Choosing a non-metallic, OE-quality cat for a 3" downpipe?

[wmorrison65]

I told myself not to sink too much money into the GP... but I broke down and ordered a 3" downpipe (along with a plog and some other stuff.) I ordered a catless only so I could take my time to choose the right cat for my goals, then have it welded in, because the manufacturer didn't have any choice of cats.

I'd like to find one that will not throw OBD codes, and will not require spacers for this. I hope this should be possible, because it was on my '01 Audi TT Quattro (3" catted downpipe for almost a year and no codes, no spacers.)

I can't find any specs on the OE cat. If I look up bolt-in replacements on Magnaflow or Walker, they don't list specs (displacement and test weight) for the bolt-ins that include a stock-like downpipe.

But if I look up universals recommended for my application, those do list specs, for universals that are 2.5" inlet and outlet.

If I look up other universals that are 3" inlet/outlet, but have the same displacement/test weight specs as the recommended ones, is that a good way to find a match? Or is there more to it than that?

[Scottydoggs]

what year is your car? what cat you need will be decided by your year.

i got a ebay 3 inch hi flow cat for 50 shipped. needed it to pass inspection here in NJ. but i also have a tuner so i killed the codes.

[wmorrison65]

what year is your car? what cat you need will be decided by your year.

i got a ebay 3 inch hi flow cat for 50 shipped. needed it to pass inspection here in NJ. but i also have a tuner so i killed the codes.

'01 GTP.

I don't want to tune out the codes. I want the cat to do its job, even if it costs a few horses. But still, 3" in/out should be less restriction than 2.5".

[Scottydoggs]

with a 01 you need a oem type cat, and even then, you may get a cat code.

[EHB]

there's a good chance any high flow cat will throw a code. it's doing its job , it's just the sensor thats not convinced.

[idrivejunk]

I may not be up on my year to year changes, but I run an emulator alongside my hi-flow cat (2.5") on a '99 and have no issues. Is that an option for you?

[Scottydoggs]

he needs the cat test shut off in tiny tuner. then he can run what ever he wants to. 01 up cars are messed up when it comes to changing the cat. the cat test just dumps fuel and thats no good.

[wmorrison65]

he needs the cat test shut off in tiny tuner. then he can run what ever he wants to. 01 up cars are messed up when it comes to changing the cat. the cat test just dumps fuel and thats no good.

Thanks, I found the thread for that. Just ordered an AVP interface anyway, which I think works with Tiny Tuner, right?

[Scottydoggs]

if its to be used with dhp your all set.

[J57ltr]

You know what's really odd Scotty? With my 04' PCM it doesn't do the cat test the same as the 05' did. You would notice that the vacuum level would change when sitting at an idle every once in a while. I have not seen this in the in the 04'. The other thing I notice is that the base timing in the 04' is 18* but in the 05* the base timing was 5-6*.

Jeff

[Scottydoggs]

wot on a 05 is 18 degrees timing.

i dont think you can feel or even know the cat test is happening. evap may make vac change.

[J57ltr]

No I mean at idle it is 18* on the 04' PCM the 05' was 5-6* at idle. I couldn't even feel it either just see the boost gauge go from 19 to 10" then back. May have been the evap, but I did notice the 02 would go rich or lean, can't remember now at that time then start switching again that's why I was thinking it was the rear O2 test

[Scottydoggs]

iirc when i was running 05 timing it was around 19-21 at idle. it was like 42 cruising on the highway too.

[J57ltr]

I do see 38-42 or so on the freeway with both years. I need to put the 05' back in there and try it again since my problems been fixed for a while now. Then try again. Lol

Jeff

[blueguy]

I was going to say, I highly doubt the timing is set that low at idle.

[wmorrison65]

BTW, if you look up universal cats for 2000-2005 Pontiac Bonneville SSEi (which also has the L67 engine,) you can find some that are 3" in and out. At least on Walker Exhaust's website. (I searched specifically for 2001.)

Other sites still come up with 2.5" cats for that vehicle. I looked at a direct fit though, and it was 3" in and 2.5" out, at the flanges. Makes sense, same 3" manifold outlet and 2.5" exhaust, just a lot shorter downpipe/cat in between them.