I'm about ready to order my exhaust for my XJ. I'm getting a flowmaster super 44 muffler, magnaflow CAT, and tail pipe. I have everything priced out for both 2.5" and 2.25" from the CAT back. I'm just wondering, is there really that much of a difference in sound between the 2.5" and 2.25"? I'm considering both options, just not sure if the extra cost for 2.5" is really worth it. Any opinions would be great!

 

don't get a system just dump it and go Magnaflow

 

I got a 2.5 because for one it's mandrel bent unlike the stock pipe, and i needed a new one anyways, but it just gives it a little deeper tone. nothing that is crazy different. just remember your going to need an adapter to go from 2.25 od to 2.5ID pipe unless you go a converter that has is 2.25 in and 2.5 out.

 

So I decided to get a 2.5" mandrel bent tail pipe, and 2.5" muffler and cat. So why would I need an adapter then? Isn't the piece behind the cat 2.5" on the newer Cherokees?

 

I have a 99 and it was 2.25 O.D and replaced it with another 2.25 O.D. Yours could be different i guess. measure it.

 

Here's a picture I saw once, I thought it was 2.5" before the CAT, and 2.25" after. If I get a 2.5" CAT won't I just be able to clamp it to the existing downpipe then?

 

Don't think about sound when selecting exhaust plumbing - it don't matter. Sound is a product mainly of the muffler.

For a stocker 6-242 or even 6-258, stick with 2.25" ID plumbing. For a stroker, go with 2.5".

The reason is that going with an exhaust setup that is too large will actually harm performance (it has to do with internal turbulence of the exhaust gas pulse.) This is something that the ricebois never do figure out - and why they're wondering that they can't get good "oomph" with a 3.5" exhaust on a 1600c/c engine! In order to make 3.5" exhaust useful on an engine that small, you're going to have to spin it up around 14,000-15,000rpm or so.

Diesels have the very large exhaust because they're such "deep-breathing" engines - hideously oversquare. (This is to maintain the high CR needed to ignite Diesel fuel.)

The numbers have been crunched by a few of us on some forum or another (I don't recall which, and I was one of six people doing the work,) we decided that 2.25" is just slightly oversized for the stock 6-242, but closer to the optimax than going down to 2.00". Similar conditions applied for the 6-242 stroker (midline stuff, 260-280cid) and 2.5" exhaust. Going up to a 3" exhaust was only really considered for someone doing a 300+ci stroker, which is a lot of work (find a block you can bore to 4.000", reweld and grind or just offset grind a crank to about 4.000", ...)

Sound is nice - but it's a product of the muffler, not plumbing size. Select the muffler separately (I like a little engine noise, but don't get silly. I use a cat spec'd for a BBChevvy vain - single unit application - and a Walker Turbo II muffler when i redid the exhaust on my 88XJ. It makes enough noise to let me know it's running, but it's not so overbearing as to trip car alarms as I go buy - unless I pull the cut-out and dump it before the cat...)

 

Cool, thanks for the info!

 

Don't mean to thread jack... I was considering a standard cat, or the Thunderbolt spin cat from performance curve.

I just don't know which one to get. Do I get the ones with the o2 sensor drilled into them? Or not? Also, what's the difference between these two cats?

 

OBD-II will require provision for an "after-cat" HEGO sensor - this can be either in the body of the cat proper or in the pipe immediately after.

OBD-I and pre-OBD don't require it. You can use an OBD-II cat, just put a plug (M18-1.5) in the sensor bung.

OBD-II is 1996 and up, with the #0630 head (the #0331 head uses a split exhaust manifold, pre-cats, and other complications - I'm not sure how that works out.)

 

From my experience (and i think that these can be quite a pain) for the main cat that is right before the muffler, there is no o2 sensor in or after it, the sensors are farther upstream in the system and work off of a set of pre-cats.