Ive been doing this to the customers darts for a few months (done over 30 abarths) now and i usually do my downpipe mod with the magnaflow cat back on the darts works great.
Lots of people here want to pass emissions and get better performance and the stock one is more than up to the task. If you look at the oem downpipe its MASSIVE in size EXCEPT at the exit where it goes to about 1 7/8 at the opening but above that goes up to like 6 inch round cat.
My mod consists of just filling in the area above the downpipe and then opening up the hole to match. If your running a stock exhaust its a waste of time to do this that being said here are a few options.
ONLY THING YOU WILL NEED TO BUY IS A 2.5 OR 2.25 VIBRANT FLANGE
OPTION 1
Open up oem downpipe to 2.25 inches and use the magnaflow cat back
OPTION 2
Open up the oem downpipe to 2.5 inches and do you own custom 2.5 cat back.
LETS move onto the oem cat
The stock one flows good only downside is the exit so here we go
stock inlet 2 7/8
pipe after inlet OD 3 1/8 INCHES
CAT OD 5 1/8
And here is the killer oe exit ID 1 7/8 INCHES
PORTED 2.5 VS STOCK
Here is a pic of the finished version
Once again id like to say DO NOT do this if you have stock exhaust waste of time AND people who have gone from a non catted downpipe to my modded oem one have stated that loss of power really isnt felt. Car still pulls great just less noise
I do these cars for fun and this is one of those projects I feel guys like me who want good hp on least amount of money spent is well worth it. If you have questions just ask guys im always around to help. Sidenote my 1.4 made 207/227 at wheels on 91 octane car pulls so good now.