I will be upgrading my exhaust soon. I have a 2000 xj with front catalytic converters. I am having trouble finding a 2.5" inlet and outlet. I want the whole system to be 2.5 inches. I am probably going with banks torque tubes and monster exhaust. They offer down tubes but they are 2". Anybody know who makes quality down tubes and cats for my 4.0 that will mate with 2.5"? I figure if I try to put stuff in without the sensors its gonna run like crap, so I want it right. Need some help!

 

There is no advantage to 2.5" exhaust unless you have a stroker. 2.25" is ideal for a 4.0.

Regarding cats, I don't shop anywhere but Performance-Curve.com

Down tube will have to be fabricated by a local exhaust shop--no other way to get what you want.

 

2" tubes on the header are fine.

 

Stick to stock size down pipe bigger will have less back pressure and kill some low end torque.

 

Yes keep it all the stock size that way everything will fit properly and you won't lose ground clearance either the way you do with larger exhaust pipe.

 

Backpressure has nothing to do with it. It is more about maintaining good exhaust gas velocity.

 

This. And also what the other's said.

Exhaust flows faster in a smaller pipe, kind of like putting your thumb over the end of a hose. When you have a pipe that's too large the exhaust moves slower and has more turbulence, thus having negative effects on low end power. The 2.5" pipe would be good for a stroker or if you were only after peak numbers at like 5k rpm.

I posted a question in the exhaust thread in regards to 2.25" vs 2.5" and didn't get any responses but I'm still curious as to whether there is any benefit to the larger pipe anywhere in the exhaust with intake mods also.

 

It is assumed that strokers have intake mods, and even then the mild strokers sometimes still use 2.25". I'm confident that even a free-breathing 4.0 only needs 2.25". Now, if I had the chance, I'd like to try 2.5" to the cat then 2.25" after the cat. Hot exhaust is Les dense and has more volume than cooler exhaust. Starting at 2.5 then tightening to 2.25 might be of small benefit. Maybe.

 

Sounds like a good theory. I would be very interested to see results of somebody experimenting with this. I think it would be difficult to find headers that dump out to 2.5" and you would need to get a 2.5" downpipe custom made. I would also like to see results of 2.5" up to the cat and also all the way up to the muffler.

All said and done, 2.25" the whole way probably is just cheaper, easier, and more efficient. The 2.5" would probably sound mean though.

 

Right. The best header choices imo are all 2.25 at the collector. Borla MIGHT not be... Anyway, lack of a desirable header with 2.5 out is why I didn't bother when I did my system. I had to have a custom downpipe made and encourage everyone to do the same regardless, so that could have easily been 2.5 if I had needed it to be. Both cats and mufflers come in many combinations of in/out size, so its really just a matter of getting the right header. I really wanted Thorley and didn't want to pay to have it hacked up and modded to 2.5" collection just for an experiment. I'm extremely happy with my system as it is.

 

OP has a 2000. The headers for the 2000 have TWO collectors that feed into the twin downpipes. On OP's CA exhaust, the twin downpipes have the precats in them. Then they join together in a collector just before the first flange near the firewall. Stealing this image again:



That shows the stock manifold, but all the headers for the 2000 are made to bolt directly in place of it. Posted all that to get to this:

You wouldn't want 2.5" collectors on the header, nor would you want 2.5" pipe in the downpipes, because there are TWO collectors on the header and TWO downpipes. You already have plenty of volume there. If you wanted to mate that with a 2.5" pipe after the precats, you would just get an exhaust shop to cut off the final collector on the bottom of the downpipes and weld in a new one with a 2.5" outlet.

 

Howcome the exhaust pipe calculator online says 2.25 inches is only good for 208hp then or is that wrong? And with the curves and crunches that would mean it's good for even less power. I see nothing wrong with a full 2.5 inch exhaust.

 

Good clarification about 00-01 vs the rest. When I refer to 2.5" at the collector, I'm referring to the point at which everything becomes one pipe. I consider the pre-cats and their surrounding pipe to be more like the secondaries of a header. That said, header options for 00-01 are even MORE limited unfortunately which really forces an 00-01 owner to take what they can get.

 

Sort of, but we have some pretty good choices. Here's a thread on another website that shows several of the headers available. They generally fall into two styles, torque tubes that look like steel pretzels and more equal length pipes like Borla. The torque tubes modeled on the Banks design have a pretty extreme difference in the length of the pipes to boost torque, hence the name. The straighter headers like Borla are all about flow, with more of an emphasis on horsepower.

With that kind of lineup, I'm not sure we 2000 and 2001 owners are really having to "settle" compared to 1999 and earlier. Our choices are definitely different, but not necessarily inferior. I went with Banks Torque Tubes on mine, but I would love to see a side-by-side comparison with a Borla or Thorley header on a dyno. I will say that from a standpoint of pure vanity, the Torque Tubes sound better than just about anything I've heard on an earlier XJ.