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I recently replaced my exhaust downpipe when I noticed it had a crush in it just above the O2 sensor.  The truck is a 1991 Shortbed 4.0L AX-15 4WD.

 

20200917_181926.jpg

 

20200917_181959.jpg

 

I compared this to my 1995 YJ 4.0L AX-15, which uses an almost identical downpipe (differences are on the catalytic conveter side).  The YJ pipe was straight and uncrushed.

 

20210209_100119.jpg

 

I assumed the MJ's pipe was damaged at some point in the past, so I replaced it with a new Walker unit.  I wrote about this in my build thread, to which I received numerous replies stating that people believed the downpipes were crushed like this at the factory (for some unknow reason).

 

The question for the community is:

 

Who has seen the same crush on their factory downpipes, and who hasn't?  If this is really something they did at the factory, why?  Pics welcome.

 

I'm hoping some of the members who posted in my build thread would post back here in tech (@Minuit, @olddude, @fiatslug87, @89 MJ, @Jesse J) as well.

 

Thanks.  This is more of a poll than a tech problem.  I replaced the downpipe and my truck is running great.  Mods should move this if necessary.  Thanks.

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Here's mine. And for what it's worth, this downpipe is going away fairly soon (it's nearly rusted through after the oxygen sensor) so I'll be able to say if replacing it makes a difference.

 

Py2GZKvl.jpg

 

nfxcgtml.jpg

 

 

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Well, I wasn't on the list but I've never seen one crushed. That is a huge difference between collector and downpipe size there! I run a JBA TJ downpipe which wraps around the front of the oil pan. But my stock 91MJ downpipe didn't have that crush nor did my YJ, TJR, XJ, ZJ or KJ.

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23 minutes ago, ghetdjc320 said:

That is a huge difference between collector and downpipe size there!

 

I think you're referring to the YJ pic, and that's just the angle of the pic making it look like a big difference between the two.  They're really close in size.  It's a 2.5" collector on a Banks revolver header funneling into 2.25" exhaust.

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7 minutes ago, kryptronic said:

 

I think you're referring to the YJ pic, and that's just the angle of the pic making it look like a big difference between the two.  They're really close in size.  It's a 2.5" collector on a Banks revolver header funneling into 2.25" exhaust.

Ahh ok, looks like at least 50% smaller in the pic lol

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If I had to guess as to why they did it, I'll put forward the two guesses I've heard most often:

 

1) the crush combines with the shape of the manifold for a scavenging effect, sucking the exhaust gases down into the rest of the exhaust system more quickly. If true, replacing the "restricted" downpipe would actually lower power. This is something that would almost certainly require a CFD analysis to confirm.

 

2) the crush actually restricts the flow of the exhaust gases on purpose to heat the catalytic converter up quickly after engine start, and the engineers considered any performance loss to be worth it for emissions compliance. If true, replacing the restricted downpipe would increase power at the cost of emissions until the converter warms up.

 

I've seen no before/after dyno numbers showing a difference in performance. If someone has them, I personally would love to see them.

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You guys are thinking way too into it.  Mine is crushed there as well as further down, from what I can tell the lower one is to clear the front driveshaft u-joint.  I had to crush mine a little more for the blue truck due to some rubbing right there.  The crush you are referencing above the O2 sensor is very likely to add clearance to something.

 

As far as the performance difference theories, the Engine Masters guys tested crushing your exhaust because it was a common thing to do to get everything to fit in the old muscle cars.  Their testing resulted in no changes with the exhaust uncrushed vs crushed, even on high performance engines.

 

"Back pressure" here we go...:twak:

 

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All factory XJ/MJ downpipes I have seen had the crush.  That's XJs with the single outlet manifold, not the later dual outlet type.  That's on the 4.0.  Can't recall ever noticing on 4cyl models.

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12 hours ago, Minuit said:

If I had to guess as to why they did it, I'll put forward the two guesses I've heard most often:

 

1) the crush combines with the shape of the manifold for a scavenging effect, sucking the exhaust gases down into the rest of the exhaust system more quickly. If true, replacing the "restricted" downpipe would actually lower power. This is something that would almost certainly require a CFD analysis to confirm.

