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Unread 11-09-2012, 03:14 PM   #1
kbrawlz
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1995 YJ Wrangler 
 
Join Date: Dec 2009
Location: Portland, Oregon
Posts: 59
4.0L downpipe fix !?....

I'm not sure why, but every 4.0L downpipe I see below the exhaust manifold has a huge dent at the bend.

4.0L downpipe pre-weld by kbrawlz, on Flickr

I don't know if that is to restrict the exhaust flow and give it more torque. But for some reason every downpipe I had (from a HO 4.0L) didn't fit/align my Renix MJ after doing the head swap.

So, since I was going to cut the pipe to make it fit better, I thought I'd just weld in a new section with no dent. I tried heating the pipe to it's austenitic range and bend the dent out from the inside, but just didn't have the right tools, mandrels, or clamping devices to do it. So welding was just the easy way out.


4.0L downpipe pre-weld 2 by kbrawlz, on Flickr

4.0L downpipe pre-weld 3 by kbrawlz, on Flickr

4.0L downpipe pre-weld 4 by kbrawlz, on Flickr

I had to do a little bending, shaping and flaring for everything to fit seamlessly. But I tacked em together and test fit the pipe on the manifold and flange support on the cross-member. It looked good, so I burned it in. I must have had the heat just right cause I got full-pen on all the seams except for one spot that I touched up. Then cleaned it up with a flap wheel. I didn't take photos, but i painted it with 1200 degree flat black stove paint for whatever reason.


4.0L downpipe after weld by kbrawlz, on Flickr

4.0L downpipe after weld 2 by kbrawlz, on Flickr


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Unread 11-09-2012, 10:22 PM   #2
SnowYJ
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1988 MJ Comanche 
 
Join Date: Jun 2009
Location: OR
Posts: 1,058
Wow man nice wrote up!
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Unread 11-09-2012, 11:00 PM   #3
whitexj98
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1998 XJ Cherokee 
 
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Location: Sterling Heights, Michigan
Posts: 813
It's there to clear the front drive shaft... on a stock vehicle. You think you gained much performance?
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Unread 11-10-2012, 06:36 AM   #4
cruiser54
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1990 MJ Comanche 
 
Join Date: Oct 2009
Location: Prescott, Arizona
Posts: 10,322
Quote:
Originally Posted by whitexj98 View Post
It's there to clear the front drive shaft... on a stock vehicle. You think you gained much performance?
Not really there to clear the driveshaft. It was made to APPEAR that way. The dent was put in there on a waiver from the EPA to heat up the catalytic converter sooner.

That said, if you replace your headpipe with a dentless one from a parts store, you can hit it with your front driveshaft when wheeling at stock height. At 3" of lift, you won't.

I have the local muffler shop make and install a new headpipe that travels in two 45* angles instead of the factory 90* angle and it clears the shaft no matter what. That's what teh factory coulda done in the first place if they didn't need to put the dent in the headpipe.

Oh, and YES, you will feel the power difference.
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