Jeep 4.0 Down pipe Mod.

[streetxj]

I don't remember if I have made a thread similar to this before, but just in case, I have a question about increasing the exhaust diameter from the header outlet to the part in which the factory 96' 4.0 downpipe increases to 2.5 inches. My down pipe is just like this one:

Even though the first half of the down pipe is 2 inches, I've read that the stock header diameter is 2 inches and some other after market headers like the Gibson and Banks Revolver have a 2 inch outlet while some others like APN for example have a 2.25 inch outlet. My stock header has no cracks whatsoever and I would like to keep it since it's a decent flowing header from factory. I got rid of the infamous exhaust dent/pinch. I'm just wondering, is it worth increasing the 2 inch section of the down pipe to 2.25 inches?
I'm planning on sending my ECU to California to get christuned in the future. Hopefully this winter depending on the financial situation.

 

[Dr. Dyno]

I'm just wondering, is it worth increasing the 2 inch section of the down pipe to 2.25 inches?

I'd go even further and increase it to 2.5".

 

[streetxj]

I'd go even further and increase it to 2.5".

Sounds tempting. The 1/2 inch difference between the header outlet and the downpipe won't have any negative impacts in flow? Thanks.

 

[Dr. Dyno]

The 2" diameter section is definitely restrictive.

 

[clean4drxj]

The 2" diameter section is definitely restrictive.

on a stock a stock motor(w/bolt ons)?

 

[streetxj]

On a stock motor with no airflow enhancing mods, it might not affect the flow much. But, once you start increasing the power ratings, it becomes a choke point. In my case, I have a "improved" stock intake system with K&N panel filter, 62mm throttle body, 2003 4.0 intake manifold with thermotec heat wrap covering the under side of it, 2.5 inch high flow cat w/ 2.5 inch Aero Turbine muffler which exits into a 2.5 inch pipe that extends into the 2.25 inch mandrel bent dynomax tail pipe with 2.5 inch tip. I also have precision auto fuel injectors and taylor thundervolt 8.2mm ignition wires, accel cap & rotor, and jegs performance ignition coil. Here is the difference between 2 inch and 2.25 inch:

 

[Dr. Dyno]

on a stock a stock motor(w/bolt ons)?

Yes, a 2" pipe is still restrictive even on a stock motor. You'll definitely notice a gain in performance and a throatier exhaust sound by increasing the pipe size to 2.5".

 

[clean4drxj]

Yes, a 2" pipe is still restrictive even on a stock motor. You'll definitely notice a gain in performance and a throatier exhaust sound by increasing the pipe size to 2.5".

interesting but won't that move your torque curve ?

 

[Dr. Dyno]

interesting but won't that move your torque curve ?

It'll move peak TQ to slightly higher rpm but you'll still gain something down low. The puny, restrictive exhaust system is one of the reasons why the mighty 4.0 came out of the factory with a measly 190hp.
On my 4.6L strokered XJ I have a 3.0" diameter header outlet and front pipe (2-3/4" at the bends) followed by a 2.5" system further down, and it makes stump-pulling torque from right off idle.

 

[streetxj]

Some pics of my current exhaust..

 

[Truckedup]

I'm thinking the worst restriction on a stock exhaust head pipe is the flattened area close to the manifold flange.
I had a hi flow cat back exhaust on my 96 XJ. I never really noticed a power increase. In fact it felt a bit slower.............

 

[streetxj]

I'm thinking the worst restriction on a stock exhaust head pipe is the flattened area close to the manifold flange.
I had a hi flow cat back exhaust on my 96 XJ. I never really noticed a power increase. In fact it felt a bit slower.............

 

[streetxj]

^Why can't I see my reply?

 

[richslezak]

Is that custom made? I am replacing my cracked header with Banks revolver and will throw in two MORE motor mounts and tranny mount. Have a full cat back system and recently installed a flex pipe. I think the header cracked because there is no flex to the exhaust as it's all welded.

The only thing that is not right is that damn awful downpipe with that nasty factory dent. I looked all over and can't find a good quality downpipe.

 

[AnthonyF]

That pipe is sex.

 

[Dr. Dyno]

The only thing that is not right is that damn awful downpipe with that nasty factory dent. I looked all over and can't find a good quality downpipe.

You'll need to have one custom made. Any competent exhaust shop should be able to make a mandrel bent 2.5" pipe for you.

____________________________________________
1992 XJ 4.6L "Poor Man's" Stroker
202rwhp @ 4700rpm, 258rwtq @ 3400rpm
1/4 mile: 14.63 @ 94.38, 3450lb curb
AX15, NP231, D30/D35
Jeep Performance, Jeep Tech, Junker to Stroker

 

[ehall]

Ask around for performance exhaust shop that can do their own bends and stainless welding, might take some looking but they are around. Most of the shops near me failed one or the other, eventually I found one a couple of hours away from me, worth a day trip. Prepare to spend a couple of hundred bucks.