i have 2 choices.

1. 3 inch catless downpipe with a 3 inch exit bolted to a 2.5 magnaflow cat back.--thats whats on it currently but with the stock cat back. magnaflow cat back is in the mail.

2. i have a extra downpipe laying around. it is a 2.5 inch catted downpipe with 2.5 exit.
with that one the transitition from downpipe to cat back will be better since there wont be a "wall" for the air to hit. (like there will be from a 3 inch exit to a 2.5 inlet)

i am leaning towards option 2 but wanted to see what you guys think.

 

Do not put a 2.5" downpipe on the lsj. Go with the 3" downpipe.

The stock catbacks inlet is only 2 1/16". I'm suprised you havent had any exhaust leaks running a 3" exit downpipe to the stock catback.

 

X2. Go 3" DP for sure

 

I have 2.5 inch downpipe, seems to work fine for me.

 

The stock downpipe for the most part is 2.785" O.D.

Yes, your 2.5" has "some" flow improvments over the production downpipe, but once you start flowing more air (pulley down) you will want a larger downpipe. A 2.5" downpipe works better as an upgrade on the LE5's & L61's.

I have gone over this with GM Engineers, and they claim its best to use a 2.75"-3.00" downpipe on the LSJ.

 

if your gonna stay on a modest sized pulley, go 2.5. If your going for max flow and planning on seriously gonna pulley down go 3". There is actually a calculation you can do that takes airflow through the maf in lbs and displacement and calculates what size exhaust you need for peak flow. Mine obviously was 3" lol.

 

where is this calculation at? right now i have a ottp stage 2 cai zzpheader then cooling mods

 

even with the dp being a 3 inch exit it will be better using that with the 2.5 catback?

 

Not really.

You want to match the exit of your downpipe with the inlet of the catback.

3" into 2.5" is not going to flow any better. Infact it will be a slight restriction. You will have all that flow coming from the 3" downpipe, and it will be slowed down when it hits the opening of the catback. This will cause eddies and inturn restrict the flow slightly.

 

can i order a 3 to 2.5 flange somewhere? since i already have the 3 inch downpipe?

 

You could prob just take it to a local exhaust shop, and have them make you one. It prob wont cost that much, and would be cheaper than buying a new downpipe.

 

Airflow through the MAF is not a good indication of much of anything... The number will change dramatically based upon the size of the intake piping the MAF is in... Some people think that 100whp per 10lbs of air is another calculation, which in fact is wrong again. You can persuade your number high or low with different intakes and tuning. In order to use your calculation you would need the actual airflow of the engine, not simply the skewed MAF airflow value.

To the OP, 3" DP with 2.5" catback is the sweet spot. As exhaust gasses cool they lost velocity, so the 2.5" catback is fine until you start making lots of power.

Also various places make a 3" dp to 2.5" flange (bolt to 3" dp, cut off 2.5" flange and use slip fit). Clear Image Autp gave me one when I first had their exhaust, and also I believe Vibrant makes one.

 

^^^ this is one man opinion, just cause you say something is wrong infact does not make it wrong, you right cause my intake piping in 2.654678899765" ohno! My exhaust setup is skewed forever, what shall I do? I didn't say it mas a mathmatical be all end all, just that it would put you in the general area of what size exhaust you need

 

Call me crazy, but here's my experience with downpipes..

I had ported stock manifold, 2.5" dp w/ cat, gmpp catback.. was able to pull 25* of timing w/ methanol..

I then got a shorty header w/3" collector and matching dp that necked down to a 2.5" exit.. similar timing..

Header cracked, went back to manifold still using 3" downpipe.. Suddenly I was knocking all over the place and only able to command a max of 22*, with everything else the same.. I'm assuming turbulence = crappy flow = more heat = knock

Now its a pacesetter and 3" all the way back and i'm able to put my timing back up to 25*...

So, based on that.. I think hitting the 2.5" wall with a 3" pipe is a very bad idea.. I'd go straight 2.5 before doing that.. but if you can get a 3" to 2.5" flange, then thats fine too.

 

Yep, thats what i said. It will def cause issues with the exhaust flow.

 

ok. cool thanks guys. time to call around

 

If it's not a 'mathmatical be all end all', then why bother mentioning it in the first place, when you could simply use the real world experience gained by many, many people in the 'Cobalt scene' and suggest the correct setup for his mods? Not quite sure why you listed something as a certainty when it is not, then argued that its not a certainty, and in fact would put you in the 'general area of what size exhaust you need'. I only said anything in the first place because your statements about the MAF airflow are not 100% accurate and would probably further confuse the OP.