 

2) the crush actually restricts the flow of the exhaust gases on purpose to heat the catalytic converter up quickly after engine start, and the engineers considered any performance loss to be worth it for emissions compliance. If true, replacing the restricted downpipe would increase power at the cost of emissions until the converter warms up.

 

I've seen no before/after dyno numbers showing a difference in performance. If someone has them, I personally would love to see them.

 

I am only quoting Minuit because what he is suggesting has some validity to it but his hypothesis is not quite correct. 

 

Background: I am an automotive engineer with a 4yr degree and 6 years of experience doing dynamometer based engine test cell work.

 

#1: Increased back pressure does not scavenge exhaust gases the way the internet thinks it does for some reason. Even if the engineers wanted this idea to work they would only be able to get it to work within a very narrow throttle position and RPM. For a low performance engine like the 4.0, an effort to do this, especially in the 80s would have been expensive and arguably useless. 

 

What backpressure does do is introduce more natural exhaust gas recirculation (EGR) into the combustion process. Higher backpressure will likely decrease your peak power by effectively reducing cylinder volume by allowing more exhaust gases to remain in the cylinder after the exhaust valve closes at the end of the exhaust stroke. The 4.0L non-pass through intake and exhaust flow on the head is so abhorrent that I seriously doubt that the crushed exhaust pipe is going to have any measurable impact on the performance of the engine compared to the flow profile of the head...

 

#2: Maybe, but unlikely. Cold start warmup is going to happen primarily at idle and almost certainly below 2500 RPM. The flow profile is so low that there would be little to no appreciable difference in the flow velocity in the pipe at that point. In most cases you would want unrestricted full flow to the catalyst face at startup for fastest warmup. Primary reason why they started using two catalysts (close coupled and a downstream under floor) was to decrease the cold start emissions. 

 

 As Dzimm posted... Almost certainly just to clearance something. I cannot think of any other reason why they would do that. 

 

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11 hours ago, Dzimm said:

 Mine is crushed there as well as further down, from what I can tell the lower one is to clear the front driveshaft u-joint. 

 

this is what I thought is was for. doesn't seem precise enough to have a good enough effect

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1 hour ago, Torq_Shep said:

 

I am only quoting Minuit because what he is suggesting has some validity to it but his hypothesis is not quite correct. 

 

Background: I am an automotive engineer with a 4yr degree and 6 years of experience doing dynamometer based engine test cell work.

 

#1: Increased back pressure does not scavenge exhaust gases the way the internet thinks it does for some reason. Even if the engineers wanted this idea to work they would only be able to get it to work within a very narrow throttle position and RPM. For a low performance engine like the 4.0, an effort to do this, especially in the 80s would have been expensive and arguably useless. 

 

What backpressure does do is introduce more natural exhaust gas recirculation (EGR) into the combustion process. Higher backpressure will likely decrease your peak power by effectively reducing cylinder volume by allowing more exhaust gases to remain in the cylinder after the exhaust valve closes at the end of the exhaust stroke. The 4.0L non-pass through intake and exhaust flow on the head is so abhorrent that I seriously doubt that the crushed exhaust pipe is going to have any measurable impact on the performance of the engine compared to the flow profile of the head...

 

#2: Maybe, but unlikely. Cold start warmup is going to happen primarily at idle and almost certainly below 2500 RPM. The flow profile is so low that there would be little to no appreciable difference in the flow velocity in the pipe at that point. In most cases you would want unrestricted full flow to the catalyst face at startup for fastest warmup. Primary reason why they started using two catalysts (close coupled and a downstream under floor) was to decrease the cold start emissions. 

 

 As Dzimm posted... Almost certainly just to clearance something. I cannot think of any other reason why they would do that. 

