Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Exhaust modifications

Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby tkdbboy » Fri Aug 12, 2016 9:07 am

Hey guys,

Been just doing some reading up on the stock setup of the STi exhaust system and just need a bit of clarification.
Some parts of the system seem more restrictive than others, and some aren't that restrictive up until certain power levels.
Originally I assumed that to get noticeable power (lets say 50kw) above stock levels, I would have to replace the whole system but from reading some threads, even just replacing the dump results in a good increase in potential for gains.

Below is my understanding on the OEM setup (% means how important it is to replace):
(not sure if the centre pipe and Y are separate in the stock setup, was working off a HKS diagram)

Dump pipe - (2.5") Most restrictive part (80%) restriction most noticeable @ Stock
Centre Pipe - (2.5") Not too bad (10%) restriction most noticeable @ ??
Y-Pipe - (2.5" to 2x2") Not too bad (5%) restriction most noticeable @ ??
Sti Quads - (2x2") Least restrictive (5%) restriction most noticeable @ > 200awkw

If anyone can clarify/ confirm the information above, that would be great.

Around what power (awkw) is an aftermarket dump with stock CBE good for?
Around what power (awkw) is an aftermarket dump + centre pipe with stock Y pipe and mufflers good for?

For the GT guys, I'm guessing the only difference in the system is the mufflers ... Is the restrictiveness of the GT and STi muffers the same?
Thanks
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby ferret89 » Sun Aug 14, 2016 3:59 pm

Hey mate

You are correct that the dump pipe is the single most restrictive part of the stock exhaust and replacing that will prob get you about 70-80% of the gains. I'd say that you could get close to 200awkw until the other parts of the exhaust restrict power much and if you are hitting 180-190 you'd probably notice a benefit to upgrade the whole system if that makes sense. The difference at that would be pretty small, you might notice a 5-10kw difference between aftermarket dump and rest stock and full aftermarket TBE.

Here are some photos of my stock Y and centre pipes for a bit of reference:
IMG_16861.jpg
IMG_16861.jpg (235.79 KiB) Viewed 8025 times

IMG_16871.jpg
IMG_16871.jpg (223.51 KiB) Viewed 8025 times


I have aftermarket dump, centre and Y pipe to my stock STI mufflers and I'm pretty happy with how it turned out. I did that for purposes of keeping drone to a minimal amount and am most likely losing only a few kw's due to keeping the stock mufflers.

AFAIK GT mufflers and STI quad mufflers are basically the same and have no significant flow/power difference too
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby tkdbboy » Fri Aug 19, 2016 8:04 am

Thanks for that info and photos Ferret.

I have always held the very basic belief that your exhaust system is only as good as its smallest point.
If the dump is upped to 3' but the Y is still 2.5" to 2x2", I assumed that the same air restrictiveness seen at the 2.5" dump, is now moved to the 2.5" part of the Y, but I guess its not as basic as that.

What dump, centre and Y pipe did you end up getting?
Regarding the Y pipe, I guess you went to a 3" to 2x2" split to connect to the stock mufflers?
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby ferret89 » Fri Aug 19, 2016 6:35 pm

tkdbboy wrote:Thanks for that info and photos Ferret.

I have always held the very basic belief that your exhaust system is only as good as its smallest point.
If the dump is upped to 3' but the Y is still 2.5" to 2x2", I assumed that the same air restrictiveness seen at the 2.5" dump, is now moved to the 2.5" part of the Y, but I guess its not as basic as that.

What dump, centre and Y pipe did you end up getting?
Regarding the Y pipe, I guess you went to a 3" to 2x2" split to connect to the stock mufflers?


I didn't take a photo of my stock dump pipe when it came off but it literally has half the turbo outlet almost covered that looks extremely limiting, much much more than any other part of the stock exhaust. Found this photo on google though:
DP_02.jpg
DP_02.jpg (78.07 KiB) Viewed 7955 times


I put a lot of detail of my exhaust mods in my build page here: http://forum.liberty.asn.au/viewtopic.php?f=10&t=28511&start=90 so you might be interested to have a read and a look through.

Basically though I bought a mild steel X-force cat-back matched to a CES dump pipe and had the exhaust shop redo my flanges to taper down closer to the stock muffler inlets like this:
IMG_1720.jpg
IMG_1720.jpg (77.48 KiB) Viewed 7955 times


Obviously not a perfect solution but I'm pretty happy with how it turned out and it has to be better than not tapering to smooth the flow right... :lol:
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby bigBADbenny » Sat Aug 20, 2016 3:51 am

Don't forget that it's not just about pipe diameters, but also the baffling in the mufflers and resonators, and the bends in the exhaust system.
Generally the three factors combine to create the characteristics of the exhaust, where you would look to promote, balance or limit either the factors of flow, sound or torque.

So with my current conservatively tuned stockish wagon setup, I have a 3" dump for flow, 3" mufflered mid for flow and noise reduction, and modded Wrx/STi sedan mufflers for more noise reduction, bc wagon daily driver and I hate it droning at suburban speeds.

As regards just the stock system, to promote flow and sound by adding a bell mouth to the dump and replacing/deleting the mid pipe resonator, next would be the cats in the dump.
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby tkdbboy » Tue Aug 23, 2016 4:47 pm

That dump pipe hurts to look at haha.

