[scottyho]

The short of it is I have a 2002 RS, 3.8L V6. It has an aftermarket exhaust that's larger than the stock 2.25-2.5" diameter. I need to put a CARB certified CAT on it for Colorado emissions. The CARB cats are all sized at 2.25-2.5" diameter.

Am I going to ruin anything if I have a welder taper the exhaust pipe down to fit the smaller diameter of the cat?

The background is my dad bought the car new in 2002, he's passing it down to my son to drive as his first car (pretty cool right?). Well we live in Colorado and follow the CARB emissions standards. The cat on the car right now is bad and must be replaced. A new cat should get me through emissions.

BUT years ago, I don't know exactly when, my dad had a larger aftermarket exhaust installed on the Camaro. The diameter of this exhaust and cat is over 3". The catalytic converters that are CARB certified for this Camaro are all 2.5" or 2.25". I have a good fab guy that can taper things down to fit the new cat. I just don't want to do something to damage the car by tapering the exhaust down to fit the cat.

TIA!

[AZIROC]

Im pretty sure if you find any quality exhaust shop, Or even performance shop, they would be able to find and install the correct size cat you need. But to answer your question, a 2.5 inch in/out cat will fit a 2.5 inch in/out exhaust. But a 2.5 cat won't work with a 3" exhaust.

That being said, does CO have any kind of "historical" vehicle emissions exemption? May wanna look into that if it meets CO requirements .

[bishopts]

I didnt know colorado followed CARB emissions standards.

Any neckdown smaller than the largest opening becomes the point of maximum flow. So for example, if you have 4in exhaust all the way out but have a 2.5in neckdown at a flange, that entire exhaust will flow the equivalent of a 2.5in exhaust system. There will be oddball pressure variance before and after but overall it will be a 2.5in exhaust that looks like 4in.

Get under it and measure the smallest points and if any of them are 2.5 already then adding a 2.5 neckdown wont change anything that isn't already happening. Being higher altitude and naturally aspirated 3.8 its gonna be a dog already. Try and limit anything that makes it worse.
Or do like California people do to escape CARB standards, register it in Montana lol.

[Gunkk]

re: CARB in CO

Pretty sure it depends on where you are in CO. Some cities (Denver, etc.) and counties do, some don't.