 

I guess that post really, for once, was only half as long as it needed to be - I needed to write a couple more paragraphs of disclaimers. I thought when I posted two opposing points as to its purpose I was clear enough when I said that those two points were "guesses I heard most often" , not "my own conclusions on the subject, based on my education and my experience" - and frankly, I didn't think this stupid piece of crushed metal pipe was worth much discussion, but here we are.

 

#1 - Don't attribute that "backpressure" nonsense to me!  I know that you know that we both know that exhaust restrictions don't magically create torque out of nowhere. Ditto for any type of cylinder scavenging, and that any effect of the two is much smaller and works under more specific circumstances than the internet at large thinks. And in the other direction, yeah I've seen the video where the guy smashes headers with a hammer and it does pretty much nothing, even on a 500 hp engine.

 

#2 - 'tis a fair point, and I'm certain you know more on this subject than me.

 

But besides all of that - I can't see any situation, short of a very strange production line quirk that I can't think of off the top of my head, that the pipe would need to be crushed to make clearance for something. Installed in the vehicle, that pipe has plenty of clearance in every direction, and certainly doesn't need any of the free space created by the crush.

 

So I'm comfortable with leaving it exactly as I mentioned it in OP's build thread - No clue why, but I'm sure they had their reasons.

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14 hours ago, Dzimm said:

As far as the performance difference theories, the Engine Masters guys tested crushing your exhaust because it was a common thing to do to get everything to fit in the old muscle cars.  Their testing resulted in no changes with the exhaust uncrushed vs crushed, even on high performance engines.

That was a good episode

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1 hour ago, Minuit said:

I guess that post really, for once, was only half as long as it needed to be - I needed to write a couple more paragraphs of disclaimers. I thought when I posted two opposing points as to its purpose I was clear enough when I said that those two points were "guesses I heard most often" , not "my own conclusions on the subject, based on my education and my experience" - and frankly, I didn't think this stupid piece of crushed metal pipe was worth much discussion, but here we are.

 

#1 - Don't attribute that "backpressure" nonsense to me!  I know that you know that we both know that exhaust restrictions don't magically create torque out of nowhere. Ditto for any type of cylinder scavenging, and that any effect of the two is much smaller and works under more specific circumstances than the internet at large thinks. And in the other direction, yeah I've seen the video where the guy smashes headers with a hammer and it does pretty much nothing, even on a 500 hp engine.

 

#2 - 'tis a fair point, and I'm certain you know more on this subject than me.

 

But besides all of that - I can't see any situation, short of a very strange production line quirk that I can't think of off the top of my head, that the pipe would need to be crushed to make clearance for something. Installed in the vehicle, that pipe has plenty of clearance in every direction, and certainly doesn't need any of the free space created by the crush.

 

So I'm comfortable with leaving it exactly as I mentioned it in OP's build thread - No clue why, but I'm sure they had their reasons.

I wasn't attributing it to you, your post was just convenient to build off of. Its also not a completely false argument.

 

I consider you to be pretty intelligent and this was in no way meant to insult you. 

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Anecdotal evidence suggests this crush in the pipe was present on 4.0L MJs and XJs when they left the factory.

 

As far as the clearance argument, there's nothing to clear in that area.  As such, I can't see clearance as the reason for these crushed pipes from the factory.

 

As far as the emissions argument, I find it very hard to believe that Jeep's answer to improved emissions was a guy on the assembly line with a hammer crushing downpipes.  If there needed to be a restriction in the pipe for emissions, they would have manufactured that into the pipe for the 190,446 Comanches and 2,932,013 Cherokees they built.

 

How many licks does it take to get to the center of a Tootsie Pop?  We may never know.  Same with this.

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All three are ho.

first one is my 92mj20210210_151549.jpg.7d70d69dfc29e9e8c1a1a428f0a09a5b.jpg

 

94 xj20210210_151640.jpg.9600c2a7077486190e0ba8c285beff83.jpg

 

??xj

20210210_151507.jpg.bf1036b3f588205930437b1f5b6954ef.jpg

 

I also had a 95xj that had the crush. 

 

I always thought it was to clear the front yoke when the suspension compresses. 

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