How much will the stock ECU adjust the fuel map with intake/headers/exhaust
 

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.

Save your money and trade up to an S. That's what I wound up doing and never looked back

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.

Actually, bolt-ons will net me about 25 hp (exhaust, headers, intake) at the wheels (this has been shown with many many dyno sheets - Che's exhaust/headers gave 19 hp at the wheels alone). This will cost 1,000$ (see ebay). I will have 29 more hp ontop of my 201 hp (15% loss) = 230 hp

OR

Your suggestion is to sell my car and buy an S with 250 hp. Average increase in price for an S model option: $3,500 for 49 hp. (yes that's right, compare two cars with exactly the same options, year, and mileage)

I like my option, price /hp is better. Plus Id rather have my car because I know it's in great condition and I love the options it has and they are pretty rare.

Save your money and buy an "S" or drop in a 3.4L motor if you really need more power. Big power gains with bolt-ons are a pipe dream with a 2.5L.

Installed new headers and mufflers. Also installed a cold air intake which of course implies desnorkeling. Not sure on power gains but the sound is absolutely intoxicating. the 2.5 L sounds absolutely MEAN. Much MUCH better than a factory 996 or even 997.