio checked if nothing else. Changing to an aftermarket cat-back will alter fuel ratio a bit as well but not to the point that the ECU cannot correct or manage. The same probably could apply to high flow catted mid pipe. A tune would be required if you change the cams, pully, or headers and probably a good idea to have the A/F checked after you change the intake plumbing if you are using one that has not been fully tested and evaluated. You would probably be fine with JLT but others types I would question it.... the reason for the suggestion is in case the airflow bypasses the meter tube or is getting tubulance due to its position relative to any bends in the intake tract before or after the MAF. If it is off and out of the main stream of air flow your motor could see a lean condition at WOT which would lead to detonation, this is probably more of a problem for naturally aspirated motors than for one with forced induction.