My setup: Turbo d16z6, 22psi on a t3/t4 turbo, some headwork, sfp type manifold.. external gate. Lots of other things that shouldn't matter.

With a 2.5" downpipe, is it worth running 3" exhaust? I mean, 2.5 isn't small by any means but I'm not sure if 3" would be worth it since it would bottle neck either way. Should I just get a 3" fabricated or should this be alright? Thanks for any information,
Matt

 

or you could just run stock exhaust and have an ecut out on your dp. that way no one knows, specially those pesky coppers!

 

Yes, run 3" all the way back if your running 22psi. upgrade your downpipe as well. Ask KTeller8 for a 3" mandrel bent exhaust plumbing....

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hatchback19 &raquo;</TD></TR><TR><TD CLASS="quote">Yes, run 3" all the way back if your running 22psi. upgrade your downpipe as well. Ask KTeller8 for a 3" mandrel bent exhaust plumbing....
</TD></TR></TABLE>

So does it depend on what PSI level you're running...what would say a 2.5" cat back system be good up to? i have heard some people recommend a 2.5" for reasons of noise when running low amounts of boost.

 

In his case running 22 psi he will notice a significant difference. My opinion if you dont want to lose a/c get a 2.5-3" downpipe and get your 3 inch exhaust. just my .02 cents and if you dont car about a/c upgrading to straight 3" will be good for some nice power.

 

I just got off talking with hytech....and he confirmed what i've learned from post from a Garrett Engineer in the past on this board.....2.5" ideal for 250 down.....3" is ideal for 300 and up....with 22psi....you need 3" all the way.....

 

3 in downpipe back piping will still help spool up the turbo quicker.

 

Have you dyno'd it at 22 psi? also are you running race gas with 22pounds? sorry to jump off topic, but I want to build up my z6 for high boost

 

2.5" DP to a 3" catback works just fine, I used it

 

thats what im runnign right now... 2.5dp to a 3inch...

 

another question...whats the difference in weight between a 2.5" and 3" percentage-wise...20-30%???

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ffgeon &raquo;</TD></TR><TR><TD CLASS="quote">or you could just run stock exhaust and have an ecut out on your dp. that way no one knows, specially those pesky coppers!</TD></TR></TABLE>

don't think it's a good idea since on a stock exhaust you'll have A LOT of backpressure, unless you run with low boost

 

My tuner says that you'll see 30WHP gains from running 3" DP rather than 2.5" DP with a 3" exhaust. Do you guys agree?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Cyphear &raquo;</TD></TR><TR><TD CLASS="quote">My tuner says that you'll see 30WHP gains from running 3" DP rather than 2.5" DP with a 3" exhaust. Do you guys agree?</TD></TR></TABLE>

I dont agree but I have never done this before...personally I have chosen to go with a 2.5" everything including DP, Cat, Exhaust & muffler...I have seen charts/graphs from many reputable sources that say that a mandrel bent exhaust at 2.5" will properly support upwards of 275 WHP which my goal is to fall just short of.

I figure that a little bit smaller exhaust will help my off boost low end torque, wont affect my spooling time much at all and will provide a much quieter managable sound while cruising. I have my e-cutout for wide open situations that I can control from the "cockpit"

This is what my research has taught me, but like anyone else...much of my decision has come from going with my "gut feeling"

 

I'm aiming for 350 - 400+WHP in the end... so maybe that would help with the 30WHP. I'm really curious about this, so bump.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mamaboy &raquo;</TD></TR><TR><TD CLASS="quote">I just got off talking with hytech....and he confirmed what i've learned from post from a Garrett Engineer in the past on this board.....2.5" ideal for 250 down.....3" is ideal for 300 and up....with 22psi....you need 3" all the way.....</TD></TR></TABLE>

That is the way that I've understood it...

-PHiZ

 

About how much does going to a 3 inch help with spooling if you are running at 2.5 dp?

Thing is, I have a 2.5 DP, but I only have my old 60mm cat back exhaust that I had ever since before the turbo. I get full boost at 3500rpm. What kind of increase could I see in going to a 3 inch?

BTW I'm running only 5psi.

 

2.5" cross section = 4.90859375
3.0" cross section = 7.068375

difference = 44% more flow from the 3", very significant

 

Pictures are worth a thousand words...

From Corky Bell's Book, "Maximum Boost"

 

Good pic. I don't think there is really a set diameter for an exhaust size though. But then again, i do believe that is whats recommended.

 

That graph means nothing.

It would be totally different mechanics from a I-4 to a V-8.

That graph means nothing without context. Am I wrong?

-PHiZ

 

I dont think it means "nothing" out of context. For a specific HP level I believe that exhaust CFM will be relatively close for all engines (you have to have a specific volume or air/fuel in every variation, although compression levels could be different)

I think its more meant as a guide and not a do all, end all. But I highly doubt 2.5" wont flow enough for a 300 HP 4 cylinder b18...as many people adamently claim...