So I have 93 sport with 4.0, and I am going to be putting on a cat-back exhaust, I am used to mustangs and what not and with some exhaust systems you have to drop rear axle to fit exhaust that curves over the rear axle, is it the same on xj's, or if I just lift vehicle up by chassis and not axle can I snake it over rear axle?

Now question for the experts, I am getting ready to install cat-back, and I am curious as to how important back-pressure really is on these motors. On all my other cars the exhaust is straight through so no back-pressure but they either have a turbo (which provides back-pressure itself) or tuned headers, and my NA cars with tuned headers and many miles on them with zero problems, they love the exhaust and make most power that way, but none are a OHV straight 6, one is a DOHC 4 banger and the other is a OHV v8 mustang with tuned headers, x pipe and turn downs and they have no problems. But I have read on this site that running exhausts with no backpressure can burn out valves on these motors, and I am not familiar at all with OHV straight 6's, and I know a cat provides back pressure but I know its nowhere near ideal, and it seems to hinder the scavaging properties of a good exhaust. I have heard you want a more laminar flow to create a low pressure zone behind each exhaust pulse due to the fact we have high velocity pulses. I mean I have read Bernoulli's principle and it seems that no back pressure would be ideal, it just that the stock tune of the computer is tuned for backpressure so with no back pressure the fuel curve wouldn't be ideal and can burn out valves or whatever, but can't that be taken care of with a better fuel curve through something like the JET performance module? And I am not saying this just cause I want an obnoxiously loud vehicle, I mean I can install resonator to quiet it and what not, I just want the most HP and TQ at as little cost as possible as the jeep is to pull my track car and winter driving, and some people say a little back pressure is good for torque but can't that be improved with the revised fuel curve? Also I am not saying I advicate driving a vehicle with no cat on the street, I am just asking for general knowledge.

 

these engine definitely need back pressure without it you will lose performance. you dont need alot of back pressure, but you do need some. i dont advise driving with out the cat that can hurt performance.

 

But if you have proper tune doesn't that violate the laws of fluid dynamics, thermodynamics and the law of the conservation of energy?

 

Also one of the reasons I am also asking is I will be getting a 2.5" cat-back exhaust, and on my cars I usually run Borla products as I love the sound, I just hate the tip the come with so I usually just cut it off and weld on stainless pipe of same diameter as rest of exhaust, and I had planned on getting just some 2.5" cat-back, then buy a 2.5" borla muffler to put in since the Borla kit is only 2" after the muffler, and from my experience all the mufflers I have seen from Borla are of the straight through design, and if you want back-pressure the cat is definetly not the best place to get it from as it is not designed for that (on any vehicle I can think of anyways)