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!

 

[WES_393]

3" exhaust has been thought to increase boost creep with 16g turbo's (if you ever decided to go that path). But porting the wastegate would eliminate this all together so my vote is 3".

 

[Rob10_99]

3" from the turbo flange for 16g 7cm flange and you can figure out what you need from there after additional power adders.

 

[Leonthefaded]

Go with a 2.5

 

[Thirdspazz]

I have a 2.5 exhaust, with a 16g turbo and 650cc injectors, smic at 15psi sounds great and custom fit. turbo spools up nice no problems, go 2.5"

 

[AK Thunder]

If your planning to go for a turbo upgrade by all means go for the 3'', no sense in buying a 2.5'' now then getting a 3'' once you've got a new turbo.
As of right now for myself, I'm still stock with a 13G and stock exhaust -_-; but I've got 10 feet of straight 3'' SS piping and a 3'' to 4'' muffler that I'm going to cut, bend, and set-up for cat-back until I can afford a 16G and 2.5'' DP.

 

[dsmspick]

Id say 3 inch. Will give you room if you decide to go bigger. Also look at the megan racing exhaust. Knew a kid who had that and liked it.

 

[FIBA M - EC17PSE]

Im going with a 2.5 - 3 inch as i heard that sometimes even the stock turbos get boost creep but i dont want to take the chance! So you could to that and then its only a case of replacing the DP or maybe even less then that depending on how you go about it, dont worry about power as so many people run 2.5" and produce 400-600 hp

 

[NHerron]

I vote 3". The 2g o2 housing should help keep creep at bay from what I have read and noticed on my previous setup. Maybe the Catalytic Converter helped too
(16G - 2G o2 (2.1") - bottleneck adapter(2.5" to 3") - DP - CAT. Basically a full 3" turbo back exhaust)

But then again if you know for sure you won't go big power in the future, I might consider the 2.5" system

Also, besides porting out an Evo3 o2 housing I don't believe there is a true 3" o2 housing anyway. Closest I found was with Megan Racing and it's a true 2.5" outlet compared to the stock 2G housing which is only 2.1". Someone correct me if I'm wrong there please

I've seen lots of solutions on Tuners for creep so check that out if you're still concerned with creep

 

[PDX-GST]

Go with a 2.5

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?

 

[AK Thunder]

Someone correct me if I am wrong.

But from what i've been informed(from an ex rally driver) is that for a good street/weekend track car, on a turbo such as a 16G, not small but not huge, you want to have for initial exhaust output smaller then your exit output, example a 2.5'' DP going to 3'' after DP or cat depending on your setup.

The smaller diameter DP creates more turbulence in your exhaust system which results in faster spool-up time than going straight from the turbo to full 3'' or more.

 

[bryanwheat]

Someone correct me if I am wrong.

But from what i've been informed(from an ex rally driver) is that for a good street/weekend track car, on a turbo such as a 16G, not small but huge, you want to have for initial exhaust output smaller then your exit output, example a 2.5'' DP going to 3'' after DP or cat depending on your setup.

The smaller diameter DP creates more turbulence in your exhaust system which results in faster spool-up time than going straight from the turbo to full 3'' or more.

With a turbo setup the less back pressure the better. You will make more power and have a faster spool. There is really no overkill with a turbocharged engine. Having more turbulence in the exhaust is the last thing that you want.

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?

You will have no drop in performance. You can possible have boost creep issues, but that is easily taken care of by porting the o2 housing and wastegate hole.

 

[AK Thunder]

With a turbo setup the less back pressure the better. You will make more power and have a faster spool. There is really no overkill with a turbocharged engine. Having more turbulence in the exhaust is the last thing that you want.

How so, Wouldn't more turbulence cause cause the exhaust to have more vacuum, thus with each pulse of the exhaust valves pull the exhaust away from the turbo faster, resulting in a faster spool up?

 

[PDX-GST]

You will have no drop in performance. You can possible have boost creep issues, but that is easily taken care of by porting the o2 housing and wastegate hole.

You know what Bryan, you just made my decision. Just because you were right about my misfire I am going to trust you on this one. Well and because you gave me exactly the answer I was looking for too. Thanks again.

 

[bryanwheat]

Not at all. The exhaust isn't going to have a negative pressure at all. It will be lower than the pressure in the manifold, but it will have a higher than atmosphere pressure. The pressure will increase closer to the turbo. The pressure will also increase the smaller the exhaust is. The bigger difference in pressure , the better the turbo will spool.

 

[Fast-Orange]

How so, Wouldn't more turbulence cause cause the exhaust to have more vacuum, thus with each pulse of the exhaust valves pull the exhaust away from the turbo faster, resulting in a faster spool up?

No, more turbulance more backpressure equaling less exhaust flow and a lack of power it will feel like your bogging.

 

[AK Thunder]

I'm sorry, I've mistaken my wording, its late for me and I've had a couple drinks this evening.
I meant velocity, turbulent airflow is erratic and will cause back-pressure you are right.

"Too great or too abrupt of a transition, and you get flow separation and turbulence, reducing flow. Ideally, the best flow would be achieved by a trumpet shaped downpipe that exits into an area below the car, but this is obviously not legal or safe because of exhaust noise and the exhaust fumes that would surround and leak into the cabin. You want the highest exhaust velocity after the turbine, and while bigger normally equals better, too large of an exhaust or bad routing could cool the exhaust, reduce its velocity, and create excess backpressure."

 

[NHerron]

That makes sense, like if you were to hook up a 3" DP DIRECTLY to 2.1" 2g housing then it could cause sort of a low pressure zone beneath the smaller flange.

Like a violent underwater current beneath it's surface

 

[AK Thunder]

That makes sense, like if you were to hook up a 3" DP DIRECTLY to 2.1" 2g housing then it could cause sort of a low pressure zone beneath the smaller flange.

Like a violent underwater current beneath it's surface

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

 

[NHerron]

Whoops yes I meant low pressure zone beneath the flange as in bad- and resulting in turbulence

 

[Jorgen]

You guys clearly don't know what you're talking about. Water and air have a lot different flow properties. The OP asked a very simple question, to which the answer is most likely 3". The reasons that 3" is better are:
1: all things being equal, a turbo car wants as little back pressure as possible. The 3" will make more power at just about ANY power level, but it really becomes more noticeable when you start to make 325+.
2: do it once, do it right. If you plan on having more power in the future, that's even more reason to just do this mod once.
3: sounds better imo

 

[Leonthefaded]

OP keep it simple. Go with 2.5.

 

[NHerron]

You wanna go a little more in depth as to why you believe someone doesn't know what they're talking about?

Or perhaps add something useful for the OP that hasn't already been said?

 

[Leonthefaded]

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.

 

[Thirdspazz]

when it comes to exhaust, everyone has your own idea, power goal, turbo size, setup, dd, or weekend warrior, and for me money, full time job, school, but i"m happy with 325hp on 93 oct gas, i have school books and living alone, go with what info you have here, from all of us, I doubt anyone here will say you made a bad choice. 2.5" or 3" find a good deal and go mount that bad boy exhaust.

 

[Bud92gsx]

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