Bypassing the Catalytic...

Pil9rotten · Sep 12, 2004

I am thinking about removing my catalytic converter, welding flanges onto it and then replacing it with just a straight exhaust pipe that would also have flanges. The idea is to get some more power but having the option of putting the cat back on for highway use and smogging. Anyone do this? Any ideas?
Will this affect the computer since the sensor going to the cat will be unplugged?

My set up is:
96 XJ, banks headers, flowmaster muffler, bored TB, K&N FIPK, 4" RE lift (5.5 RE long arm going on next spring), Dana 44 front and rear both 4:56 with ARB lockers, 31" tires (35" tires coming in the spring), ARB front winch bumper, 8000 Warn winch (soon), rear Olympic air tank bumper, full skid protection,

Thanks.

scoobyxj · Sep 12, 2004

unless your present cat is plugged up it is a waste of time. newer cats flow more than enough for the 4.0 exausting requirments in fact i have learned that unrestricting the exaust to much on the 4.0 acualy [sp?] will lower your peformance. not to metion it is eleagle, and most likely will light your check engine light. my old lady had a pos grand am [96] and the cat was removed because it was thought to be pluged [it was not even remotely] and it ran like sh*& and got horable gas mileage untill it was put back on.

xtremewlr · Sep 12, 2004

Alot of the ricers and import car fans will do this. I had a Mitsubishi 3000GT for a couple years and ran a straight pipe (also called a "test pipe") on it without issues. You will notice that the exhaust note will be louder and you might feel a slight performance gain but nothing significant. I highly doubt you would get a CEL from removing the cat as there are no O2 sensors back that far. It is true that newer cats flow pretty damn good. It wont hurt anything to give it a try and see what your results are. If it sux, you can always bolt the cat back on.

Overdriven · Sep 12, 2004

An easier way to do the bypas is to take a pipe and clear out the exsisting cat. THat way you won't have to cut the flanges off. Gut the cat and bolt it back on. I've been running a hollow cat and Thrush glass pack on my cherokee for better than a year now. It sounds like a small V8. I've noticed a little mpg gain but I did loose a little low end torque. One side effect is the exhaust smell. It's much worse when you are not running a cat. Now that I'm starting to drive slower over rocks I'm looking for a cat to put back on.
Good luck though.

XJJPR · Sep 12, 2004

Don't do it. The 4L likes a little back pressure.

The cat gives that back pressure to make the 4L happy.

On your 96, it DOES have a O2 on the cat.

hinkley

brent · Sep 12, 2004

been there done that, you loose a ton of torque, talk about slow off the line!!!

Eagle · Sep 12, 2004

Standard disclaimer: NAXJA

provides an open forum for the expression of ideas and opinions. NAXJA

is not responsible for the opinions expressed by individual posters.

What you propose to do is illegal. Aside from the fact that it probably will reduce rather than enhance performance, the official position of NAXJA

is that we do not recommend or condone modifications which violate Federal or state laws. The catalytic converter helps clean the air. I like clean air. You should, too.

And, yes, the 96 does have a second O2 sensor downstream from the cat, so your "test pipe" likely would trigger a problem code.

Matt S. · Sep 12, 2004

It would do more than trigger a code. ALL 96' vehicles are OB2 which is a much higher level of computer controls. By removing that o2s you could very likly have problems with fuel milage, power, bad running engine and other small issues. It is NOT recomended, of course you are able to do what you want.

IMO, you live in SanFran. I happen to live in Fresno. I get all your cities smog. Please please please keep your cat, at least in consideration for our health!

atriot:

Matt

XJJPR · Sep 12, 2004

Eagle said:
Standard disclaimer: provides an open forum for the expression of ideas and opinions. is not responsible for the opinions expressed by individual posters.

What you propose to do is illegal. .......,

the official position of is that we do not recommend or condone modifications which violate Federal or state laws.

Eagle,

Where is this official position written down?

Or is this a non-written position?

Did the BOD vote on this? Meaning is this an official position?

??????????????????????????????????????????????????????????????????????????

hinkley

XJ_ranger · Sep 12, 2004

just make some exause dumps (also illegal) and then you can switch your exaust to not run through the cat whenever you want... dont remember anyone doing that on a cherokee, but it happens all the time with street racers....friend of myne put some in his lexus mall crawler SUV

Eagle · Sep 12, 2004

Mark Hinkley said:
Eagle,

Where is this official position written down?

Or is this a non-written position?

Did the BOD vote on this? Meaning is this an official position?

??????????????????????????????????????????????????????????????????????????

hinkley

If it was voted on by the BOD is was prior to my election to the BOD. However, it has been discussed by the BOD during my term.

From the Forum Guidelines legalese:

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Also from the Forum Guidelines:

takes no responsibility and assumes no liability for any Content posted or uploaded by you or any third party, or for any mistakes, defamation, slander, libel, omissions, falsehoods, obscenity, pornography or profanity you may encounter. As a provider of interactive services, is not liable for any statements, representations or Content provided by its users in any public forum or interactive area on the Site.

