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!

 

[boost97gst]

Go ahead and do it big. Go 3 and be done with it forever

 

[RamenPride]

I have read that I shouldn't go over a 2.5" on a stock turbo. I plan on working up to a turbo upgrade in the future, but may not be for a year or two. What would be the downfall of putting a 3.0" exhaust on my car now?

Your stock turbo will be fine if you go with a 3" exhaust. If you plan on upgrading in the future, then there's no point getting a 2.5" now. You'll just end up wishing you went with the 3" to begin with.

 

[GoCanes777]

Okay, so I am sort of at a loss of what to do. I want to upgrade my stock exhaust system on my 99 GST and I have read that I shouldn't go over a 2.5" on a stock turbo. I plan on working up to a turbo upgrade in the future, but may not be for a year or two. What would be the downfall of putting a 3.0" exhaust on my car now?

I was looking at this one if I go with 2.5"

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

Or this one for 3.0"

eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

I ran a 3" with a stock turbo and full bolt ons for a while and had zero problems, i agree with the 3 inch. I'm not sure what your money situation is but the apexi dp works great for me and 90% of exhaust shops can finish the rest of the exhaust and you pick your muffler and your golden!

 

[DSMnOOb00]

Go ahead and do it big. Go 3 and be done with it forever

+1

do it right the first time, I have a 16g with small amounts of porting done to the waste gate and o2 housing ZERO Issues...

like some of the other guys already stated, it wont hurt your set-up and will only help you down the line...

the only thing youll have to watch out for if you do decide to get a custom 3inch exhaust, make sure the downpipe is as far away from the oil filter as you can get it... a good 2 1/2 inches or more will work.

I have seen people install custom down pipes only to find out 2 months later they didnt leave enough room for remove the oil filter

 

[pboglio]

Go with a good quality 3" exhaust system. Something with alot of muffler size volume and a decent resonator. A 2.5" downpipe or 3" downpipe as well, they both make the same power up to at least 400 w.h.p. I see no reason to run a 2.5" catback as it makes noticeably less power than a proper 3" catback and there are 3" catbacks that are reasonably quiet.

 

[Alan11]

Ill tell you what OP. Go to a thread I created titled "Rally Exhaust" read some of the posting gofer made in their. You can find some very useful info.

Just search for it.

no wonder why you still have a red chicklete !! You expect for him yo go with a 2.5" if you give him no input???

OP like bud said are you replacing all the way from your o2 housing or just catback??
It al depends on the turbo setup your aming for..
I personally im getting the 2.5"megan o2 housing, 2.5" custom made DP, with my thermal 3" catback.. by no means am i cutting the thermal flange ... thats why im not getting 3" all the way...

Search a lot before you spend your money!!

 

[LxiMax]

I say the best thing is to port the 2g O2 housing out to 2.5", pick up a Megan Racing or RRE 2.5"-3" downpipe, and any 3" catback of your choice. That should give optimal flow and effciency IMO.

 

[GsxAZman]

What is your reason for suggesting the 2.5? I am getting mixed reviews here.

Pros and Cons for both would be great.

My biggest question I guess is if I went with the 3.0 now will I regret it because of performance issues with too big of a pipe on a stock turbo?

I have a buschur 2.5 " in one of my dsm's as well as a 3" turboback exhaust on my 2g with a 16g. The 2.5" is much quieter IMO and you get a little more backpressure with your smaller turbo. You can always just add an exhaust cutout down the road just like im planning on doing. Adding the exhaust cut out still brings the price cheaper than I would have paid for a 3" unit 5 years ago. With my 3" exhaust as stated I needed to port the heck out of my exhust housing to eliminate boost creep. In the future I will only buy 3" kits due to how cheap they are now, but I had gotten a great deal on the buschur 2.5 and thats all I really needed at the moment.

 

[jpmxrider489]

I have a 3 inch exhaust from ebay. Im having no regrets so far.

 

[avefenix]

yes and no, you want a gradual change not an abrupt change. With an abrupt change from a 2.1 to 3 yes you are getting a low pressure zone, but you'll have erratic exhaust flow(turbulent) which can cause back-pressure.

The way I figure it, the optimal setup would be 2-2.5-3 (or 2 1/4-2.5-3...etc.)preferably all expanding from the beginning of the DP to the exit of the DP. Thus creating maximum velocity resulting in really fast turbo spool-up

ABSOLUTELY !!! This is what ive done with an upgraded turbo system of course 2.5 to 3 so my vote is on this!!

