Anyone Done this?

[kawaboy13]

Has anyone ever cut off their catalytic cinverters and ran without one? I have 1n 87 Renix 4.0. I'm wanting a loud, deep rumble. What muffler shoould I choose? I was thinking magnaflow or flowmasters. I'm not sure. I want it to sound deep and not like a race car. I was thinking about taking the cat off for a deeper, louder sound, but will it mess up my computer system or have any affect? Thanks!

[mvusse]

It won;t mess up your computer, reduced back pressure will result in less low end torque and may possibly detrimental to fuel economy. It is also a federal crime with a fine of $10,000 (up from the $6,000 it used to be a few years ago). Don't get caught!

[Pete M]

the cat won't affect sound much if at all. that's the muffler's job.

[kyleag89]

I ran my 89 MJ with no cat and a stock muffler for about 6 months. It sounds exactly the same with the new cat on it but there was a noticeable difference in performance. So I say leave the cat on there.

[dasbulliwagen]

I gutted and sleeved the conv on my 2.5, and installed a flowmaster 50 series with no noticable change in performance or milage, but then again, the 2.5 is a different story than the 4.0 in this regard. don't worry about your conv, and just put on whatever muffler you want. Good luck.

[kawaboy13]

Alright well i think i'm going to leave the cat. What'd ya'll think for sound?

[deziped]

Hearing is good.

[Geonovast]

If you really want a different kind of sound, snap one of the bolts holding the manifold to the rest of the exhaust system, then drive 550 miles.

 

 

 

 

 

 

 

 

 

 

 

 

 

But don't do that.

[TrailReadyMJ]

If you want to get an idea of the sound of different setups, check on youtube. An XJ will sound about the same, and there are many exhaust sound clips on there.

 

Mine has a pretty deep, loud (but not annoying loud) sound with a cat that practically emptied itself (I just helped finish it), and a flowmaster 40 series dumped under the bed. Dumping it under the bed probably made the biggest difference in giving it the deeper tone at idle, but you'll have to check your local laws. In VA it just has to extend 13" past the passenger compartment if not ran all the way out.

 

I will admit I am starting to get a bit tired of the drone that comes with chambered mufflers, and I will likely change it to something quieter soon. If I wasn't going to be driving it cross country in a few months, I doubt I'd worry about it though. I will also be replacing the cat, just cause I will be towing a camper and will need all the low end torque I can get!

[86ComancheXNate]

mine came with straight exhaust, not cat. The previous owner took it off. Would it be a good idea to get a new one? would I gain any power or torque? I'm only running the $#!&tly little 2.8 V6

[whowey]

No matter what combination you try.. you are never going to make the 4.0l sound like a V-8...

 

You will end up with a much flatter exhaust note, and not the classic rumble at all.

[Pete M]

mine came with straight exhaust, not cat. The previous owner took it off. Would it be a good idea to get a new one? would I gain any power or torque? I'm only running the $#!& little 2.8 V6

 

 

hard to say. you should get something from it, but since your engine is a carbed engine not a computerized engine, I cannot say for sure what will happen. 2.5s and 4.0s have a computer that controls everything and it expects to have a certain amount of backpressure there. if the pressure goes away, the computer won't know it and carries on it's merry way with its original fuel curve and timing.

 

You should at least get a muffler. :thumbsup: and if you have any sort of emissions testing in your area, you should definitely get a cat.

[mkbruin]

On our '88 the previous owner had removed the cat. We never knew until we failed an emmissions test (barely) and then failed the visual.

 

Tossed a new cat on the truck, it ran like a new vehicle. Passed the e-test with flying colors.

 

Its there for a reason, and they are relatively cheap to buy.

 

There is absolutely no upside to removing it.

[87Warrior]

My donor XJ has the exhaust cut right past the cat. At idle it is not real loud, just a steady 'brrr' drone. In the test drives I made around the farm grounds, the constant drone gave me a headache. Not to mention it sounds like a meth-head trying to drink water and yell at the same time when tached up. :ack:

 

I will note that the pipe diameter is much larger than the factory MJ pipe. A larger pipe will generally give the exhaust note a deeper sound as well.

