Ran into a Problem at the Exhaust Shop.

[streetxj]

Here's an update on the situation. Before I changed my wheels, my dad took the truck to the exhaust shop where they have done work on my jeep before. They have been there for a long time and they do a great job. The part of the stock down pipe that had the infamous dent/pinch was replaced with a smoothly bent piece by them a little more than a year ago. Later on, I noticed that my stock downpipe is actually 2.00 inches in diameter which then grows to 2.5 inches suddenly. The whole thing is mandrel bent at the factory, so that's a plus. That's how all factory 96-99 4.0 XJ down pipes are. So, I wanted to replace the 2 inch section with a 2.25 inch pipe from the header collector to a bit after the point where it turns into 2.5 inches right before the 90 degree bend before the cat. When he got there they looked at everything and even unbolted the down pipe from the stock 96' HO header. Then they told my dad that the stock header collector was 2 inches (I've read that the stock header outlet is 2 inches so it's a legit statement) and that they couldn't make a larger diameter pipe that would fit properly on the stock header collector. Then they've mentioned that if I want, they can install a larger pipe right under the stock diameter flange of the header (leaving a small section of 2 inch pipe), but it was going to be 60 bucks.
Thats when they told my dad that he would just be wasting money in putting a pipe that is larger than the header outlet and that removing the factory pinched pipe was good enough, which they already did when they first worked on my exhaust. At least they were honest. They also told me that in order to install a larger diameter pipe, I'll need to install an aftermarket header with the 2.25 inch outlet/collector to make it a worth while modification. I would like to get your opinions on that.

 

[Alexia]

Having a short section of the small diameter pipe is similar to having a dent in it. There is really no point in installing a larger down pipe if the header does not support it.

 

[streetxj]

Having a short section of the small diameter pipe is similar to having a dent in it. There is really no point in installing a larger down pipe if the header does not support it.

That's exactly what they've mentioned. The first part of the downpipe has always been 2 inches in diameter. But now there is no large dent, which used to penetrate into the exhaust path. Now it's all smooth. As of now, the jeep has a nice power band. It's very torquey across the whole rpm range. Basically, it doesn't feel like it's getting choked. Maybe I should just save $$$ for a chris-tuned ECU. Thanks for your feedback.

 

[Dr. Dyno]

They also told me that in order to install a larger diameter pipe, I'll need to install an aftermarket header with the 2.25 inch outlet/collector to make it a worth while modification. I would like to get your opinions on that.

Your exhaust shop is right and it's good for you that they're honest. Props to them.

 

[O-Gauge Steamer]

The shop I use, here in the Springs, is an honest "hole in the wall" type ran by a guy about as old as I am. He does not hold with taking money under a false flag.

To that end, I ran replacing the pipe with the dent and it was his opinion that very little to no gain would be in the offering.

Anyone have dynamometer proof that doing this results in a gain?

 

[yossarian19]

I have no dyno proof. I can say with certainty, though, that at stock height it is very difficult to get the pinion yoke to clear a modified down pipe. Also, my butt dyno noticed no real improvement after the mod. By contrast, I can feel my cold air intake.
If I recall my reading correctly, the exhaust scavenging effect is controlled mostly by the first couple-few feet of the exhaust system. Header tube length, down pipe length / shape / diameter, up to the cat. A larger header collector and a higher flowing down pipe will move that scavenging effect to higher RPM, sacrificing a little bit of lower end power. Overall power will be improved. I recall also, from whatever source, that the scavenging effect is really only moved from 900 RPM to 1400 RPM when you put headers & down pipe on a jeep. I wish I could remember all this with footnotes and more details but I just don't.
Basically, the real restriction to flow in a Jeep exhaust system is cat back. Modifying the cat-forward section is good for a streeted Jeep or a racer but for the DD / off road use that most of them see, stock is good enough (and it clears the blasted pinion yoke just fine)
Somebody correct me if I'm wrong.

 

[streetxj]

I have no dyno proof. I can say with certainty, though, that at stock height it is very difficult to get the pinion yoke to clear a modified down pipe. Also, my butt dyno noticed no real improvement after the mod. By contrast, I can feel my cold air intake.
If I recall my reading correctly, the exhaust scavenging effect is controlled mostly by the first couple-few feet of the exhaust system. Header tube length, down pipe length / shape / diameter, up to the cat. A larger header collector and a higher flowing down pipe will move that scavenging effect to higher RPM, sacrificing a little bit of lower end power. Overall power will be improved. I recall also, from whatever source, that the scavenging effect is really only moved from 900 RPM to 1400 RPM when you put headers & down pipe on a jeep. I wish I could remember all this with footnotes and more details but I just don't.
Basically, the real restriction to flow in a Jeep exhaust system is cat back. Modifying the cat-forward section is good for a streeted Jeep or a racer but for the DD / off road use that most of them see, stock is good enough (and it clears the blasted pinion yoke just fine)
Somebody correct me if I'm wrong.

Mine is not a 4x4 so I don't have to worry about pinion clearance. I have a 2.5 inch thunder bolt high-flow cat and a 2.5 inch aero turbine muffler with 2.25 inch dynomax tailpipe w/ 2.5 inch polished stainless tip. Originally I had a 2.25 inch dynomax super turbo catback installed along with the smooth pipe to replace the pinched section. Then I went back to that same exhaust shop and had the aero turbine installed on there. After that first time I had the exhaust work done, it felt like it lost a bit of low end oomph. Then I upgraded to a 62mm throttle body, then to a 2003 4.0 intake manifold (with thermotec heat wrap under it). Felt a huge gain there. Then I changed over to the 2.5 inch AT2525 muffler. I actually felt a small gain in the lower rpms and it seems to pull harder now in the higher rpms. Guess that vacuum effect of the aero turbine actually works.