Does Size Matter? 2.5" vs. 3" Exhaust discussions. [merged]

[Boostin18]

2.5" vs. 3" exhaust discussions are merged here.

I have a 90 GSX and when i bought it the guy said it was full 3" turbo back with a EVO3 O2 housing (ported), now here is my question. When I took the exhaust down, it had a 2.5" exhaust flange at the top of the down pipe and then flares for about a half inch out to 3", if that makes any sense. This is my first 3" exhaust and I was wondering if this is right? I was thinking of porting the o2 housing because it is 2.5", making it 3". Then bringing the exhaust to a shop and having them weld a 3" flange to about a half inch long peice of 3" ehxaust pipe and then welding it on to the whole exhaust. So basically getting rid of the 2.5" flange and the half inch long flare and just welding on the 3" flange with the new piece. Would this make a diffrence if i do this or not. I don't know if this would make any diffrence or not, but when i put a boost controller on, i would try and get it to set at 15psi, but never would it would just spike and go down to like 12 or 13, but never above 15, but never at 15. I also have a ported 2G manifold (havent had it off to make sure it really is ported). Any help would be great!

 

[a_scobel]

It is 'normal' for the downpipe to be like that. When I get my exhaust later this month, my downpipe's going to be the same way, but I'm still going to tell people it's a 3" exhaust. A lot easier than explaining every time that it really starts off at 2.5" then flares out...although the guy really should've told you it wasn't a TRUE full 3" exhaust when he sold it.

Is it the stock turbo?? Update your mods list, please! Porting out the 02 housing & making the downpipe a full 3" would work. You may still get boost creep though. Read up on it here.

 

[doug]

Performance-wise, that little stretch of 2.5" won't make any difference at all. Save yourself the hassle.

 

[lordnikon_04]

I am looking at buying a turbo-back exhaust w/ O2 Eliminator and external dump from RnR Racing. I was wondering should I go with 2.5" or 3" and Stainless or Aluminized Steel? I have a 1991 GSX with stock everything and I am wanting to be able to run at least a 13.5 1/4 mile. which do u think would be better for my goal?

 

[WI_99'GST]

just go 3". because if you eventually want to upgrade to a large turbo you are going to need a 3" setup anyway, and it will work fine with everything else stock.

 

[icedearth15]

Yes, go with 3" and never look back. I would go with a stainless system because or its strength and how durable it is. To run a 13.5 in the 1/4 you will also need a boost gauge, and boost controller, at least. Make sure you have a clutch in decent shape so you can get a good launch without frying it. Also think about buying a K&N air filter, Walbro 190 LPH, or Walbro 255 LPH with a regulator. Good luck with the 13.5--Dave

 

[tsistock4now]

im not sure what the difference, besides size, is between 2.5 and 3 inch 02 elim., catback and downpipe. and what would be better?

 

[greeneclipse]

go for a 3in now so you dont have to upgrade later on if you decide to go for more power.

 

[MyEclipse5]

go for a 3in now so you dont have to upgrade later on if you decide to go for more power.

Yeap, and if you want a loud car, get a O2 dump also.

 

[leet]

Depends on your goals.

If you want to save money, go 2.5".

If you want to go all out, spend a little more and go 3".

 

[WI_99'GST]

really depends on your future plans...if you plaan on making a bunch of power it is a better idea to go 3inch right away. that saves the possibility that you will get a 2.5 inch and then notice that you need to go bigger. SAves you money, and it doesnt hurt to have a 3 inch system with evberything else stock.

 

[militiadsm]

Was just wondering if an exhaust upgrade would raise my boost. I am currently having a Turboback setup put on my 98 GST. It's 2.5" to the flex section on the dp and from there back it is a full 3". I wasn't sure if this upgrade would raise my stock boost from 14 to 15 or 16 psi. I know that 15psi isnt a very good idea on the old T25. Are there any dangers associated with this? and if so, are there any solutions? I heard about just buying a MBC, but am not sure. Any help would be much appreciated, thanks in advance.

 

[funniegus]

A 3in exhaust will give you better exhaust flow. It will help your turbo spool better, but it wont give more boost. Thats what a boost controller is for.

 

[Quasimondo]

Larger exhausts will clear restrictions and lower your spoolup time. It won't raise your boost levels unless you you have boost creep. Lots of posts all over about what it is and how to fix it.

 

[I-luv-em-all]

it wont give more boost. Thats what a boost controller is for.

not true. many people that get 3 inch exhaust get boost creep which does increase boost, and a boost controller cannot help that.

 

[L0G1C]

I've been reading the posts and tech articles about exhausts and have come across quite a few different opinions on this. No real answers, just a lot of speculation. I consulted with my "regular" exhaust guy and he was under the impression that going to a 3 inch exhaust would be impractical with my Talon. He said that it's probably better to go with a 2.5 inch cat back system with a Magnaflow straight-through not-stainless-steel muffler (pretty much the same system on my Contour, but it's 2.25 inches... Sounds great, but not really loud).

What I want out of this exhaust:

• Good flow for up to (no more than) 300 HP eventually
• Louder than stock, noticably, but not "OMFG that's loud!!" or "rattle the windows a block away" loud...
• Stock appearance so that when chumps in hon-duhs roll up they don't think I want to race them just because I have DSM with an exhaust tip

Most of the speculation that I have read about 3 inch exhausts on turbo'd DSMs is that it's "OMFG" loud. There is other speculation that it's not that bad...

Can anyone give me some guidance on this?

 

[SexyDSM95]

Apexi N1 is holy crap car alarms go off loud

- or -

If you can find this and good luck the Apexi World Sport

 

[iowaboostin]

I've never seen a problem happening with a 3" exhaust on a turbo DSM. I've got a full custom 3" turboback on mine, it's got a highflow cat and one resonator and it's nowhere near "OMFG" loud. It's only a little louder than stock exhaust at WOT, but rumbles at idle

 

[SexyDSM95]

I've never seen a problem happening with a 3" exhaust on a turbo DSM. I've got a full custom 3" turboback on mine, it's got a highflow cat and one resonator and it's nowhere near "OMFG" loud. It's only a little louder than stock exhaust at WOT, but rumbles at idle

If you have a 3" Exhaust including DP without a ported or larger 02 housing you are asking for back pressure and boost creep....those two are deadly for a turbocharged car.

 

[L0G1C]

So how loud do you think a 2.5" cat back system without a resonator and a straight through muffler would be, would it be raspy, and would it flow well enough for a 14b?

 

[pballernh]

A 2.5" will support your goals and will not be bad at all sound wise. A resonator and straight through muffler should do you good.

 

[L0G1C]

Awesome! Thanks!

 

[jonk00000]

I have seen plenty of dsm's with a " full 3 inch turbo back, " but yet at the same time they have a 1g or 2g ported o2 housing. Correct me if I'm wrong, but porting the 1 or 2g o2 housing will get no more then 2.5 inches. The point of 3 inch exhaust is to lessen back pressure creating faster spool up. So would the o2 housing be a form of air restriction? Any advice will be appreciated thanks
jon

 

[leet]

You are correct. A true 3'' turbo-back exhaust system would have a 3" O2 sensor housing as well.

 

[soul]

If you want true 3"tb then order a 02 eliminator downpipe. That way there's no more o2 housing. Thats what I'm running.