XJ-NC

Anybody toy around with types of exhaust on the I6 for increased HP?

Does the I6 require back pressure from the cat or would removing it add a few HP?

Would Flowmaster or straightpiping add HP?

huntingman2706217

i pulled the exhaust bolts off from in front of my cat and moved it down and rebolted it so its all still there but it just comes out right before the cat instead... just to play with it... actually doesnt sound that bad but its loud as crap... lol... it probly helped my HP but i cant tell... nobody probly will tell the difference in just exhaust... only on the dyno really... my cats busted so its probly better... got a single chamber muffler im gonna stick on it once i get it back on the road... just to see... lol

radio flyer

4.0 engines perform better with some back pressure unless you build a stroker and are planning to run 5k plus rpms. 2.5 inch pipe is pretty much as big as you want to go if you want to maintain the the low rpm grunt that these motors are known for! taking off the cat is wrong and can get you a ticket! Shop around for a cat back system---

kamokid

without a cat will be fine, the muffler provides enough back pressure.. Gives more pep, sounds cool too.. I have a gibson and it rox!

bigbadxj

X2 on cut the cat it's not wrong and with a good muff it's sounds good and maybe a little power.

grn99

I cut the cat off on my YJ and it seemed to give it a little more power, also used a three chamber flowmaster. On my XJ I've kept the cat and have a Hooker "aero chamber" muffler on it. The exhaust is tucked up high and dumped at rear axle. Sounds good. Dont think it has changed much power wise.

gearheadette

I got a cat back flow master and its LOUD!!!!

98 JeepJeep

sound clip?

Metmknice

You can run without cat**, but depending on year you might get codes and/or run rich. I would recommend replacing with a new high-flow cat and runnig straight pipe as far back as you want to go. I guess it all depends on how loud you want it.

**note, running without a cat is illegal unless it is strictly an off-road vehicle. Figured I would say it nicely before someone else does..

bigbadxj

Well then all my cars without them are off-road only HAHA

mike37

thats a myth

radio flyer

Is that so

bigbadxj

I'm with you I have been building street/strip drag cars and off-road trucks for a long time and I believe the whole back pressure thing is B.S. I have read a hundred articles and charts for one way or the other and it just doesn't make that big of a deal to worry about. just like being able to feel a 5-HP gain. It's all in your head

radio flyer

Taking off the cat is still the wrong thing to do, if he has an upstream o2 sensor and a down stream o2 sensor the computer will run in default mode without the cat. Not the way to run any engine for any period of time. Cats are not a punishment they serve a positive purpose by reducing pollution! High flow cats are the option to go with if you want more flow.I dont think that i have all the answers, i just believe that everyone should be responsible for what they do........

mike37

it helps on some corroborated motors depending on cam duration and
overlap

but on a comp controlled motor you want no back pressure
but you can still over size the egsost and ruin the scavenging effect

I think you should run a cat just get a hi flow one