08's have two cats in the downpipe. Anyone know the reason behind this?

I personally suspect it is to help meet more stringent exhaust emissions. I understand the two cats are smaller than the previous single cat, but same inside diameter, (although I haven't pulled mine out yet for a better look.) I also suspect that two smaller cats heat up faster than a single larger one. But of course the big question is, do two smaller cats adversely affect flow characteristics?

I am thinking about upgrading to a LSJ downpipe, but don't want to be bothered with the changed configuration throwing cels or whatever.

 

emissions. The first one us still pretty big second one is considerd a half cat

ur gonna throw cels just get a tune

 

Wrong. I changed my 08 DP to a LSJ dp. No CEL's.

 

i thought only 1 cat was on the DP and the 2nd cat was after DP. i wanted to get a catless dp and then just have 1

 

No the second cat is on the DP too. I haven't had any problems having an LSJ DP and I'm pretty sure I would of had a CEL by now. It's fine trust me.

 

I have the lsj downpipe on my 07. No cel's for me either

 

Was installing the LSJ a worthwhile upgrade? I picked up a GMPP honed manifold for a good price. Combining it with the LSJ downpipe should work well, (in theory at least).

Thanks guys for the replies.

 

I don't really FEEL a difference to be honest. I never dynoed my car before so I can't give you before or after dyno numbers. There is a slight sound difference though that is most pleasant I can tell you that. As for exhaust manifold I decided to keep it stock as I read gmpp manifold doesn't do much, and I don't want aftermarket cause either CEL or warranty. I am trying to find just a GMPP muffler tho to keep warranty for that if anyone knows where I can get just a GMPP touring muffler. Cause pretty much that's all our exhaust needs as we already have 2.50 piping. And as a surprising find, the gmpp cat-back exhaust is 2.25 piping! So technically GMPP cat-back is actually a downgrade! That's why I want just a muffler.

 

I didn't want to install headers either. The trick with headers, is that you get what you pay for. High end, long tube equal length tuned headers are not cheap, plus you need a corresponding downpipe. You want the headers tuned to the power band of the engine. I very much doubt many currently available on the market are.

The GMPP cat back is mandrel bent. The OEM stock isn't. The OEM stock also decreases to 2.25" over the axle. Even though the GMPP system is only 2.25", you can be sure it flows better than stock. The question is by how much? And is the benefit worth the cost?

 

I have a zzp dp and I felt a pretty noticable diff

 

Sorry to sound dumb, but I don't know THAT much. What's mandrel bent? Also couldn't you just cut the 2.25 section off the exhaust and weld a 2.50 pipe on? Or is that stupid?

 

I guess the simplest way to explain it, is that madrel bent curves maintain full volume where as a pressure bent OEM exhaust may loose a quarter inch of diameter give or take.

 

Ah I see. What about cutting the 2.25 section off the exhaust and welding a 2.50 pipe on?

 

^this mans onto something here

 

I assume the stock exhaust decreases to 2.25" over the axle for clearance reasons, or maybe it is simply due to the fact that it is pressure bent. (But knowing GM, who really knows?) It is probably more money than it is worth to replace that section only, and the muffler would still be a 2.25" inlet. Secondly, you would still want the replacement section to be mandrel bent, otherwise why bother?

BTW, for curiosity sake, how long have you had the LSJ installed for? (and why did you install it? Your questions suggest that you are not that experienced, but upgrading a downpipe is not something a novice normally attempts, unless instructed.)

 

Umm about a week ago. And I installed it cause it's less restriction and 60% bigger cat which means more performance! I don't really feel a difference though. But I'm guessing I probably would if I had the gmpp touring muffler as I'm sure a muffler is prob the most restrictive of an exhaust. Plus I don't want a real loud exhaust, and I like the sound of it. Can't I just take my car to midas or something and have them put a 2.5 pipe in and mandrel bend it then weld it? It sounds pretty simple. Doesn't sound like it should cost much either sounds like it wouldn't take more than 30 minutes of labor.

 

the OEM exhaust is mandrel bent on the SS/SC and SS/NA .... it is not pressure/crushed bent. My OEM cat-back on my SS/SC maintains its diameter at every bend .... I'm not being an ass, I just don't know where you are getting that it is crushed bent ???

 

Ah forget it. I might as well just get a gmpp cat-back and make **** less complicated Not like I'm trying to make my balt a race car anyway just want a few extra ponies from some bolt-ons.

 

the muffler isn't a huge bottleneck. by the time the exhaust gets to that point it's not much of a bottleneck. i don't know about the turbo's downpipe, but i do know in the ss/sc, the main bottleneck is the downpipe as the diameter goes to around 1.75". the header also isn't all that great either. while the exhaust is definatly worth looking into upgrading, it's really not too bad overall... again, on the ss/sc. can't speak for the turbo. but i honestly don't see it losing a quarter inch at the muffler as being too big of a deal.