[streetjeep2.5]

I have a 2.5 with single in, dual out the quietest Flowmaster available in 2004 with tail pipes coming straight back to under the bumper with turned down chrome exhaust tips. The turned down adds deepness to the sound. At idle sounds not like a 4 popper, but definitely not a v8. When driving, hardly any sound, except in the cab!

Think with a 4.0 same setup would sound decent.

[Dirteatr717]

gutting your cat actually results in power loss and if you want a mean deep rumble, get a flowmaster super 44 and keep the stock cat and stock sizing.. 2.25 all the way back and it sounds like a little mac truck LOVE IT at like 2,200 rpms

[Myron]

I've cut cats off all kinds of things and it normally adds a mile or two a gallon and better times at the track. The only time it hurt anything was in a buddy of mine's CRX, cost him 2 tenths in the 1/4 but it was a completely stock D15.

Only thing I've noticed as far as sound goes is it makes it a little louder, pitch stays the same.

[mvusse]

On my 94 Suburban with a 350 it cost me about 7% in gas mileage.

[TrailReadyMJ]

On my 94 Suburban with a 350 it cost me about 7% in gas mileage.

 

That, and many other vehicles, have a downstream o2 sensor, so if you remove the cat, that sensor will read false and give poor fuel mileage. I am not the biggest chevy guy so I can't confirm it, but you may be able to correct this like I did in my stang.

 

Please note this is not relevant to our MJs, and I will use my old 01 GT stang as an example of this (i miss that car!)... They came with 4 converters on the as well a 4 o2 sensors. 2 before the cats, and 2 after. When swapping to an aftermarket "off-road" H pipe w/ no cats, you will get a check engine light, and it will run rich without the proper fix. To fix that most aftermarket programmers (SCT, Diablo, etc..) have a feature to turn off the rear sensors. Basically all it does is make them give a false reading showing normal conditions, then relying on the primary sensors to provide the necessary info. They also have fake sensors that can be manually mounted called MIL eliminators, but these often cause problems and trigger CELs. NOTE: You would NEVER want to eliminate the primary sensor(s) this way, just the secondary sensor(s) that are usually only relevant to "fancy" emissions systems.

 

In General:

I agree that if this is not an off-road only truck, you should just run everything it came with, and just make sure it's all in good condition. At the same time, keep in mind even if it's a trail only rig, most places require a "spark arresting exhaust", so straight pipes, and technically even chambered mufflers (w/no cat) would still be illegal. If you want sound, just swap the "cat back" with your system of choice.

 

We only have one sensor, and it's before the cat, so chances are removing the cat won't have a huge effect on it. There may be a slight difference in backpressure, but if you've ever looked through a new converter, they are pretty free-flowing to begin with, it is the carbon buildup overtime that causes any real blockage.

 

An I mentioned, i plan to install a new converter in the semi-near future, so I can figure the mileage difference between with and without. In fact, I can get a rough HP measurement as well!!! It's not going to be the most accurate thing, but my Kenwood stereo has a G-meter built in and can estimate HP :D It was a fun toy in the stang, especially for measuring handling G forces (pulled 1.17 lateral Gs in a modded GT.. with street tires...off the track ).

[mvusse]

On my 94 Suburban with a 350 it cost me about 7% in gas mileage.

 

That, and many other vehicles, have a downstream o2 sensor, so if you remove the cat, that sensor will read false and give poor fuel mileage. I am not the biggest chevy guy so I can't confirm it, but you may be able to correct this like I did in my stang.

 

Nope, single O2 sensor, just after where the Y pipe comes together, a ways before the cat.

 

I believe downstream O2 sensors were introduced on the Vortec engines, starting in 1996.

[TrailReadyMJ]

 

Nope, single O2 sensor, just after where the Y pipe comes together, a ways before the cat.

 

I believe downstream O2 sensors were introduced on the Vortec engines, starting in 1996.

 

Ahh, my mistake, I was thinking they had them that early, I am fully admit I'm not a big chevy guy.

 

I am curious though because I haven't heard of an actual loss from removing a cat. Did you still have full length pipe after the cat? If not, that could explain it due to loss of scavenging (often falsely referred to as back pressure). Otherwise :dunno: :hmm: