I have a 90 XJ with 4.0l, AW4 and 231. It is on 5" of lift with 33s. 4.10 gears. The exhaust is leaking at the manifold gasket. I've had the jeep 5 years now and haven't touched the exhaust forward of the cat. I am contemplating changing out the manifold, down pipe, and cat - they're getting a bit crusty. The EGR is already deleted. I see the 91+ stuff is more readily available and cheaper. I can weld an O2 bung to the down pipe. Looking for recommendations on a good flowing setup. Also, anything I should change out while I'm in there?

 

An alternative on exhaust manifolds:

As far as exhaust, you can use the Renix exhaust manifold and be fine.
If you want to use the HO exhaust manifold, you must go with an HO headpipe and screw your O2 sensor into that headpipe. Standard Renix harness is plenty long to do so.
An additional trick is to NOT use the standard downpipe with the "crush" in it. Have a muffler shop make you one.

 

I was hoping there was an aftermarket piece without the 'crush' in it. I don't know of a good muffler shop around here I can trust, will have to find one.

Also, is there a power advantage to using the HO manifold?

 

Someone may chime in with the part number for the downpipe with no crush.

 

bump. does anyone know of a downpipes with no crush>

 

The only downpipe I know of is from Walker and the fitment on it is horrible.

 

https://upsw.com/1999-jeep-cherokee-...ont-pipe-a9042

 

x2 that the Northern Exhaust front pipe is pretty solid. I just bought one. Stainless steel and mandrel bends look nice. I ended up not using it because it does hang a bit lower where the pipe crossed from driver's side to passenger's and I wanted to prioritize clearance. It bolted up just fine and looked proper though.

 

That is a nice looking pipe, but it shows for the later years. They do not have one for a 90 (I have a 90). Anyone know what the difference is? I can cut/weld as needed to make it work if it's just a little difference. I'm going to have to get creative with the exhaust pipe anyways to clear my long arm setup.

 

The difference is everywhere, the location of the collector on your 90 is in a different location and angle and the routing of the down pipe is completely different. I have this downpipe in my parts collection due to it not working on my 88, saving it for when I swap to header when my manifold finally cracks.

 

DOH. My bad for missing that, sorry. I was just excited to share about something I just went through.

FWIW, the pipes are cheap enough that they might be worth buying with the sole intention of cutting and modifying. However, if I were in your shoes, I'd probably take it to an exhaust shop.

A word about downpipe size:

Several years back I had a shop make me a custom mandrel bent down pipe/front pipe/mid pipe that was 2.5" all the way. The rest of the system was also 2.5" all the way out to the tip. After installation, I definitely noticed my torque curve shifted to higher rpm. I lost a bit of the low end grunt. It was mostly noticeable going up long grades at higher speeds.

When shopping for a down pipe replacement this time around, I got into the weeds a bit with the sizing. Did some research on different forums and engineering resources trying to determine the best pipe size for what I wanted. Long story short: the OEM size is best for low end torque, and starts to become a restriction around 2800 rpm.

The OEM downpipe (on my 99, at least) is 2" ID out of the manifold collector and through the first couple bends. It then transitions to 2.25" ID as it crosses under the transmission.

The reason the smaller ID is better for low end torque is because the gasses flow faster which aligns the scavenging effect of reflected pulse waves with a lower RPM. This is a compromise of low end torque vs high end power because if the pipe is too small for the given volume of gas, the power required to push the gas through the pipe begins to have a non-negligible effect on output.

I arrived at the conclusion the OEM pipe is optimal up to approximately 2800 rpm using a rule of thumb proposed by David Vizard which states a lossless exhaust needs to flow 2.2 CFM per horsepower at the flywheel. The calculation is laid out in this forum post: https://forums.justcommodores.com.au.../#post-2487281. Note its just a rough estimation and should not be taken for gospel as this makes a lot of assumptions, rounding, and does not incorporate the losses due to bends.

 

No worries on the minor oversight XJlimitedx99, I appreciate all of the information. I originally posted this in March of last year. I came to the conclusion I would take the jeep to an exhaust shop once I get the long arm setup done back then. abeb123 revived this thread from the dead. I'm building mine as a rock crawler, not a bouncer - so high horsepower isn't a concern to me. I'll keep it the oem size. Hell, the crushed in part hasn't hindered it much to this point, may just fabricobble it into working around the long arm setup.