Newbie muffler question

So I replaced the exhaust manifold/downpipe on my 1996 auto...now the catback that's on there is an aftermarket (Flowmaster i believe), and the downpipe outlet is 2.5", and the pipe to the cat converter is 2.5" as well...how can I mate these up? Or do I need to get a new catback that slides into the outlet of the new downpipe?

I'm a newbie :doh:

Sorry, what's the problem. 2.5 to 2.5. Stock is 2.25, but 2.5 to 2.5 matches.

on some applications the diameter is "the same" but may actually be flared slightly outward or inward depending on if that end is an inlet or outlet. so try checking for this even though both pipes may read as the same diameter. if both are actually exactly the same and won't mate at all, you could always consider a pipe expander (under ten bucks at harbor freight, but it was a POS and seized/broke in the expanded position inside my pipe; thus get a decent one somewhere else) or you could go the ghetto way and cut slits in the overlapping pipe so it goes on, then exhaust tape it or weld it

you either need a coupling that both pipes slip into, or flare one or the other so they slip together or i guess you couple butt weld them together in a pinch but not recommended....

Where would I get a coupling for it? Wouldn't it need to be like 2 3/4 for it to fit right? Schucks only has 2 1/4, and a 3" coupling piece. I didn't check AutoZone though.

Winter, the problem is, the new downpipe I got, is the same diameter as the rest of the exhaust, my issue is getting them connected. I'm new at this and want to do it right the first time.

The downpipe is flared out at the end, but flared to 2.5 which is the same as the aftermarket rear exhaust.

Exhaust pipes, mufflers and fittings are made for ID and OD (inside and outside diameters). Normally you want the ID coming from the front and the OD facing the front, but this is probably not crucial. You need to pay attention to the ID and OD difference. Trying to understand what you have, it seems like you have two pieces that are 2.5 outside diameter. You have to get an adapter coupling that is 2.5 ID to 2.5 ID. Don't plan on a muffler clamp to pull down a 2 3/4 pipe. It might be possible to weld, but not to clamp. It will be a mess. And forget about the "muffler mender type paste". I tried that when I was a teenager. It doesn't work for shit.

Here is what you need. http://www.summitracing.com/parts/WLK-41964/?image=large In the text is says OD 2.5, but the enlarged picture and the title says 2.5 ID. That's what you need. And it has the slits to help the clamps tighten down without collapsing the pipe.

Most places sell reducers to deal with this problem. OD to ID reducers are usually fairly easy to come by ( http://www.summitracing.com/search/?keyword=exhaust pipe reducers&dds=1 ). Looks funky with two clamps so close to each other, but they just have to seal. It's also a good idea to point the threaded ends of the clamüs to the side and not down, may save future scalp injuries or hanging on obstacles and ripping the whole works out.
Or go to a muffler shop and have the guy make you a reducer, or if the down pipe isn't in the XJ yet, have him expand the end.
You can adjust the down pipe for clearance on the cross member by putting a 3/4" (or maybe 1/2") piece of wood between the pipe and th cross member before you tighten everything up.