Weak links in Gen4 Exhaust. Is the Y pipe?

[Shaheenis]

Hi,

I am planning for the upgrade of my exhaust but before doing this I would like understand more about the y pipe. I have STI quads now and they are staying. I am planning to replace the dump and mid pipe but why would I replace the Y pipe? Does this section have any issues or is it simply small diameter pipe? I am not planning on running big powar just sub 200 probably stock turbo will stay.

Cheers,

[zch91]

Well.. For a start you can't replace/retain the Y pipe with a stock exhaust, as it is part of the midpipe.

Exhibit A:

[Shaheenis]

Mm ok. Is that a resonator under the shield?

So can you get a dump that bolts straight up to the factory mid+y-pipe? Looking at that exhaust the Cat back section looks pretty clean (depending what's under that shield).

[PoidaGT]

Mm ok. Is that a resonator under the shield?

So can you get a dump that bolts straight up to the factory mid+y-pipe? Looking at that exhaust the Cat back section looks pretty clean (depending what's under that shield).

All dump pipes should mate up to the stock centre and y pipe (the standard flanges are ordinary).
I upgraded my dump first and left the stock centre, y pipe and mufflers and I made 170kw (auto)
When I upgraded my centre & y pipe, I made no extra top end but my low end and mid range torque jumped a fair bit and was very noticeable.
I definitely think that the centre and y pipe is mandatory upgrade when you first start modding your lib.

[Shaheenis]

Thanks man. I am guessing whatever is under the shield is the restrictive bit as the size of the pipe from what I can tell should not be such an issue. I would have expected the exact reverse of what you have experienced though. Dump pipe only should in theory improve overall flow but top end gains should be improved by opening up the entire exhaust. Either way good feedback for me to consider thanks . If I can save some dollars I will mating a turbo back TCP to my STI muffs.

[mynameisdaniel]

Big difference with the dump pipe. After a tune revision there was also a huge improvement after going to a bigger centre pipe for me. I'd say the opposite, dump pipe improved low end, but opening up the middle section has given more outright power. Wheels now lose traction where they wouldn't before...

Keep the STi mufflers, sounds great with a bigger centre pipe and dump

[Shaheenis]

Yes that's actually what I was trying to say as different to PoidaGT's experience. Dump pipe will improve overall performance and flow from bottom end to top end without any other exhaust mods as the turbo can spool a lot more easily (quickly) by dumping exhaust with very little immediate back pressure. As engine revs climb the actual flow of the entire exhausts becomes a point of restriction typically with mods to the rear of the vehicle making the least difference as exhaust gas has cooled (and condensed) considerably by this point. It of course depends entirely on just how restrictive the difference components are though. I remember reading a very interesting and technical thread on the physics of this.

Yes I am hoping for some more sound out of the STI muffs, they sound great but quiet.

[Boostaddict]

I have the dump and center and its cut just before the split into the y bit at the back and left stock from there. So its like having a stock diff back exhaust. It is FAR too quiet and in fact only makes noise on light throttle higher in the revs.

If i give it anything at all i hear nothing from the exhaust as my pod takes over. It will be even worse when my fmic goes back on and i install my perrin inlet. All boost and no exhaust. So my stock y pipe and sti mufflers will be going . Would love to keep the sti tips tho but dont want to hack them up. Thinking just twin singles but paint them black so u really cant see them.... Sorta stealth sorta.!!!

[PoidaGT]

Thanks man. I am guessing whatever is under the shield is the restrictive bit as the size of the pipe from what I can tell should not be such an issue. I would have expected the exact reverse of what you have experienced though. Dump pipe only should in theory improve overall flow but top end gains should be improved by opening up the entire exhaust. Either way good feedback for me to consider thanks . If I can save some dollars I will mating a turbo back TCP to my STI muffs.

Hey Tim

How much does TCP charge for a Turbo back system without the mufflers?

