Remove "dent" on downpipe?

[Telefunken]

Most Xjs have a factory dent in the exhaust pipe right after it meets the header. Ive read on jeepforum its there to create back pressure or to clear the control arm at full stuff. Not sure which is true.
But, are those of you with strokers cutting this out and replacing with a straight piece or leaving in the crushed piece?

[Cheromaniac]

I had the original 2-1/4" outlet of my Borla header cut off and replaced with a 3" outlet. My custom made headpipe is also 3" but since it isn't mandrel bent, it narrows down to 2-3/4" at the bends.

[5-90]

Most Xjs have a factory dent in the exhaust pipe right after it meets the header. Ive read on jeepforum its there to create back pressure or to clear the control arm at full stuff. Not sure which is true.
But, are those of you with strokers cutting this out and replacing with a straight piece or leaving in the crushed piece?

It's there to clear the control arm at full stuff - "creating backpressure" is something that makes me die a little bit inside everytime I hear it in a performance context (although it has been done for emissions control reasons, to encourage exhaust gas reversion to replace the EGR valve. Both ideas are equally silly, and there are better ways to handle reduction of NOx...)

If you've got a lift, you can get away with replacing the downpipe with one that doesn't have the dent. However, if you go full stuff at stock height, it's even odds that the UCA and downpipe will "self-clearance" - and the job won't be anywhere near as clean (it's a nicely-formed "dent" in the pipe from the factory, so the interference with laminar airflow and the resultant turbulence is minimal.)

@Cheromaniac - what's the displacement on your engine? 3"ID sounds almost like overkill - which can cause more trouble than it's trying to fix (there is an optimax exhaust diameter to consider when dealing with naturally-aspirated engines. Too small, and it will inhibit exhaust flow due to insufficient flow area. Too large, and the uneven cooling of the exhaust pulse will cause turbulence in the gas flow and inhibit scavenging. That's why I always laugh at the ricebois with the 3" exhaust on their little NA Civic...)

[SilverXJ]

It's there to clear the control arm at full stuff -

Are you sure about that? I don't remember what year they did away with it, but my 2000 XJ doesn't have the dent. It also came with the LP D30. I'm inclined to think ifs for the drive shaft.

[5-90]

It's there to clear the control arm at full stuff -

Are you sure about that? I don't remember what year they did away with it, but my 2000 XJ doesn't have the dent. It also came with the LP D30. I'm inclined to think ifs for the drive shaft.

I know some changes were made to exhaust routing over time - they may have moved the downpipe enough to eliminate the need for the "clearance dent."

[Cheromaniac]

@Cheromaniac - what's the displacement on your engine? 3"ID sounds almost like overkill - which can cause more trouble than it's trying to fix (there is an optimax exhaust diameter to consider when dealing with naturally-aspirated engines. Too small, and it will inhibit exhaust flow due to insufficient flow area. Too large, and the uneven cooling of the exhaust pulse will cause turbulence in the gas flow and inhibit scavenging. That's why I always laugh at the ricebois with the 3" exhaust on their little NA Civic...)

See my sig. Displacement is actually 4563cc so it's not quite 4.6L. The 3" headpipe probably IS overkill but since it isn't mandel bent, it narrows down to a little over 2.5" at the bends so I'm OK with it. The rear half of my exhaust is 2.5" anyway and it's not much louder than stock. There's no drone at any speed; just a deep powerful tone under acceleration.

[Muad'Dib]

Moved to general tech...

Also, the "dent" was there for driveshaft clearance reasons. Not control arm. Not much you can do to get passed that without a lift or a custom exhaust routing.

[5-90]

Moved to general tech...

Also, the "dent" was there for driveshaft clearance reasons. Not control arm. Not much you can do to get passed that without a lift or a custom exhaust routing.

Thanks - picture in my head probably got confused (I've just obtained a motorhome built on a 1978 Dodge M500 chassis - between that and the work I've been doing on my wife's 2005 Suzuki Verona, mental images can get crossed up...)

[cruiser54]

Well guys, here's "the rest of the story". This comes directly from my buddy who was at JeepTech working in the emissions division when the 4.0 came out. I was Service Manager and Shop Foreman at a dealership when we met in the mid 80s. The "crush" in the headpipe was made to appear like it was for front driveshaft clearance. And, they routed the headpipe so it "needed" the crush spot. It's true purpose was to slow down the exhaust gasses to preheat the catalytic converter as a last ditch effort to get an emissions exemption from the Feds.

I have new ones made by the local muffler shop. He routes them in two 45* angles instead of the 90* angle like they were stock and they clear the driveshaft even if you're not lifted.