Critique my exhaust plan

[kastein]

doesn't work quite that way... too small a pipe will restrict flow.

The whole point is to keep a significant amount of gas flowing as quickly as possible. The momentum of the gases in the exhaust system results in low average pressure in the system which remains below the pressure of the gases in the cylinders, which helps scavenge or suck the gases out of the cylinders.

If you put too small a pipe on the system it will in fact increase velocity, but it the gas will back up in the pipe, pressure will increase, and there won't be enough scavenging to really clear the cylinders.

At least that is how it makes sense to me.

I know my '98 only got more gutless when I pulled the muffler and tailpipe for testing, and worse when I bored a hole through the cat element to see if it was plugged. Turned out that the head gasket is smoked, and I have a new muffler, new cat, new tailpipe, and parts to put together a downpipe sitting around for installation after I swap a new motor into it so I can rebuild the old one outside the truck on spare time.

 

[ehall]

It's like trying to blow through a straw vs a garden hose, more effort to move the same volume of gas in the same amount of time--if you are able to do it then yes the gasses will have to go faster by necessity, but you will expend way more effort on getting to that point

The thing is to time the exhaust so that one exhaust wave is working down the pipe when the next cylinder expels its gasses, so that the negative pressure behind the first wave pulls the second wave through. Too large of a pipe and there wont' be any negative pressure to provide the scavenging affect, too small of a pipe and you will use up the power created on pushing the gasses out.

Optimal diameter is actually relative to RPM. If you are running 2000 RPM like us a smaller pipe will be better.. If you are running 4000 plus RPM then a larger pipe will be better. I use 2.25 on my jeep and my cadillac because I am usually in the 2-3k range, need the torque to start moving.

 

[twodoorXJ]

Ive got a summit high flow cat, a super 40. I get no drone, it sounds superbadass, and all the chicks love it.

 

[Monster Mopar]

Don't really want this thread to turn into another pissing match about back pressure guys. I know all about it, and there's tons of information out there.

Yes back pressure is a myth, yes you can have to large of exhaust, and we all know it can be to small. There's a sweet spot where size is maximized as well as exhaust gas velocity. From my experience 2.5" is to big on a mostly stock 4.0, it looses low end.

 

[clean4drxj]

I'm running an APN header stock down pipe(getting rid of the crimp by the collector) and a borla catback, and my jeep sounds awsome, not to loud but sounds just right with a deeper tone.

 

[Monster Mopar]

Be advised... I originally ordered a set of these this style Pacesetter header....

Thanks for the heads up, I've read your story in another thread there just doesn't seem to be any othr options for renix guys. Unless you can steer me in the right direction.

Monster Mopar,

I am have been researching a new exhaust system for my 98 also, here is what I have found..... I would go APN...

Actually I'd love to go with an apn but they don't (to my knowlege) make a header for the renix motor

www.performance-curve.com has good deals on cats and cat-backs. You could a stainless steel setup for less than $200 shipped.

I second the performance-curve.com recommendation.

thanks for the link guys and the input, I'll check it out!

 

[clean4drxj]

So if your running no muffler or cat a 2" pipe would be good?

usually when you're odering an exhaust system/piping they give you a size to horse power chart so what RC man is telling you is the truth. Unless your running a a stroker you shouldn't need anything bigger than a 2.25 inch exhaust.

 

[Monster Mopar]

Magnaflow cats flow well and they won't fall apart after a few months. I've seen some others where the brick started rattling within weeks

Yea I was just looking at their spun converters, interesting idea, seems stout.

 

[Monster Mopar]

If I go with a header designed 91+ Xjs then the only thing I need to do is weld a bung in for the egr tube, correct?

Thinking of going with the APN header now

 

[kastein]

I believe you might have to swap the downpipe as well. Not sure. And make sure the manifold gasket will line up properly - I forget what year you use to make the 91-99 manifold ports seal to the 87-90 head.

 

[Monster Mopar]

I'll do some searching, I know I've read the info on the gaskets, but don't remember anything about swapping down pipes though.

 

[Monster Mopar]

Okay during my searching I've found conflicting information on the apn header fitting the renix.

Has anyone expereinced fitment issues between the header tubes and the renix intake runners?? Some people say it fits and others say no it doesn't.

 

[Johnnie Walker]

If I go with a header designed 91+ Xjs then the only thing I need to do is weld a bung in for the egr tube, correct?

Thinking of going with the APN header now

Who needs egr? If you have visual/function emission test then yes you would need it. But its not necessary. Just block off the port at the intake. My EGR hasn't worked since I got my jeep. Mine is simple fix, but it runs better without it IMO.
I have a 14" 4x9 OVAL Maggy on mine with a turndown. Nice tone, no drone.

 

[Monster Mopar]

Either way I'm going to make the necessary modifications to run an egr if I choose to go with a 91+ header.

 

[Jeff-97XJ]

www.performance-curve.com has good deals on cats and cat-backs. You could a stainless steel setup for less than $200 shipped.

I would not use Performance-Curve for anything !!!!! Worst service that I have ever had, completely unresponsive to my requests.

 

[Monster Mopar]

hmm, most folks on here seem to like them.

Since this thread has been rebummed, I'll give a quick update

I went ahead and purchased the pacesetter headers and I'm more than happy with they way they fit together. and have no regrets with my purchase. I still need to purchase the rest of my exhaust pieces and will be doing that soon.