I've only driven this car since I finished the turbo build maybe 15 miles. It's still not tuned so I haven't been driving it. I noticed today that the rear side of the cat is noticeably discolored. Does this mean anything? Is it normal?

The cat is a universal 3" hi-flow unit

 

It just looks like the way the metal reacts to the heat

 

was it brand new? i agree with the above. that's just what happens to new stainless when you heat it enough and the back half of a cat gets really hot.

 

Yes I realize it's discolored from heat (obviously) but I should have been more clear in asking why it's only discolored directly along the casing where the substrate is located internally?

I'm wondering if perhaps the cat is too restrictive and thus causing intense heat concentration at that point in the cat? The metal isn't discolored prior to the cat.

Also, that cat in the picture is a 400 cell count, ceramic substrate unit. After posting this thread I did some looking around online and there seems to be a rule of thumb of sorts to only use metallic substrate cats for turbo applications.

I have since purchased a 300 cell count metallic substrate cat that I can weld in this weekend. I plan to do so, and even if the discoloration is normal I figure the lower cell count unit will give me better flow anyway

 

Why not go with a 200 or 100 cell count cat?

 

Lol why not go with no cat

 

I have to pass emissions, I don't really want to run no cat because I'll have to switch out and crap for testing. Beyond that, I'm no eco nut, but I hate the smell of no cat especially driving around town and whatnot. This isn't a race only car or anything. So that being said, I would prefer to just lose a few hp and have a cat.

The other day when I was looking the 200 cell cats were all like $250 which I didn't care to spend. Of course now with my luck I found some for $120 ish tonight. I have to have all my changes done this weekend since it's (supposed to) getting tuned next week, so I'll have to go with what I have at this point.