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Discussion Starter #1
On the subject of "pointless" mods that involve more sound can anyone else weigh in on this topic regarding, specifically the Forester STi exhaust system as it still seems to be quite vauge from just about everything I've brought back in the search.

On my 2004 STI I have an HKS cat back, the rest is OEM from Japan. I would like a little more volume/subaru gargle to come through and there seems to be a few options, different mid-pipe, or a downpipe. Given the downpipe is £500-1000 and certainly plus a retune vs £150-200 for a new mid-pipe and maybe a re-tune, and although the downpipe will make a lot more difference in terms of sound/power I'm currently interested in the two boxes on the mid-pipe and where the catalyst(s) is/are actually located as I'm thinking of picking up a H&S mid-pipe, much easier to change come MOT time as opposed to a downpipe.

I had the car up for an oil change this weekend but forgot to get a proper look at this at the time so these are photos I have from the importer during the undersealing process, but the thing I'm interested in here are these two boxes...

So one of these is most likely a resonator but is the other a cat, be it a 2nd or only cat?

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From this angle (below) I can see an O2 sensor, I can check this at the weekend but is this in the downpipe or is it in the mid-pipe as from what I can see there is no bung for this on the H&S midpipe for the fSTi? (99% sure its in the downpipe looking at the above photo).

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I understand the fSTI exhaust system is Frankenstein of the Impreza parts bin but can we clear this up with what is exactly on this car, is there two cats? Whilst I understand the O2 sensor is behind the downpipe cat on the Impreza STi one of these boxes looks very much like a cat and is behind the O2 sensor on this fSTi which to me implies its a second cat and the ecu isn't that interested in whats coming out of it meaning it can be fitted without a re-tune (backpressure changes dependant of course).

This is the midpipe I'm looking at:

 

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The OEM lambda sensor is definitely in the end of the downpipe. The two centre exhaust boxes, one is the main scrubber CAT & the other a resonator. There is also a CAT in the downpipe as you probably know.
I can't remember the center pipe length you need to order to directly replace but it is on this forum somewhere.
 

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Discussion Starter #3
The OEM lambda sensor is definitely in the end of the downpipe. The two centre exhaust boxes, one is the main scrubber CAT & the other a resonator. There is also a CAT in the downpipe as you probably know.
I can't remember the center pipe length you need to order to directly replace but it is on this forum somewhere.
Thanks, I thought as much, excuse my ignorance but when you say main scrubber cat does this mean I can't remove this one for MOT as its soley responsible for emissions? Or that its simply responsible for picking up stuff the first cat may not catch?
 

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Discussion Starter #4 (Edited)
Just a follow up incase anyone else searches for this stuff, I bought this in the end...


(H&S fSTI Centre Pipe with Bubble Silencer - 2.5" Pipework)

It should be 53" in length to fit the fsti, unfortunately they sent the wrong one out the first time which was 46.5" for a UK XT. I didn't know this until it was bolted upto the axle back however, I looked at the other end and thought no chance those spring bolts will make up 3" haha! Good job I didn't do a pikey job and cut the old one off. Anyway, props to H&S upon realising their mistake sent a 53" one out that came the very next day.

Its all fitted up with new gaskets and bolts, really easy to swap out. It now and sounds deeper, bit more baru gargle on throttle and the occasional, much less muffled bang so it compliments the HKS Hi-Power axle back nicely (although the welds, whilst adequate, are not as pretty as the HKS). Also despite some reports it does, it doesn't contact with any part of the body or standard diff brace (not sure about the group N) on mine.

Will be doing a 150 mile stint today so we'll see if it drones or not but from the short test drive, which did get up to motorway speeds, it seemed fine.

I will update with a sound clip at somepoint when I eventually get round to it.
 

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Don, King of the parts diagram
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The mysteries of the FSTI three piece exhaust, "2nd cat or not" has been around for years


With yours removed can you confirm that the first box is a cat or just an expansion box.
 

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Discussion Starter #6
The mysteries of the FSTI three piece exhaust, "2nd cat or not" has been around for years


With yours removed can you confirm that the first box is a cat or just an expansion box.
It looks exactly like the one in the middle as expected, it certainly looks like a cataylst but I fitted it on a Friday evening so haven't had a proper look at it yet to make sure. Other than cutting it up (might need it if MOT guy complains) I'm not sure how I could tell really.
 

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Don, King of the parts diagram
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It looks exactly like the one in the middle as expected, it certainly looks like a cataylst but I fitted it on a Friday evening so haven't had a proper look at it yet to make sure. Other than cutting it up (might need it if MOT guy complains) I'm not sure how I could tell really.
Having reread your original post you state that you are running a "cat back" HKS system but the photos you posted show the standard OEM mid pipe. And as far as I can tell HKS do not do a mid pipe for the forester. They do have a muffler on their website, so is not your system an "axle back" HKS setup?

The centrally positioned box on the OEM mid pipe in my picture above, is definitely a resonator/ silencer, the left hand box is a ?
You should be able to view it's contents from the open end. If it looks like this.......


it contains a catayitic converter, if not it's just an expansion box.
 

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Dont know if it helps but you can fit a newage impreza downpipe, 2nd cat delete pipe and centre section and it mates up perfectly to the forester axle back rear section.

I fitted a Prodrive PPP impreza downpipe which I bought used for £150, then had it ceramic coated for another £165, and a £138 M2 Motorsport resonated centre pipe, along with my Apexi axle back rear section it's really improved the sound, it's perfect to be honest and sailed the MOT.



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Discussion Starter #9
Having reread your original post you state that you are running a "cat back" HKS system but the photos you posted show the standard OEM mid pipe. And as far as I can tell HKS do not do a mid pipe for the forester. They do have a muffler on their website, so is not your system an "axle back" HKS setup?

The centrally positioned box on the OEM mid pipe in my picture above, is definitely a resonator/ silencer, the left hand box is a ?
You should be able to view it's contents from the open end. If it looks like this.......


it contains a catayitic converter, if not it's just an expansion box.
I'm running an HKS axle back, I put cat-back by accident. I had a look at the centre on Saturday and it has a honeycomb centre simliar to what you posted, I've seen this before on other cars so I know that it is certainly a catalyst.

Muzza is that an STI? I looked at the PPP downpipe and it doesn't have a place for the O2 near the bottom and is much shorter than the one pictured above in Dons earlier post. The centre pipe I have from H&S looks just like M2 one and is 53" long.I have a 2004 (Zenki) STi.

Update on the sound after a 300+ round trip and its fine, a little louder in the back for passengers but not drone, sounds great through the gears though.
 

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Don, King of the parts diagram
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Nice to know for future reference that its a cat - thanks.

You are running the standard 3-piece post turbo "classic style" exhaust as used on the classic imprezas and most early JDM ( before things went twin scroll)


Mozza is using the newage impreza/ european SG Forester 4- piece style exhaust



The only joint than is in the same location for both systems is the rear (or middle pipe) to muffler
 
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