2000 XJ cat and muffler question

Sweetwater

Well-Known Member
Joined
Aug 29, 2012
Location
The Great State of Mecklenburg
While messing around under the XJ I couldn't help but notice my OEM cat and exhaust tubing is about to fall to pieces. Here's the specs on XJ:

2000 "sport" I6 with auto. OME suspension, K&N FIPK and flowmaster muffler with close to 100k and 7 years of DD use.

So, I swung my an exhaust shop and the very friendly manager advised they could/would install NC smog inspection compliant cat (I think we were talking about MagnaFlow brand) while replacing bad tubes and using flowmaster that's there. I'm thinking this sounds pretty good...

...but a friend who's an experienced GM tech warned me that the MagnaFlow cat may create an excessively smooth flow, causing codes to be thrown constantly and then the dreaded FAIL upon inspection. He's also of the opinion it won't add any real performance to the XJ's 4.0.

Any feedback gentlemen? Any experience in this area? I would even consider a complete exhaust if it weren't for the binding laws...so a header is pretty much out too unless it was compliant. All I need is a cat and some tubing that'll work as well or better than stock. I'm open to opinions and advice, including shop referral.
 
Your friend is wrong. Flow has nothing to do with how/what the oxygen sensors monitor. The measure oxygen content, not flow. No worries on the Magnalflow cat.
 
Thanks Jody! I trust the GM guy, but he's more conservative than many car guys and his experience is limited to GM cars and trucks. He's had trouble with those vehicles and exhaust mods, but my guess is the mods may have been more extreme? Who really knows? Erring on the side of safety?

So, I've done some quick searching for a header as well, and they are pretty steep in price (my other projects will suffer if I buy a header too!). Do these mods help? The marketing hype is pretty convincing, but pushing a brick into headwinds isn't efficient anyway I slice it. Any real experience with these mods?

FWIW, our TJ has a complete MagnaFlow system (it passed inspection this year!) but I can't relate it to the XJ since PO also did other mods and the 33" tyres, et cetera, add up to a different experience- it is also a brick in the wind, but still different all together!
 
I have lots and lots of Jeep 4.0 performance. IMO, installing a header on a 4.0 with stock internals is a waste of money. The early 4.0 HO's had a tubular header style manifold that flowed very, very well. The late rmodel ones (like yours) have a 2 piece manifold that flows quite well itself. For the MINIMAL gains you would get from a "performance header" the cost outweighs any gains.
 
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