Originally Posted by wparsons (Post 909643)

There's two cats, one in the header and one in the front pipe. The Factory O2 sensors are around the cat in the header, so no catted front pipes need bungs.

If you're not worried about the CEL (or will have it tuned out), just get the header with bungs in it. You only need pre-cat O2 for closed loop, the post-cat O2 just tells the ECU how well the cat is working, and has no effect on how the car runs.

Originally Posted by wparsons (Post 909643)

There's two cats, one in the header and one in the front pipe. The Factory O2 sensors are around the cat in the header, so no catted front pipes need bungs.

If you're not worried about the CEL (or will have it tuned out), just get the header with bungs in it. You only need pre-cat O2 for closed loop, the post-cat O2 just tells the ECU how well the cat is working, and has no effect on how the car runs.

Originally Posted by alex_86 (Post 910279)

The second O2 sensor does determine how the car runs if the second o2 is telling the ecu that that certain elements are more than others such as CO it will give the engine less fuel and vice-verse.

Originally Posted by AznKirby (Post 2318328)

Sorry to bump an old thread - but there's something I might not be understanding. If the primary O2 sensor determines how well the car runs - does that mean that I can't install my (catless) header in my driveway, then drive it to get tuned?

Originally Posted by steve99 (Post 2320182)

Put an O2 spacer or defouler in the REAR 02 sensor this should eliminate the P0420 car efficiency cel code due no cat in header .

some headers it shows up quickly some it takes a week or so, just be prepared to reset cel occasionally with andriod torque app or obd scanner reader or pull a battery terminal for 30 secs or so.. If you get cel other than P0420 investigate.

P0420 wont force ecu into limp mode but it will disable cruise control and may drop you to safe afr and timing maps reducing power.

it wont blow up without a tune it just wont generate optimium power due fueliing ignition and cam timing not being optimal for your setup.

stock tunes run rich

your more likely to get issues by changing intake and upsetting maf scaling

Originally Posted by DM7 (Post 2318405)

On a stock header the primary O2 sensor is the one closest to the engine between the engine and the header cat. The secondary O2 sensor is located right after the header cat and in front of the front pipe cat.

In an aftermarket catless header, the primary O2 sensor will see the same exhaust gases that come right out of the engine. The secondary O2 sensor will now see gases straight out of the engine as well.

As far as the primary O2 sensor is concerned, nothing has changed. It will continue to feedback AFR information back to the ECU.

The secondary O2 sensor will see that the exhaust gases have not been cleaned by a cat. This is what will cause a cat efficiency CEL. As far as I know a cat efficiency CEL will not effect the way the engine runs.

The engine will run just fine without a tune optimized for an aftermarket header. It may or may not gain any power. The ECU will throw a cat efficiency CEL.

A tune that is optimized will adjust the intake and exhaust cam/valve timing to take advantage of the exhaust evacuation characteristics of the new header. A well designed header will allow exhaust gases evacuate the cylinder at a higher speed. Adjusting the exhaust cam/valve timing takes advantage of this.

Originally Posted by AznKirby (Post 2318647)

That is an awesome response. Thank you, Sir.

Originally Posted by steve99 (Post 2320182)

Put an O2 spacer or defouler in the REAR 02 sensor this should eliminate the P0420 car efficiency cel code due no cat in header .

some headers it shows up quickly some it takes a week or so, just be prepared to reset cel occasionally with andriod torque app or obd scanner reader or pull a battery terminal for 30 secs or so.. If you get cel other than P0420 investigate.

P0420 wont force ecu into limp mode but it will disable cruise control and may drop you to safe afr and timing maps reducing power.

it wont blow up without a tune it just wont generate optimium power due fueliing ignition and cam timing not being optimal for your setup.

stock tunes run rich

your more likely to get issues by changing intake and upsetting maf scaling