 

[PieEyedPiper]

Go with a good quality 3" exhaust system. Something with alot of muffler size volume and a decent resonator. A 2.5" downpipe or 3" downpipe as well, they both make the same power up to at least 400 w.h.p. I see no reason to run a 2.5" catback as it makes noticeably less power than a proper 3" catback and there are 3" catbacks that are reasonably quiet.

I like this.

 

[Leonthefaded]

no wonder why you still have a red chicklete !! You expect for him yo go with a 2.5" if you give him no imput???

OP like bud said are you replacing all the way from your o2 housing or just catback??
It al depends on the turbo setup your aming for..
I personally im getting the 2.5"megan o2 housing, 2.5" custom made DP, with my thermal 3" carback.. by no means am i cutting the thermal flange ... thats why im not getting 3" all the way...

reasearch a lot before you spend your money!!

At least I can spell. Like I said op. Check out the rally exhaust thread. There is buttloads of info there about exhaust systems. More facts then what you will get elsewhere

For those who dont know how to search Click Me

 

[Alan11]

For those who dont know how to search Click Me

finally something usefull

And sorry english isn't my first language, at least i know how my car works

 

[avefenix]

2.5 inch down to 3 inch catback you wont regret.....

 

[PDX-GST]

Op are you only going with the catback? Or o2 housing back(turboback)?

For now just the cat back, but later a down pipe and either a new O2 Housing or just port it.

I am trying to follow the upgrade path found on this forum here:

Tech Guide: 2G 4G63t Upgrades: Stage 1 Power - DSMtuners

 

[NHerron]

"Does size matter?" by the way, !

OP I'm still stickin with my guns in favor of 3". Especially since your only thinking [factory] cat back. I'm 100% sure you wouldn't need to worry about creep with that setup. Just make sure the pipe routing doesn't end up ghetto hanging by 1 pipe hanger or something...

Do it once do it right would definitively apply here

 

[pboglio]

The 2.5" downpipe to 3" catback is proven. I ran 3" open downpipe and WOT power was the same, but the low end felt a bit off. That might be a result of actually not having a catback attached, but looking at the data acquisition I gained zero peak power, zero turbine response improvement, and a shit load of noise. I hated it so much I tossed the 2.5" downpipe back on and was happy again. As far as boost creep goes, gonna be hard to stop it on a 16g, no easy solution other than an external wastegate. I just live with it.

 

[PDX-GST]

Installed my 3.0" Intake today and I am extremely happy. Fit great with no modifications needed. I have to say that removing the muffler portion of the stock exhaust was a bi*** since it goes over the rear cross member. After fighting with it for a long time, I finally dropped the cross member a bit and it was off.

I got it from japanimportshop on ebay.

 

[DSMnOOb00]

Any pictures?

 

[PDX-GST]

Any pictures?

I took a couple.

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[DSMnOOb00]

I'm glad you decided in the 3". Good choice!

 

[91fwdrst]

Hi guys i just recently got a full 2.5" tbe and I have a big 16g turbo on yhe car. I cant seem to get a straight answer from anyone on if I'm going to exceed what the exhaust can flow with the turbo I have now. My goals for the car are 350whp and being able to get the full potential out of the turbo. I keep getting told that the 2.5" is fine unless I want over 400whp or no I need to have a 3" tbe and that I can only make 300whp if im lucky with the current exhaust. Thank you for your help.

 

[Awd-laser]

You will be just fine with the 2.5" exhaust. My tuners made 641 on a 2.5" exhaust.

 

[Talyn]

You will be just fine with the 2.5" exhaust. My tuners made 641 on a 2.5" exhaust.

QFT, Unless your name is brent rau you probably don't need a 3" exhaust on your dsm.

 

[mitsubishikid]

You want to use 2.5'' to 3''inch for the most street power from a 16g MHI turbo.
3" inch is more likely to rattle then 2.5", but some tuners have used/custom made 3" that does not rattle, it just takes a lot of time test fitting, and mounting it all on and off a few times, to get it just right.

Your 2.5" setup, is at least better then the flow limited, stock pea shooter setup.
I think since you already have 2.5" setup stick with it, and raise the boost [after you do a boost leak test] and see if you can reach your target of 350hp, because buying/making a 3" inch setup is gonna cost you more money$$$, and if you can do it with 2.5" it wont cost you anything you haven't already spent.