Cat or No Cat???

[702_MJ]

Yeah there was a loss of power when i ran open cat (no pipe after the cat) for a few days. But once i put the new exhaust back on it ran fine to me. My engine is tired already loss of compression from being abused (engine came from a state vehicle, truck towed a damn boat long distances up hills and other stuff). I plan to build a stroker motor and put higher compression pistons in it. Once you get higher compression opening the exhaust is fine. Maybe I'm just the build it big performance junkie :dunno:

[ComancheKid45]

Thats possible.....basically i was just running an open Cat b/c there wasnt much to my 18" glasspack.......but i defently noticed a loss of power with that setup. Theres nothing qwrong with building anythng big, I'm sure different setups perform different for different people

[Pete M]

Our engines were designed around a certain amount of back pressure. Individual experience will differ when opening up the exhaust.

[702_MJ]

why glass packs? everyone uses glass packs, Isnt a glass pack basicly just straight through with like some fiberglass lining or something.

[ComancheKid45]

Yeah basically.....not much difference unless its a decent length

[mvusse]

My new to me XJ has a cat, followed by a 30" or so Cherry bomb glass pack. Surprisingly it isn't really any louder than my MJ with a stock muffler, no cat.

[702_MJ]

never used a glass pack before. The only thing i would use straight pipe on is a diesel, my friends dad had a 2002 cummins 5.9 ram straight pipe, and like i think 5.5" or 6" from where the muffler was supposed to be. Man that thing was a BEAST!!!!! You could hear it from about 5miles away and it poured smoke like a damn train!

[CWLONGSHOT]

never used a glass pack before. The only thing i would use straight pipe on is a diesel, my friends dad had a 2002 cummins 5.9 ram straight pipe, and like i think 5.5" or 6" from where the muffler was supposed to be. Man that thing was a BEAST!!!!! You could hear it from about 5miles away and it poured smoke like a damn train!

 

It has no effect on me, but I gotta say, you need a bit more education on what and how to make a internal combustion engine make power...

 

Hogging out a perfectly good hi-flow cat to make a free flowing exhaust is ... well.. stupid. Running a larger pipe, no cat and a free flowing muffler WILL destroy lo-end torque. No matter what you think and have seen. The laws of physics exist everywhere and for everyone...

Open free flowing exhaust are for only advantageous in W.O.T. situations. If you drive like that, it will offer an advantage to a stock type setup. But virtually nothing else in a jeep is made for this kind of driving. Jeeps are generally not go fast vehicles. In order to achieve a favorable return, from a free flowing exhaust, you need to increase INTAKE, side. no matter how well the exhaust flows, if the engine cannot get the air/fuel in, how is it supposed to get it out faster?

 

If you like it loud, more power to you. But don't try to justify it with grossly untrue misinformation. Thats not cool.

 

CW

[BREEZE1]

So I guess in a round about way CWLONGSHOT just answered my question. A hollow cat or no cat with a freer flowing muffler destroys your low end torque!!! Dang, I really need to do something with my cat, because you can hear the stuff raddle around in it. I may just gut it this weekend and let y'all know the results!! :ack:

[702_MJ]

First of all everything you said CW makes sense, but see i have all that. Air intake, Chipped it to adjust the air to fuel ratio and what not, and now a free flowing exhaust and theres alot more to come. Yes your right jeeps are not fast, but with those mods on the result ive found is acceleration. I can get to highway speeds without putting my foot into it.

 

Any you build it your way ill build it mine. We all have different opinions on what and what not to do.

[glundblad]

I'm old school and this post really surprises me. When I was a kid, I had an old POS Datsun 240z. The muffler went out on it. I drove it for awhile without a muffler. After I put a new factory muffler on it, I bet I lost 10% hp.

 

My 4.0/5sp commanche runs pretty well. I bought it new in 87 and bought back used in 2008. THe exhaust was rusted from the engine to the tailpipe. It also didnt have the torque I rememered when I owned it originally. I replaced the muffler & exhaust with factory everything except for the cat. I can tell by the loud sound the cat is empty now due to the years of sitting and low mile driving.

 

I would have thought for sure a new cat would reduce my hp even further. So I wasnt even considering replacing it. I always thought more back pressure reduced horsepower. The posts here indicate I am wrong in my assumptions. I think I am going out to get a new cat. I always liked the factory sound of the 4.0 anyway.

 

So is it generally agreed that lack of backpressure robs torque?

[CWLONGSHOT]

IMHO,

Much of what we think we get from a free flowing exhaust came from the the time up to the smog years and primairly form the small V8's. Most had very restrictive/poorly designed exhaust manafolds and small dia pipes. SO, slap a set of headders on a '60's 327/350 along with some glass pac mufflers and watch that motor wakeup. I did it in high school, just like thousands upon thousands of other kids did. Well, these auto engineers advanced in how they design and build enjines. Some of our thinking on the subject has stuck in the 60-late70's.

 

Don't get me wrong now, some exhaust systems still do have restrictive systems. Most are much better and will not nearly see the seat of the pants boost we did 30 years back. Look at the exhaust manufolds on your 4.0. They are tubular, much like the headers we put on that old Nova/Chevelle/Mustang. Not nearly as restrictive as cast manafolds of old. Cats are much better too. Now cheap mufflers, the cheapies still can be quite restrictive. As you would surmize with the boon in custom muffler manufacturers in recient years. Air filter systems too can restrict the air getting into your engines. Minor perf boosts and larger economy boosts can be had addressing these.

 

So, with a realitively stock jeep set-up. Meaning STOCK cam, TB, injectors and exhaust manafold. Little is gained from removing the cat or adding a better air filter system or even just a "chip". Now of those, a properly setup chip will alter the air/fuel ratio and adj the entire computer controlled fuel system for optimal performance. Not something the factory could do. Because they needed a good balance with economy. After all, like I said Jeeps are not thought of as performance vehicles. With that stock system, a certain amount of back pressure is a good thing in the overall driveability and performance/economy of that engine system.

 

Wanna get a boost in HP? Stroke it, cam it, add a bigger TB and injectors AND chip it!! Throw on a air filter system along with a good header and exhaust system WITH a hi-Flo cat. Don't go crazy with pipe, 2.5" is just fine. Now GEAR IT!! (A big one many people skip right over) Fineally feed it and you will have something rivaling many small V8's.

 

Just my humble opinions...

 

CW

[BLHTAZ]

:agree: with CW. Chopping put parts of the exhaust is nothing but detrimental to the 4.0L engine. Keep the cat and run a free flowing muffler if you want the sound and it will perform better.

 

DEFINITELY regear the axles if you wan to see a real difference in the "get-up-and-go" :thumbsup:

[mvusse]

Yes, for optimal low end torque you need some back pressure from the exhaust; a free flowing exhaust can help top end HP, but will rob low end torque.

 

Since our engines are designed for low end torque instead of top end HP, a free flowing exhaust will not gain you much. You will lose torque and might even lose fuel economy as well.