Losing Cats 3 & 4

DeeDubya · 07-25-2005, 03:30 PM

When installing the Banks I noticed that the pipes reduce to about 2" maybe smaller just behind cats 3 & 4. Quite a restriction, not to mention the secondary cats themselves. I also noticed the flanges just behind cats 1 & 2. Has anyone replaced the secondary cats with a straight pipe between the 2 sets of flanges? I wonder how this would effect emissions, leaving only the primary cats? If I remember, the 02 sensors are in front of the first flange so that shouldn't be a problem.

Armada · 07-25-2005, 03:53 PM

Yes, folks have done this. There is a lot here. I don't have time to do a search for you right now. Someone else may have the links. Try searching "secondary cats" or phrases like that. It's been done - there are even photos, if I recall correctly.

DeeDubya · 07-26-2005, 12:18 PM

Thanks Armada, "secondary cats" got me there. I guess the unanswered question would be about emissions. Texas starts the emissions testing in September.

**gr8titan** · 07-26-2005, 03:17 PM

Quote:

Originally Posted by DeeDubya

When installing the Banks I noticed that the pipes reduce to about 2" maybe smaller just behind cats 3 & 4. Quite a restriction, not to mention the secondary cats themselves. I also noticed the flanges just behind cats 1 & 2. Has anyone replaced the secondary cats with a straight pipe between the 2 sets of flanges? I wonder how this would effect emissions, leaving only the primary cats? If I remember, the 02 sensors are in front of the first flange so that shouldn't be a problem.

I have replaced the secondary cats in my 4x4 (4x2's have restrictive resonators in lieu of the secondary cats) with 2 1/2 inch straight through resonators and had O2 "bungs" installed to take in the secondary OE oxygen sensors. Since these sensors were upstream to the secondary cats, the removal of the cats did not make any difference to the air (exhaust) quality passing by them and therefore never affected the ECU.

A few TT members replaced their cats/restrictive OE resos with straight pipes and the common denominator is the "raspy" tone it produces. Installing a straight flow resonator instead produced a superior old V8 non-raspy deep mellow tone.

Check my links below my SIG or here for install pics.

RockyMtnTitan · 07-26-2005, 05:22 PM

Deleted my rear cats a few weeks ago....installed Dynomax Race Bullets (straight through reso) in their place along with O2 bungs, like gr8titan did. Kept original flanged cat sections in tact, just in case I need them in the future to pass the visual part of the emissions....not worried about passing the sniff test as the primary cats should take care of things there.

Of note....when they install the new flanged sections, be sure to have them use the factory metal flange gaskets on the flanges upstream of the secondary O2 sensors so you don't develop leaks causing a SES lean code....a couple of people have run into problems with this. The factory cat sections use these metal gaskets only on the downstream flange because the upstream conn has a special compression type donut seal in lieu of a gasket. I just had them use standard gaskets with high temp rtv for the downstream flanges, as the Nissan ones are like $20 each! If they ever leak, then I'll replace with the pricey ones.