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Keep the Cat

Like stated above its the responsible thing to do. While noise is not really your issue, having gone down that route with a flowmaster, the rumble gets really old. Having to shout to be heard in the cabin gets even older and not being able to hear the stereo without cranking it gets older yet. When my Flowmaster was at the end of its life i swapped it out for a Magnaflow from Kolak and I can hear again,... and the Magnaflow is all stainless steel and was cheaper than the Flowmaster.
 
If restriction is a concern, you can get hi-flow cats that will flow well beyond the hp you're putting out. They're kind of expensive (as with all cats) but you'll be legal, and responsible. I know in this state it's illegal to not have a cat, and the state I used to live in was also illegal to sell a vehicle without a cat. So, keep the cat, if you feel it's too restrictive get a hi-flow cat.

Also, on my trans am when I put in the race pipe (cat removed), it messed up the rear o2 sensor, and had to be deleted out of the computer. That's also something to keep in mind.
 
Keep your cat I have a 5.2 and I dumped the cat when I had my FM40 installed. Can't keep the Check Engine light out, and it runs really rich. Also I'm sure the OX sensor is toast by now too. As far as the sound I think it sounds harsh and raw. I have heard other 5.2s with 40 series installed with cats, and I think they sound much better.
 
Good stewardship and pissing off econazis aside, your jep has an 02 sensor after the cat. When that sensors doesnt show a relative increase in 02 after the cat it assumes the cat has failed and causes the engine to throw a code and run lean.
 
Good stewardship and pissing off econazis aside, your jep has an 02 sensor after the cat. When that sensors doesnt show a relative increase in 02 after the cat it assumes the cat has failed and causes the engine to throw a code and run lean.

idk about that mine only had one and my dakota had a 5.2 and only had two right at the manifolds. what im goin to say next mihgt make some ppl angry but mine when i got my zj it didnt even have a cat form the previous owner and being a cert. auto tech i went to summit and bought a high flow cat form magnaflow to imporve gas mileage and not burn out my valves. i gotta heavy foot :) but when it comes in im matching it to a flowmaster 70 series they sound deeper than the 40. the 40 drones too much
 
with the way the 4.0 is you don't even need a cat my brother in law had an XJ with no cat it sounded awesome the only thing you could notice is it would kinda back fire when slowing down which made it sound like a hot rod really. so if you want no cat go with no cat the emissions won't change it will not read any high with or without the cat and gas mileage goes you won't see a change.
 
While you think you dont have a problem, your computer is doing things to your timing and fuel maps that is impacting performance. It may be imperceptable from the drivers seat but it's happening. You cant simply tell the computer to ignore the signal or loss of signal from the O2 sensors.
 
Catalytic converters do a few things for the engine. True they can be spark arrestors, but mainly they are there to act as a catalyst for the gases and change them from bad green house gases to more manageable green house gases. They also act as a good area for back pressure, which your engine NEEDS to properly scavenge the the combustion chamber.

Now with that said, I have no cat. I did however weld in a section of 2" pipe to create a bit of back pressure and keep the engine happy. ALSO, I recently ran through inspection and passed with about the same numbers as my car did WITH a cat. So.....do you need a cat, yes and no. If the engine is running at its most efficient and is well maintained, then its just a backup to keep the EPA happy. And if you don't recreate the back pressure somewhere in the pipe, then you will probably notice a drop in MPG. Without the cat and modified pipe I went down to about 9.2 MPG. With the back pressure I got it back up to about 9.8.
 
My cat just crapped out, completely clogged up.
Due to cash issues I just replaced it with a length of pipe... and DAMN.
It sounds better, I have noticeably more power (which I really didn't expect), and I should be getting better mileage cause of the less restrictive exhaust flow (too early to tell, I took it out yesterday). I left my muffler in which should give me the back pressure I need... although I don't know about you Grand Cherokee guys, but my XJ gets enough back pressure from the header itself to run well.
I vote take it off.
 
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