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!

 

[asd_gst]

While I have heard of people noticing a slight drop in low/mid range torque (although I don't think it's even possible, if you search, it's out there), I've never noticed such a thing with any turbo car I've owned. Generally speaking you'll have more top end, better spool, and an exhaust note that makes the car more fun to drive without loosing anything.

yea i didnt think it would have a negative effect on a turbo car, like i said i know it can on a n/a car but this is my 1st turbo car, so i ask about everything just to be sure i dont mess up with something

 

[jakeachy]

you MIGHT get a case of small boost creep, but the gains will make the buttdyno happy

 

[meliketoball]

you MIGHT get a case of small boost creep, but the gains will make the buttdyno happy

I thought it would help preventing it. Anyway, go for it.

 

[SuzukiRidr326]

i got 3" on my lightly modified 2.0L GS, i lost some bottom end but gained at top end

 

[Dsm2nr89]

I know this topic has beaten to death and past but i am still undecided. My goal is to make low-mid 300 hp. 2.5 or 3 inch ? Thanks for any help, i am about to go crazy if i dont agree on a size asap .

 

[wret]

A few years ago this might have actually taken some thought. These days with all the low cost chinese stainless exhausts out there, you'll have a harder time finding a 2.5-inch exhaust and it would likely cost more than 3-inch.

 

[kenamond]

And someone somewhere had a plot showing that you start losing noticeable hp/torque in a 2.5 exhaust at around 350hp. I don't think it was for our car, but I vaguely recall that it was for a "comparable" motor.

 

[f_upkid1]

My friend just went from 2.5 to 3 inch on his gsx. It made a world of diffrence.

 

[blownturbotsi]

I'm not sure about a 3in yet either. I have a 2.5 TBE with a test pipe and it won't let my turbo spool much. I had only one bolt in the o2 for the longest until I found one that fit right (middle of no where) and as soon as I put the DP all of the way back on, it'd fall on it's face around 5k. I took out the test pipe to run just open DP and it was the same.

I took my car to get tuned and my tuner was able to get it to rev to red line, but it was only making 9lbs. Before with one bolt in the o2, it was at 15 and wasn't touched. We tried and tried with the boost controller, but it stayed at 9lbs. I took off the DP completely and it jumped to 27lbs on the first pull. I'm currently at 19lbs and have to run open o2, but I don't know what to do about an exhaust. If a 2.5 won't even give me more than 9lbs, will a 3in do much better? Would I have to go bigger?

 

[dsm-onster]

And someone somewhere had a plot showing that you start losing noticeable hp/torque in a 2.5 exhaust at around 350hp. I don't think it was for our car, but I vaguely recall that it was for a "comparable" motor.

Well, I've logged and plotted a significant difference with this engine going from 3" exhaust to open o2 housing with a small 16g at only 22ish lbs/min. So now I'm a firm believer in bigger is better WRT turbo exhaust.

 

[my_precious]

I did 3 in exhaust on my 6 bolt 1g AWD and was creeping to 22-25psi on the gauge. ported the o2 housing and turbine housing/wastegate and now she holds 16psi to red line. sooo much faster now. I totally recomend a 3in turbo back exhaust!

 

[marence412]

the bigger the better. better spool ups on the 3''.

 

[Razor_nl]

Ok maybe this is a really dumb question, but which aftermarket exhausts (catback) are 2.5" ?

I'm aiming for a 2.5" for some back pressure for easy city driving (which I'm doing every day, also traffic jams) but most of the "well known" brands like the Apexi N1 and Greddy Evo2 are all 3"
I couldn't really find a 2.5" catback.

In my country the Eclipse is quite rare so not many parts are easily available here.. it all has to come from the usa.

 

[crimsondragon]

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60mm. Close enough to 2.5 inch.

 

[gsxpj6428]

stick with the 3". who knows? maybe you will want to mod more in the future and that will come in handy.

 

[Razor_nl]

Well my first choice was the Greddy Evo2. (mostly because of the sound, I just love it) Until I read up a bit on the size discussion.
My choice for wanting a 2.5" and not a 3" was backpressure. Heavy car, daily driving (no dragracing at all), so I want some backpressure to keep the speed and response at lower rpm. I've had another car before with a quite large exhaust under it, drove it for a few months, and when I put the original back on I really noticed less power in higher rpm, but I just loved the power I had in lower rpm.

Oh and by the way, is there any mark where you can say like.
if you have more than xxx HP you need 3" , less HP is fine with 2.5"

 

[dsm-onster]

Pressure keeping exhaust gases from leaving the cylinder does not increase low or midrange torque. Scavenging is not backpressure. You should not measure the range an exhaust pipe savenges best by it's backpressure.

 

[RiceKiller_TSi]

What he said ^^

Backpressure + Turbo =

 

[Razor_nl]

Right ok I get it.
This is my first turbo car and with non-turbo car's backpressure kinda stands for low and mid torque at the cost of power high in the powerband.
Right, choice made, EVO2 here I come

Now just wait and see if it's possible in some sort of way to get it back over the suspension, I hate it when it hits the ground.

 

[DSMunknown]

Just wanted to throw this out there just for this thread.

Another issue may be also that you did only upgrade to a 2.5" system all the way back. Most people think while 2.5 to a 3" system isn't all the much bigger, when you do the math & work out the area of each diameter you will find the 3" is actually a good amount larger then the 2.5" system, ~4.9 vs 7.1.

 

[Blk_GST_96]

kinda a dumb question... but will a dual exhaust be better than regular exhuast on a GST??

 

[PieEyedPiper]

kinda a dumb question... but will a dual exhaust be better than regular exhuast on a GST??

A dual exhaust serves no purpose on our cars. It's just extra piping and more restriction in most scenarios.

 

[Blk_GST_96]

Thanx man

 

[PDX-GST]

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"

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Or this one for 3.0"

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[rcarbajal4]

I would say 3". Its very inconvenient to have to buy a full 2.5 exhaust system (not really cheap) and have to upgrade later down the line. That's just my point of view.

It also depends on your goals. If you aren't planning on making some serious power (400+) it wouldn't really matter.