[zch91]

Have a squiz here viewforum.php?f=43

And/or PM 'tbear'

[PoidaGT]

Have a squiz here viewforum.php?f=43

And/or PM 'tbear'

Looks like they charge $1199 for a stainless TBE minus the mufflers which i think is a pretty good price.

The only TBE exhaust that i know of that would be cheaper is an X force mild steel system.

[Shaheenis]

Yes and the TCP is a great quality system, I don't know if you would find many/any with better quality finish, welds etc. I can't wait to get mine probably in a couple of months. I need to sell one of my cars first . TCP with stock rear, match made in heaven I reckon .

[PoidaGT]

The only thing I didn't like was there was no splitter in the TCP down pipe.
Apparently the splitter stops the waste gate from disrupting the airflow off the exhaust turbine.
I'm really not sure how much difference it makes to power and torque though.

[Shaheenis]

Yes itsca shame it doesn't but I have owned 4 dumps in my Gen1 Lib from open bell mouth to splitter and two different divorced waste gate setups (CES and AMAuto). I actually think open bell mouth assists proper waste gate control as the waste gate flow is not restricted. The force of the exhaust pushed by the exhaust wheel would pick up re waste gate gas without causing any significant back pressure I believe. You can have a splitter easily installed if required.

[Shaheenis]

So after some research, I don't believe the factory Y-pipe is restrictive at all, the combined diameter equates to just over 3" at the rear which is standard aftermarket turbo exhaust outlet for most turbo cars including those making big power. The size of the mid-pipe (haven't measured but looks about 2.5") and the factory resonator would probably be the biggest restriction but I can't see this being a restriction until over 200atw and that is just based off other Subaru's I have known and the size of the exhaust (some of the old lib boys used to run 2.5 at the rear and found it to keep more bottom end without really sacrificing top end (tuned exhaust). My old 3" Fujitsubo system actually is 3" but the outlet to tip is 2.5" increasing to 4".

So I have been thinking about this a lot lately as I want to replace at least the dump pipe but didn't want to have to custom modify anything (bolt-on only). What I have realised (and stand to be corrected) is that there is simply no sensible bolt on replacement except for doing the whole exhaust including mufflers. The reason being that somewhere you will encounter a step-down (blunt edge) as the aftermarket component meets the smaller diameter stock exhaust components. This will actually be much worse at the front of the exhaust where exhaust gas velocity is higher (e.g. if you just replace the dump). However after comparing the size of aftermarket Y-pipes they exit at a minimum of 2.25" whereas the stock muffler inlet is ~1.5" which is actually a horrible step and disruptor to have in your exhaust. I have spoken with Chris from Best Mufflers to explore options and he agrees he simply wouldn't mate an exhaust that had a blunt step. If you were replacing just the mufflers however this wouldn't be an issue as you are going from smaller to larger in direction of airflow.

So given I really want to keep the STI mufflers and they won't be a restriction for my power goals I simply need to have some customisation of my exhaust . I have also looked at the pictures posted above of the stock exhaust and it actually looks very clean, the size I don't believe is a restriction for my power goals and Chris reckons he has cut open one of the factory resonators and they are a straight through design so not as horrible as I might have thought.

Given it is too tricky to try and modify the y-pipe with a downsizer and likely alignment issues I have decided to replace dump-pipe only. Problem is I want one of the TCPs or iNvidia and both have CAT in downpipe which means there won't be any gap between where the down-pipe meets the factory exhaust so the only way to create a smooth transition would be to hack the new dump pipe (I don't think so) or modify the inlet to factory mid-pipe. I have a picture below that depicts what I am attempting. I expect the mod to cost a couple of hundred if I get flanges fitted so I can revert to factory without a return visit to the exhaust shop. That would pay for the flanges and labour. I would of course need to fit the dump pipe myself and do a temporary bolt up to get to the exhaust shop. Thoughts and comments welcome.