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Originally Posted by mikstew
My two and a half litres of displacement are beginning to feel puny. I want some more ponies, so I am thinking of opening up the intake and exhaust on the car.
With a new filter, cold air box, headers and exhaust (cat-delete), I am hoping to achieve my goals.
However, doing these mods will not yield any benefits unless the ECU adjusts the A:F ratio appropriately. How much can the ECU do? +/-5%? 10%? 25%?
And with that adjustment, how much power % increase would come with the mods?
From what I know, the ECU makes adjustments based on the pre-cat O2 sensor reading, which would not be affected by deleting the cats.
I plan to prevent a CEL by installing a spark plug fouler (defouler) on the post-cat O2 sensors.
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I agree with harryrcb, the early S is a steal and will be more powerful than you can ever make from your 2.5 with bolt ons.
On the operation of the engine management system, air fuel ratios are mostly determined by the air flow read by the MAF. It takes that value and calculates the correct ratio based on modifications from a bunch of other factors (air temperature, throttle position, O2 sensor reading, coolant temperature, battery charge state, ect). The O2 sensor is mostly used to optimize fuel efficiency during cruising. So bolt on modifications to reduce pressure drop in the intake and exhaust are compensated for automatically as long as the MAF, MAF tube, and the other sensors aren't disturbed.
The answers to your other questions are covered several times in this forum. Poke around and you'll find all the info you are looking for.