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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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