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  #1  
Old December 17th, 2017, 14:11
RCP Phx RCP Phx is online now
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3" Exhaust

Can you fit a 3" full exhaust on a factory equipped(design) XJ? I just did a 2-1/2" exhaust but I think it's kind of restrictive.
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  #2  
Old December 18th, 2017, 00:29
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1990JEEPXJ 1990JEEPXJ is offline
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Re: 3" Exhaust

Why do you think it’s restrictive?
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  #3  
Old December 18th, 2017, 07:56
trippled trippled is offline
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Re: 3" Exhaust

I think boostwerks had a 3in setup for his turbo. Might look at that. That being said, I I doubt you need a 3in even for a stroker.
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  #4  
Old December 18th, 2017, 09:29
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Re: 3" Exhaust

You can but pretty sure you need some back pressure and 2.5" should be adequate. Unless you're running a turbo really no need to go up to 3".
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  #5  
Old December 18th, 2017, 09:44
burntkat burntkat is offline
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Re: 3" Exhaust

I have to agree. If anything, 2.5" is a bit excessive for a stock motor.
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  #6  
Old December 18th, 2017, 10:04
RCP Phx RCP Phx is online now
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Re: 3" Exhaust

It's 4.6 with a R.Pottenger big valve ported head/intake. My only hint is that it gets a little fat at the top end at W.O.T.! Although the entire exhaust is brand new, it was a complete re-pipe done with a 2-1/2" stock style muffler/cat on a Banks Revolver header.
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  #7  
Old December 18th, 2017, 15:02
lawagoneer lawagoneer is offline
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Re: 3" Exhaust

I agree with you the 2.5 inch exhaust is probably restrictive on you 4.6. V8 4.6 motors run dual 2.5 inch pipes. The only thing I can say about running a 3 inch pipe is that it is much more difficult to fit in spaces where smaller pipes go easily. I've put 3 inch pipes on a couple of cars where I thought it would be no problem and had some clearance issues where I had none with 2.5 inch. I see you put stock muffler and cat back on and I think changing your muffler and cat to high flow units would do everything you need without going to 3 inch.
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Old December 18th, 2017, 16:31
RCP Phx RCP Phx is online now
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Re: 3" Exhaust

Quote:
Originally Posted by lawagoneer View Post
I agree with you the 2.5 inch exhaust is probably restrictive on you 4.6. V8 4.6 motors run dual 2.5 inch pipes. The only thing I can say about running a 3 inch pipe is that it is much more difficult to fit in spaces where smaller pipes go easily. I've put 3 inch pipes on a couple of cars where I thought it would be no problem and had some clearance issues where I had none with 2.5 inch. I see you put stock muffler and cat back on and I think changing your muffler and cat to high flow units would do everything you need without going to 3 inch.
That's the original question. will it fit! As for a "free" flowing exhaust, I don't want any noise above stock!!!
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  #9  
Old December 18th, 2017, 19:35
Dragonlich1961 Dragonlich1961 is offline
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Re: 3" Exhaust

my 2.5 inch pipe is tight due to everything else in that area. a 3 inch would not fit in the stock location with a shackle relocate bracket, ext shackles, tank skid, and trailer hitch. you are going to have to look for a non stock cat and muffler with the biggest case you can fit. my recomendation would be to run a 3 inch down pipe to cat and muffler, and a 2.5 tail pipe to keep the noise and drone down. as exhaust cools it condenses and loose velocity, reducing pipe size will help to keep the velocity up. i don't think full 3" is need for a NA motor. boostworks had a turbo on hiswith 3", and switched to a 2.5 tailpipe upon remoing the turbo.
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  #10  
Old December 18th, 2017, 21:02
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Re: 3" Exhaust

3" will fit, may be a bit tricky in the back making it clear your leaf spring.

I run 2.5" with my LS (~380hp) and no probs FYI.
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  #11  
Old December 18th, 2017, 21:29
lawagoneer lawagoneer is offline
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Re: 3" Exhaust

Quote:
Originally Posted by RCP Phx View Post
That's the original question. will it fit! As for a "free" flowing exhaust, I don't want any noise above stock!!!
With the parameters you set and having a stroker try finding a really good muffler shop to fit the 3 inch. You may find it hits and some clearance issues and with the stock cat and muffler a lot of work for little or no improvement.

Remember a internal combustion engine is just an air pump. You have increased the size of the pump but put known restrictive muffler and cat---that is your chock point and hoping to fix it with larger pipe. Without decreasing the restriction, muffler and cat, you won't see any improvement.
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  #12  
Old December 19th, 2017, 01:02
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Re: 3" Exhaust

I'm going to be building a 3" exhaust for my LS. I cut and folded the rear so i'm going to run it above the rear axle and out the side. I believe boostwerks ran it the same way.
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  #13  
Old December 19th, 2017, 03:42
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Re: 3" Exhaust

Quote:
Originally Posted by RCP Phx View Post
Can you fit a 3" full exhaust on a factory equipped(design) XJ? I just did a 2-1/2" exhaust but I think it's kind of restrictive.
I've expounded on this some time ago, but installing an overlarge exhaust can actually cause enough turbulence to be as restrictive as installing an exhaust that is too small. I recall doing the numbers on this, and the 4.0L engine works just fine with 2.25-2.5" ID exhaust (it actually worked out to about 2.375", as I recall, but those are the tube sizes to either side of it.)

If you're worried about restriction, here are a couple of things to try:
- Change the catalytic converter for a unit that is spec'd for single-unit application for a large V8 engine (can typically be found in 1980's Chevvy van applications, with the right size inlet & outlet.)
- Change the muffler for a less restrictive unit - spec'd as above, or a low-end performance unit (I like using Walker Turbo II mufflers)
- There is a slight crimp in the downpipe after the exhaust collector, in most cases (I'm sure it's there on RENIX - 1987-1990 - I think it's there 1991-up...) this may be cut out and replaced with a flex joint - which is a good idea anyhow, as the XJ exhaust system is overly rigid, and this is what contributes to manifold cracks (vibrations and stresses get transferred up the plumbing into the weldments and the HAZ of the weldments, and they fail first.)

But, installing a 2.5" exhaust is fine for a stock 4.0L engine, up through a mild (through 4.5-4.6L, IIRC) stroker build. Then, go with a 2.75" setup, and 1.75" primaries.
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  #14  
Old December 19th, 2017, 13:33
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Re: 3" Exhaust

If you want to know if it is restrictive or not.....

1) make a stub pipe that comes off the downpipe and dumps out right there under the jeep

2) find a track that has a test & tune night

3) run the jeep once or twice with full exhaust, and then once or twice with just the stub pipe (make sure the engine is @ the same temp each time so as to be consistent)

if your times get better, then chances are you can use a bigger exhaust, if they slow down, not so much

You can also compare WOT fuel if you have that capability
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  #15  
Old December 19th, 2017, 23:49
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3" Exhaust

Have you read the 4643 Jeepspeed thread?
I do not recall them using 3" pipe.
Do not know of anyone that has a stroker using it either.
Only way to know for sure if you need to go larger would be dyno run/tuning, etc.
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