My 2cents from a newbie that only has research on his side
From what I have read the best set up is a header that flows from 3 to 2 to 1 like this one http://rustysoffroad.com/mm5/merchan...y_Code=headers
As far as going to a larger than 2.5 diameter pipe it has been stated that all is good with longer pipes to improve exhuasting the spent combustion chamber gasses thus allowing more fresh air into the combustion chamber giving a better explosion, BUT the larger than 2.5 diameter pipes are said to effect the back pressure of the system ( I`m guessing it has something to do with the compression like a vaccum cleaner going to big will only cause a tornato affect instead of a straight flow ) and rob the engine of it`s good gass mileage due to the combustion chamber still being full of gasses from the last explosion...
This is just what I can gather from what I have read, Myself I have decided not to go with a header as my stock manifold is still in good condition and my 96 cherokee is only a stock engine and the only noticable gain I would get is a louder growl... I have up graded my exhuast system to a 2.5 high flow from the bottom of the stock manifold to the tail pipe and this setup will help my engine breath better and improve my gas mileage a little, My next upgrade is a cold air intake and larger throttle body to better improve the fresh air flow into the combustion chamber, of course after I has researched the crap out of everything I hate waisting time and money

 

I have a header like you posted, but i spent $130. Ive heard there isnt much gain in going from 2.25 to 2.5 unless you have a stroker.

 

That`s what I`ve been reading also, alot of guys off the jeep forums are saying it would be a waist of time and money ( you would see no worthy gains in gas mileage or horse power ) & that the header was designed for racing applications and larger compression engines as a stroker would have, so the stock compression of a ho 4.0 is ok with the stock manifold unless you plan on flooring the gas peddle and racing in the Baja. But if you have the money and really like the growel it gives I would go for it, I`m just to cheap

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Back Pressure is not as much of a factor as everyone makes it out to be.

 

i call bulls#$% back pressure is very important i went from 4 into 1 headers to a set of zoomies (4 individual pipes) on my rail and lost .8 of a second in the 1/4..to much exhaust WILL rob power and can heat up exhaust valves

 

and going from the "tubular manifold" like on my 98 to a long tube header is kind of pointless ... i swapped out a cast manifold on a buddies 95 for a stock "header" i had and it picked up noticibly

 

This.

And moar this.

 

the actual backpressure is not a good thing

the scavenging effect is what you want, just enough to pull spent gasses effieciently.

 

Call BS all you want. Back Pressure is NOT neccesary on an engine that can modify its air/fuel ratio like our XJ's and every other fuel injected vehicle can.

Want my source?

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

I have more if you want them.

 

i tell ya what bring your jeep to the track or a dyno.....run it full exhaust,run it with a proper performance exhaust, then rip the whole thing off....bet you lose power wide open...too much exhaust WILL LOSE POWER!!! for that matter ANY stock motor will produce more power with the proper exhaust over a super free flowing system or open headers

 

and just to clarify i'm not saying the stock system is the way to go just saying if you put to open of an exhaust on a engine it will not run it's best

 

even with the modifying of the AFR's , the motor will still result in less power because of the spent gasses that are ALSO in the cylinder during combustion

 

Now wait, before you go crazy trying to defend your point, I did not say that running with nothing will better an engine. I stated that back pressure is not near as big of an issue as people keep making it out to be. Of course if you run any engine without an exhaust it won't be tops; that's the simple of science of chemistry thermodynamics and gas laws. However, with a good system that allows for a pressure system to build (not a major one like many say is needed for torque) the lower pressure will 'pull' the higher pressure and temperature exhaust out of the combustion chamber following the velocity of the exhaust being forced out by the piston.

 

What? The only spent gas left in the cylinder is that with which was not able to flow to the lower pressure zone in the exhaust system with the rest of the expelled gases. I'm not understanding what you're saying here.

 

all adding fuel to a lean burn due to exhaust will do is gas wash cylinders