Basically, NAXJA

's position is that of most newspapers and Internet forums: By making available a "forum" for people to promulgate their ideas and opinions, NAXJA

provides the venue without accepting responsibility or liability for the content. For what should be obvious liability reasons, as an organization NAXJA

cannot advocate any action that knowingly violates laws. If we "officially" were to advocate such, NAXJA

would become an accessory to the illegal act and could be held liable for any consequences.

Technically, anyone who posts tech advice that recommends removal of or tampering with required emissions or safety equipment is in violation of the Forum guidelines. As a moderator, I could simply nuke any such posts. However, rather than stifle technical discussion, I generally prefer to simply remind people that any advice to violate a law comes from the person posting the advice, not from NAXJA

. Follow such advice at your own risk.

BornAgainXJer · Sep 12, 2004

Mark Hinkley said:
Don't do it. The 4L likes a little back pressure.

The cat gives that back pressure to make the 4L happy.

On your 96, it DOES have a O2 on the cat.

hinkley

does a 95 have the second o2 sensor?

Eagle · Sep 12, 2004

BornAgainXJer said:
does a 95 have the second o2 sensor?

No.

96 was the first year for OBD-II

XJJPR · Sep 12, 2004

Eagle said:
If it was voted on by the BOD is was prior to my election to the BOD. However, it has been discussed by the BOD during my term.

From the Forum Guidelines legalese:

Also from the Forum Guidelines:

Basically, 's position is that of most newspapers and Internet forums: By making available a "forum" for people to promulgate their ideas and opinions, provides the venue without accepting responsibility or liability for the content. For what should be obvious liability reasons, as an organization cannot advocate any action that knowingly violates laws. If we "officially" were to advocate such, would become an accessory to the illegal act and could be held liable for any consequences.

Technically, anyone who posts tech advice that recommends removal of or tampering with required emissions or safety equipment is in violation of the Forum guidelines. As a moderator, I could simply nuke any such posts. However, rather than stifle technical discussion, I generally prefer to simply remind people that any advice to violate a law comes from the person posting the advice, not from . Follow such advice at your own risk.

Man you better go look at that picture of the speedo showing 90 and give him a ticket.

Just wanted to know if NAXJA

was getting THAT technical over sight law breakers. Just thought your post was a little over board for legalese crap! It isn't like he was going to kill someone or rob a bank or blow up someone!

hinkley

shaggys645 · Sep 12, 2004

On my grand I removed the cat to a pry-bar to the inside and busted all that crap out and replaced it so that it looks like its still there. Noticed a little increase in gas mileage and hp but not much else. By the way still passed emissions when the time came.

Rev Den · Sep 12, 2004

Do a search...this comes up every now and again. The fact is it does NOT increase power, and in fact, hurts low end tourqe.

Rev

OT · Sep 12, 2004

Rev Den said:
Do a search...this comes up every now and again. The fact is it does NOT increase power, and in fact, hurts low end tourqe.

Rev

And gives Eagle a chance to practice his vocabulay.

Remove it if you want to. You're the one who has to pay the price if you get caught without required California emissions equipment.

Promulgate away.

FrankNBrew · Sep 17, 2004

Scrappy said:
It would do more than trigger a code. ALL 96' vehicles are OB2 which is a much higher level of computer controls. By removing that o2s you could very likly have problems with fuel milage, power, bad running engine and other small issues. It is NOT recomended, of course you are able to do what you want.

IMO, you live in SanFran. I happen to live in Fresno. I get all your cities smog. Please please please keep your cat, at least in consideration for our health! atriot:

Matt

The check engine light will come on, but it will have absolutely no effect on the operation of the engine electronics. The O2 sensor after the cat is also known as the "catalyst monitor sensor". It's only purpose in life is to make sure the catalytic convertor is working properly. It tests the convertor during a cruise by momentarily sending rich and lean commands to the PCM, and the PCM makes sure the CMS reacts much slower than the O2 sensor. If the CMS reacts along with the O2 it can be assumed the catalyst does not work, and a catalyst efficiency code is set, along with the check engine light.

8Mud · Sep 17, 2004

There is an approved test pipe, for export vehicles destined for countries without unleaded gas. Looks kind of like a cherry bomb muffler with squared ends, the one that came with my 87, was necked down to 1 3/4" ID at the in end, normal 2 1/4 out.
LadyWolf also has a test pipe on her 2000 XJ with a O2 sensor port at the rear of the test pipe/resonator/whatever it is (sure doesn´t look like any cat I´ve ever seen).
The write up I read on the rear O2 sensor, said it´s function was to make sure the cat. had enough oxygen to function at optimum. In later model cats. they changed the catalyst and internals, so they worked better, supposidly less restriction, but required a little more free oxygen. I´d guess the front O2 sensor and rear O2 sensor, reach a compromise.
Messing with the cat and getting caught in California, would probably be a seriously expensive process. I´d do some serious thinking as to wether the questionable gain, is worth the possible consequences.

Prepmech · Sep 17, 2004

we took the cat off of my brother's '96 and replaced the rear 02 sensor with a 02 sensor simulator that we got from Summit Racing. It was a louder but not too much faster, if any faster at all.

We took his off because he had a check engine light on because of a cylinder misfire that put raw unburned fuel in his cat and caused it to heat up and start coming apart inside. It rattled really bad, until we took it out.

After taking it out, we didn't have any problems with the MIL (check engine light), because of the simulator.

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