I've been getting tons of PMS on this and very easy mod so here we go. 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 which is a big bottleneck
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.
You can open up oem downpipe to 2.25 inches and use the magnaflow cat back i have here
The magnaflow goes from 2.25 inches for about 2 feet to a 2.5 inch set up all the way back
You can open up the oem downpipe to 2.5 inches and run the NEUSPEED cat back section here
The NEUSPEED is 2.5 flange back just the magnaflow back section i feel flows better which you can see in the pics.
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
After you remove the downpipe just fill in the area above the flange (weld) and then get a grinder and open up the exit to 2.25 inches or 2.5 inches to whichever exhaust set up you want to go with.
I opened up mine to 2.5 inches (i have a custom 2.5 magnaflow cat back) and you can see how much bigger is verses stock
Once again id like to say DO NOT do this if you have stock exhaust waste of time and Ive installed the mangaflow on my car and deff mid to top end performance increase and ive installed the neuspeed on others also and same result. I feel due to the way neuspeed does its rear piping the flow of the magnaflow and neuspeed is pretty close. Just depends on if you want to spend the extra cash.
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.
Note I tried to keep the oem look so took it off the car and did it. If you want to get the 2.5 flange from ATP turbo they have them like 45 shipped and you can cut off oem flange and weld it on also. For me I just wanted the most oem look I could.