I installed a SMS header which as 1.5" primaries, 2.5" merge collector, 2.5" downpipe with a gutted cat and it's bolted to my stock catback.

I would have imagined I would have lost a few psi of boost, but with my 2.8" pulley, it's almost the same as with the stock manifold/downpipe/cat. I hate seeing 17-18psi, I'd hate to kill piston #4. There is no way this header flows WORSE than the stock manifold...

I know the gutted cat probably creates turbulence though.




Would getting a 3" catback help ensure that I have less backpressure, since I didn't see a large boost reduction with the header or should I just get 2.5" and not worry about it?

Thanks

 

with a 3" cat-back, and ctis header and 3" catless downpipe, on a 2.8 I see 16.7 psi

 

That's what I would be happy with! I wonder if you're seeing that amount of boost based on the header/downpipe, or the catback. I'd like to see some other members boost levels with a header and the stock catback that way I can compare.

 

Just means that header flows no better then a stock header.

 

2.79 i peak 16psi mods in sig

 

Looks like I should get a better header :/

 

your header is fine try chopping off the cat or go with a higher flow cat. example magnaflow high flow cat is a 300 cell i run a 100 cell cat mine flows a **** ton more as close to catless as you can get while staying legal.

yours looks like a magnaflow

 

The previous owner gutted the cat. I don't understand why, most high flow cats flow well enough, but apparently he didn't agree. I know a gutted cat doesn't promote velocity, but I don't see it creating backpressure.

 

turbulence slows volicity

 

I have a 2.7 and I don't get above 16 PSI (full 3" system). You may be getting restriction due to the over-axle area if you haven't replaced your catback.

 

Your exhaust is only as free flowing as your most restrictive part

 

Yeah I have a completely stock cat back. And staged07ss just posted that the stock resonator is only 2 1/6" ID, that's horrible. Maybe I should leave the header and focus on the catback. Now, 2.5" or 3"?

 

2.5 unless you go turbo

 

Not so much. I (and others) gained power going to a 3" even without a turbo.

 

3rd gear on the dyno with the stock manifold I saw 17.44 psi @ 6500rpm

I just ran 2nd out to 7200 rpm and saw 17.4 psi on my interceptor....

I guess what i'm looking for is a "this is normal with a header and the stock catback" or

"I lost x.x psi with a header and catless downpipe with stock exhaust".

That way I can either spend the money on a better header, if that is infact the issue, or leave it and just get a catback.

Thanks

 

i was seeing 15-16 psi on my 2.8" with a jbp header, a 3"dp, and stock catback

 

Ok, looks like I should get a new header lol

 

A big cork in the stock system is the downpipe and replacing this with a aftermarket unit usually lowers boost everytime. Boost does seem high but boost alone with not kill the car. I ran my 2.5" setup for 2 years with no issue i was seeing 19 psi on that through the CTI setup.

 

With my vibrant header, catless dp, and Hahn 3" catback I only see 15.6 psi on my 2.7. While on the same header and dp but stock catback with a different muffler I saw 16.4 on a 2.8

 

Yea, like I said - the OP's restriction is most likely the stock catback.

 

Very much agreed

 

So 3" or 2.5"? It's a $80 difference from mandrel bent exhausts

 

Might as well just go 3" since you will never have to replace it if you mod more

 

Only problem is it will have a 2.5" flange on it since my downpipe has a 2.5" exit.

 

You prob should have looked into a header with 1.75" primaries. 1.50" primaries are on the small side. Also, as Bill (GMPP Engineer) once stated to me, bigger is better on the exhaust side of the lsj. Now, that doesnt mean to go get a 3" exhaust, but you would def. benefit from a 3" downpipe, and at least a 2.50" high flow catback exhaust