I'm liking your exhaust setup Ferret and will use it as a guide for mine.

I'll be going an AVO dump (3" catted), centre (3" resonator) & Y pipe (3" to 2x2.5") to stock mufflers. I guess the flange holes won't line up going from 2.5" to 2.25"?

I found another member off this forum who has the same setup on an 07 and it sounds pretty damn good
https://www.youtube.com/watch?v=GZxsUpADolw


Benny what did you do in regards to the join between Y pipe and mufflers?
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby bigBADbenny » Tue Aug 23, 2016 6:38 pm

Unless mfg specific eg invidia which has non oem compatible mid/y to muffler unions/flanges (eg their y pipe and muffs are non oem location), most aftermarket muffs will bolt up to the stock muffler flanges.
Eg fujitsobo, tcp, avo, ultrex etc.

So generally, 2.5 and 2.25 use the same flange.

My fabricator used a slightly radiused inlet on my modded Wrx/STi sedan muffs...
So like the pic above but the radius on the muffler side.

It's totes fine for a stockish sub 200atw street setup...
I'd be at 150 odd on a good day ;)

Take a peek here, specifically the zorst section for a rounded intro into the science and practical application:
http://www.msengineering.com.au/tech_index.html
You can see that for a stock turbo or smaller/medium turbo setup there's no harm in leaving the main restriction in the system at the muffs :idea:

:good:
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby tkdbboy » Wed Aug 24, 2016 4:21 pm

Ah sweet, glad most of them keep it easy.

I'll have to check out your thread on what you did to your mufflers.

That MSR read was a great read. Especially the part about bends in the axleback.

I noticed that some manufacturers make a flange adapter from 3" to 2.5" that I guess smoothens the air transition from AFM to stock pieces.
Doesn't look like anyone makes a 2.5" to 2.25" so maybe can get one fab'd up even though its a very small drop.
I'd like to avoid any direct fabbing to the pieces incase I wish to put stock Y back on or put AFM mufflers.

Will have it installed next week so fingers crossed that it all goes as planned haha
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby bigBADbenny » Wed Aug 24, 2016 6:30 pm

Any step is pretty insignificant where 2.5-2.25 is concerned, likewise for dump to mid transition if any... Imho, eg compared to what you may gain from bell mouth and cats etc...
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby GTB25T » Mon Oct 25, 2021 3:17 pm

Have read some of this post and there is a lot about post cat and dump pipe but I can't find detail on the UP pipe.
Wouldn't the inlet to the Turbo from exhaust headers make a difference to turbo spool?

Can some one please clarify: Does the 07-09 post facelift GTB EJ255 have an UPpipe CAT?

Does the 07-09 post facelift GTB EJ255 STI version have an UPpipe CAT?

I'm thinking before any costly exhaust mods, to change the UP-pipe to the turbo.

Thanks,
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby Stifull » Mon Oct 25, 2021 3:52 pm

I don't know if there is any mechanical or electrical difference between a GT and an sti apart from the factory tuning. I believe the sti is only 10-15 kwts more than the GT etc and I highly doubt that any exhaust changes is going to get 200 awkwts. The down pipe is the most critical. The turbo waste gate opens and the escaping gases only go 20mm or so before they crash into the stock downpipe. This makes those gases go sideways and straight into the turbo gases that are escaping. It's like when 3 lanes go into 2. there is always congestion there. A bellmouth or divorced downpipe will make the biggest difference. Adding a 3 " system at the end of the downpipe then a good tail section with high flow mufflers will finish the job. Still won't make 200 awkwts unless you are going to wind the boost up to silly levels. You may make 140+ with a full turbo back system but until you replace the turbo and all the rest of the intake and fuel delivery hoses, pumps, controllers, filters etc 140 is as good as it gets .. manufacturers awkwts are at the flywheel of the engine, not the wheels of the car and a awd car will loose a min of 25-30% + so 200 at the flywheel (subaru measuring point) will only be 140 ish atw..
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby GTB25T » Mon Oct 25, 2021 5:54 pm

Thanks for reply.
I'm not looking for big kW gains but increase in initial torque and quicker spool of the stock turbo.
I understand, particularly in the USA, most people replace up pipe with a quality non CAT version and replace the downpipe with a reasonable cost effective bell mouth CATTED down pipe, change the rear mufflers to STI and leave the rest stock.

Stays quiet with a nice note and majority of kW gains from those sections without drone.

Up pipe replacement enhances benefits of the down pipe.
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Re: Gen4 GT STi - OEM Exhaust Pieces Restrictiveness

Postby Yowie » Mon Oct 25, 2021 10:30 pm

Keep your wits about you when considering any aftermarket larger-diameter up-pipes.

The whole point of a turbo system is for the exhaust gasses to be funneled into a very tight nozzle within the turbine housing where they can impart maximum velocity in the turbine wheel. The tightening begins earlier - noting that the cross sectional area of the up-pipe is not double the cross-sectional area of each exhaust bank's merged output.

If you swap out a factory up-pipe for a name-brand aftermarket large diameter up-pipe (but otherwise keep a stock-ish sized turbo etc.) you'll just have this large diameter, lower-velocity exhaust hitting the flange of the turbine housing and having a sharp 90 degree edge as the point of reduction to go into the turbine housing.
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