2.5" Vs. 3"in exhaust

[chase78]

what modiffication do i have to have to be worth going up to the 3"
setup?
or
Is there a benefit with stock internats to just go 3" setup?

 

[Black95TSIawd]

when u hit mid to low 11's, then u'll feel a difference of the 2. untill then, you wont.

 

[doug]

Originally posted by Black95TSIawd
when u hit mid to low 11's, then u'll feel a difference of the 2. untill then, you wont.

 

[ItsStockOfficer]

Thats funny, my car was consistently faster on a 14b at the track with the 2 different exhausts...what exactly do you have to back up that completely rediculous statement?

 

[doug]

So no other mod changes besides the exhaust, and all the other variables were also the same-- temp, humidity, track condition, fuel, etc.? I guess it's possible, but hard to believe.

Don't get me wrong, even though I disagree with some of the things Corky Bell says in Maximum Boost, he says 2.5" should be good up to like 350HP-- maybe it was 400, since it's been a while since I read it. Yes, you can make the argument that most of his experience comes from muscle cars, but shouldn't exhaust flow be exhaust flow?

 

[ItsStockOfficer]

First of all. No, no changes. Second of all, I also went from a full 3 inch, and then disconnected it at the o2 senshour housingm, but left the exhaust on, NO weight changes. Traps speeds increased about 1.5 from 117 to 118.5. Got my first 11 that way, which would not have been possible with exhaust attached (coupled with my driving skills, anyways).Thats a significant improvement in hp at the low 12 sec area, far away from the aforementioned mid 11s! Probably 10-15 hp. i would have hated to run on 2.5 inch.

What exactly does saying an exhaust is "good up to" mean? My friend ran 12.6@107 on the stock exhaust. So now stock exhaust is officially "good up to" about 290 hp. That in no way infers its better then 2.5 inch. People ave also ran 12.0's on stock intercoolers, so I guess they are "good up to" 375 whp. I see no point to go larger, do you?

 

[chase78]

"good up to"

If i had a stock car and I put a 2.5" downpipe high flow cat, WHATEVER, and HI flow cat back. Would i see a big, small, or no performance change when compared to if I put a 3" system?

 

[ItsStockOfficer]

On a bone stock car the difference would be nonexistant-minimal, becuse your at 11 psi flowing through 2 inch rubber ic piping. At 18 psi with some intake mods, yes...a minimal difference in both spool and power....once you upgrade turbos, definitley.

 

[chase78]

So doing your own port job(gasket matching and step removal) would be a better option(as far as spool up and HP) then buying a 3" system on a stock to mildly modded car?

 

[ItsStockOfficer]

Originally posted by chase78
So doing your own port job(gasket matching and step removal) would be a better option(as far as spool up and HP) then buying a 3" system on a stock to mildly modded car?

Are we refering to the o2 housing alone? You obviously can't port your exhaust sytem. As for the O2housing, I only run tubular, You lose 5 or 6 lbs on an already far to heavy car, and I think they help quite a bit. I have the RnR 3 inch turbo back in stainless

 

[chase78]

At www.vfaq.com in the "PORTING" section they talk about the three subject belew.

turbo's
http://www.vfaq.com/mods/porting-turbo.html

manifold
http://www.vfaq.com/mods/porting-exhman.html

O2
http://www.vfaq.com/mods/porting-O2.html

In the post he talked about a faster spool up after the port jobs he did.
I was just wondering you( ItsStockOfficer ) said before that the 3" system helps with spool up a little.
----So is the porting job in the links above a better solution for the stock and mildly mod car then a 3" setup?

 

[ItsStockOfficer]

the bigger the system, that faster the spool. porting makes it flow decent, but not in any way comparable to tubular.

 

[chase78]

Sounds like a 2.5 " hi flow set up is all a person under 350HP really needs. With a port job( to kill a few hours ) when they install the tubular header.
Thank You

 

[DSMJim]

Originally posted by chase78
Sounds like a 2.5 " hi flow set up is all a person under 350HP really needs. With a port job( to kill a few hours ) when they install the tubular header.
Thank You

Yes but a 350hp car will pick up 15hp going to a 3" exhaust. A 2.5" may be good UPTO 350hp but you will see gains with a 3" exhaust

Lets put it this way. We built a 2.5" turbo back exahust for a Mazdaspeed Protege (turbo) and a full 3" turbo back exahust (prototyping to see if it was worth the extra cost for full 3" or not). Went to the dyno and ran the 2.5" it gained 14hp, then put the 3" on and ran the car again it gained 18hp. 4hp extra and the car was hot, trust me my hands remember from the burn they got.

Those cars have a Garrett T-25 so if somebody cares to explain to me how a 2.5" exhaust is "good enough" I'm listening.

 

[DrBrown]

Since we are on topic here, if a person had a 2.5" downpipe and wanted to remove the mandrel 2.5 cat-back in place for a new 3" cat-back would it be worth it or does it really need to be a full turbo back 3" exhaust to get the better power?

Right now Im thinking something is restricting my car majorly as I have a very big turbo (garrett T3/T4 w/ stage 3 .63 turbines & 57 trim) and my car is far from tuned but still Im trying to squeeze every last bit of power this POS has left, well as my fuel will allow me to. So can a 3" catback get 15 whp or more on a V6 pushing 240-250 whp as is?? Chase78, Sorry to jump into your thread but the topic was already brought up and I didnt wanna repost the same questions.

I have read that a 2.5" madrel bent catback is good up to 400 bhp. That doesnt mean you shouldnt upgrade because your not pushing that much power or whatnot. If you want an extra 12-15 whp then get a bigger exhaust. Turbos hate backpresure! It can only help. And if cost is the issue build your own out of mandrel bends from summit or jcwhitney.com

 

[ItsStockOfficer]

Thats a good example, 4 hp on a T25 is a 2.5% increase at ONLY the 170 hp level...at the 400 HP level it might be a 5% increase for a gain of 20 hp. Or it might be more, depending on the path you took to get to 400 HP. This does not even include the benefits of faster spool....

Were trying to go fast people! Why are you trying to skimp on one of the fundamental mods that is a foundation of how a turbocharger operates! Is it just me or do people listen more to those who run 13's talk about running 11's then those of us who have actually done so?

 

[99 gst 420a dsm]

so all that means a press bent 3 inch turbo back will be good up to 350 horse

EDIT: sry to possably go off topic, i have a mandrel bent exhaust, but at our shop we press bend custom exhaust and some people (4 cyl tuners) have asked