Exhaust Question

91 XJ 122k
4.0l

Hi new to this board,

I purcahsed a Catco direct fit with the DynoMax cat-back. The catco inlet is 2.5 however the outlet reduces to 2.25. The dynomax is 2.5 all the way to the exit.

Should I cut the 2.25 section off the cat and make it 2.5 all the way through or should I let it reduce to 2.25 then increase back up to 2.5 at the DynoMax.

Are there any benefits one way or the other? Will I even notice anything if I cut the 2.25 section off (this is the way I'm leaning).

Thanks
Mark

The Catco is made for the OEM system, which is 2-1/4" from the cat back. If your muffler is 2-1/2" I would trim off the 2-1/4" section of the cat outlet and run 2-1/2" all the way. Be sure to test fit everything, because you may need a short section of 2-1/2" pipe either between the cat and the muffler, or between the muffler and the tailpipe.

Cutting to 2.25 section out is exactly what I want to do but before I started cutting parts I wanted to make sure I wasn't doing something I would regret later.

There seems to be a lot of discussion on keeping backpressure and since stock exhaust after the cat is 2.25 I guess I started having performance anxiety.

I have an '88. My brother used to manage a Speedy muffler shop and when the first exhaust replacement came up he talked me into a 2-1/2" system with a Walker turbo muffler.

Big mistake, at least for the way I drive. I have a 5-speed. Highway cruise RPM is around 1700. I lost gas mileage, I lost acceleration, and I lost power. It sounded nicer than the stock, but when that system rusted out I went back to the stock 2-1/4" and I think it's much better.

But you already have the 2-1/2" system. That small restriction isn't going to create any significant backpressure, so you might as well go 2-1/2" all the way.

cut out the 2.25 and go with 2.5 all the way..i cut out my cat and just bridged it with a full 2.5 mandrel bent exhaust pipe with a flowmaster series forty and a k/n intake up front. gas milage went up slightly. then i added the roof rack a an another two inches in hieght and lost it all. now i get 12mpg on the highway..tryin acetone for my drive to indy today. so we'll see.

Thanks,

With Eagles comments about losing gas mileage, acceleration and power, I started having doubts.