A friend of mine has a '98 SSEi and wants me to find him some exhaust goodies...

This is the right muffler for the Flowmaster right?
Flowmaster 80 Series Camaro Muffler 2.5 / 2.5 */D 42583

probably going to go with this cat also... same one I just recently bought
MagnaFlow Oval Catalytic Converter 2.5" - 94006

all looks alright to me.. Ig go with all MagnaFlow, but thats just my opinion

Cat for which of your i'*?

Remember his will have a rear O2 bung like your new one. Not like your old one.

The Downpipe diameter is slightly over 2.5" on the 96-99s. A shop would NOT install a 2.5" cat I brought to them for my 98 SSEi...I disagreed with them on it but I'm just bringing this up....however the 3" was a bit too big and had to use a 2.75 to 3" reducer, then a 3" to 2.5" reducer on the muffler side.

Either will work, my prefered solution afterwards was just to get a 3" Downpipe along with a 3" cat then a 2.5" to 3" reducer.

3" also allows for upgrades later like headers

Regarding the O2 bung, it does not really mean much since the high flow cat is several inches shorter than the stock version.

I bought that cat for our '03 SSEi. I actually didn't even think about the O2 location This cat doesn't have a bung on it. Is the bung on his '98 in the cat, or in the pipe after the cat like in our '03?

On the S2 cars..the O2 bung is about 4-6 inches behind the overly long factory cat. There'* no need when getting a replacement cat to worry about the rear O2 location. I'm guessing the S1 looks like it would be a problem if it had a rear O2.

On the the downpipe diameter. The outter diameter of the pipe is larger than the inner. Skip ahead to the exhaust shop putting it on. They'll want to retain the flange that is part of the cat, it'* a tight fit..but the shop I went to cut the cat after the flange and welded the new cat to the small pipe/flange that was left behind.

Cool, that is EXACTLY what we did with the '03 at the muffler shop that "The Dude" works at

It is a tight area as the flange is basically the front of the cat. A little work and they can do it nicely w/o cutting the downpipe.

Hans..do you have any pics etc you could pm me as to how they dealt with the double layer walls in the DP.