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Old 11-25-2021, 06:19 PM   #1
SatsuMX
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2005 Legacy GT

Default I'm havin a custom exhaust made, but I'm unsure about O2 sensor locations

So as some of you might now, I got a used '05 LGT that came with a pretty horrible custom aluminum catless straight pipe, so I'm getting a new custom steel one made, which will have a catless downpipe and a catted exhaust, but I'm wondering what's the ideal position for both the front and rear O2 sensors.


Front would be on the downpipe I assume, but for the 2nd sensor, should I have it before, on, or after the cat? Or should I consider removing it and have the tuner turn off the sensor in the ECU?
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Old 11-25-2021, 06:29 PM   #2
K3rm1tth3fr0g
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2002 WRX - Link ECU
being installed :)

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Catless downpipe but catted exhaust? Where are you putting the cat then?

Front o2 goes in the header. It already has a bung for it.

Rear o2 doesn't really matter a whole lot, placing it OEM distance from the turbine isn't a bad idea. I can't give you an exact inch distance but it should be easy to deduce.

A lot of people get rid of their rear o2 and/or replace it with a wideband, however I suggest keeping the rear stock sensor for tuning sake, and also installing a wideband per the manufactures instructions in addition to the OEM sensor. WBs usually go like 18" or so from the exhaust ports or turbo. But many people put them much closer, and most DPs have the pre tapped WB bung much closer to the turbine housing than 18".
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Old 11-25-2021, 06:45 PM   #3
SatsuMX
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Default

I'm glad I asked, I hadn't considered adding the wideband!

I would have the catless downpipe, but I'd have the cat that goes in the midpipe, before the resonator mostly because this is a daily driver and I don't like the strong smell, and hopefully I can pass emissions. I know the rear O2 sensor "expects" exhaust to have gone through the first cat, so I was wondering if I needed to do something about that.

I'm silly and for some reason thought the front O2 sensor was on the downpipe, not on the headers.

... went and checked my piping and lo and behold, there is already either a wideband or o2 sensor installed on the crummy downpipe, probably why I was confused.
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Old 11-25-2021, 07:13 PM   #4
K3rm1tth3fr0g
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being installed :)

Default

Just buy an OTS catted downpipe my friend. It will very likely be cheaper and use a better cat.

There is absolutely no reason to move the cat out of the DP.
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Old 11-25-2021, 09:12 PM   #5
JSR84
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Default

Quote:
Originally Posted by K3rm1tth3fr0g View Post
Just buy an OTS catted downpipe my friend. It will very likely be cheaper and use a better cat.

There is absolutely no reason to move the cat out of the DP.
Agreed, itís actually detrimental to Cat performance and longevity.
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Old 11-25-2021, 10:07 PM   #6
benflynn
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Why a cat? If you havre to then Iíd go in dp, if you donít have to then donít or a 3Ē Magna flow in the mid pipe
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Old 11-25-2021, 10:52 PM   #7
K3rm1tth3fr0g
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being installed :)

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Quote:
Originally Posted by benflynn View Post
Why a cat? If you havre to then Iíd go in dp, if you donít have to then donít or a 3Ē Magna flow in the mid pipe

Try getting a tune without a cat from someone you have never emailed before. It's very hard
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Old 11-26-2021, 02:51 PM   #8
SatsuMX
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2005 Legacy GT

Default

Quote:
Originally Posted by K3rm1tth3fr0g View Post
Just buy an OTS catted downpipe my friend. It will very likely be cheaper and use a better cat.

There is absolutely no reason to move the cat out of the DP.

Unfortunately I live in Mexico, so getting one is about 2x the price of having the custom one made. I'll check with the exhaust guy and see if he can build it with the cat on it then.
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Old 11-26-2021, 03:24 PM   #9
SatsuMX
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Default

Talked to the guy, he said yes of course he can add the cat to the DP. Thanks for all the info guys, always appreciate it!

Will have pics later
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Old 11-26-2021, 03:29 PM   #10
benflynn
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Any volume between the turbo and cat will help spool, cat will not fire off as fast, but I donít think there is a performance gain having it in the DP.
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