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Old 02-19-2019, 12:19 PM   #1
MichaelBrown86
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O2 relocation from first cat, to second?

Hello everyone. I searched around the forum and couldn't find what I was specifically looking for, so here's my question to see if anyone has attempted it:

If I remove the catalytic converter on the exhaust manifold by purchasing an aftermarket option... would I be able to place the up-stream O2 sensor on the down-stream bung... and then take the down-stream O2 sensor and (after installing a O2 bung after the second header) relocate it to that position... and fool the ECU into thinking there's still a catalytic converter on the exhaust manifold?

Has anyone attempted this? What were your results? Did your vehicle throw a CEL?

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Old 02-19-2019, 12:24 PM   #2
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relocating the first A/F sensor would be useless, you could keep it there wherr it is, why move it of a few inches?

relocating second O2 would probably work as you intended.. but why not tune it disabling the P0420 and squeeze a bit of power out?

you will have to make wiring extensions to both sensor to make them fit to the connectors, and drill a hole in the front pipe... lot of mess, tune is a lot easier and quicker
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Old 02-19-2019, 12:44 PM   #3
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relocating the first A/F sensor would be useless, you could keep it there where it is, why move it of a few inches?

relocating second O2 would probably work as you intended.. but why not tune it disabling the P0420 and squeeze a bit of power out?

you will have to make wiring extensions to both sensor to make them fit to the connectors, and drill a hole in the front pipe... lot of mess, tune is a lot easier and quicker
I come from the Miata world, and they make exhaust systems with this option (O2 sensor wire extensions included).

The main reason people do this is so that they can still pass emissions test when the inspector scans the ECU (very rarely do they visually inspect). So long as the ECU says "No problems here, Sir!", You pass.

I'd imagine the tune would simply disable the CEL by not running the test, but that wouldn't work for the purposes stated above.

I agree with relocating the first O2. Doesn't really matter if it's in the headers upstream or downstream bung, does it?
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Old 02-19-2019, 12:59 PM   #4
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ecu decides if cats work ok or not by several tests. Delta of O2 readings, temps, their change over time since cold start and so on. At very least, moving sensors more downstream will change temps to colder. Very probably cat readiness checks may still get triggered.
btw, IIRC there was some aftermarket option of your proposed design .. not recalling vendor .. fujitsubo? hks? Anyway, it was set of cylinderback of catless header, overpipe, catted frontpipe with O2 sensor bung (+sensor spacer and sensor wire extender), catback.
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Old 02-19-2019, 01:23 PM   #5
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ecu decides if cats work ok or not by several tests. Delta of O2 readings, temps, their change over time since cold start and so on. At very least, moving sensors more downstream will change temps to colder. Very probably cat readiness checks may still get triggered.
btw, IIRC there was some aftermarket option of your proposed design .. not recalling vendor .. fujitsubo? hks? Anyway, it was set of cylinderback of catless header, overpipe, catted frontpipe with O2 sensor bung (+sensor spacer and sensor wire extender), catback.
Maybe the delta of heat could justify the first O2 sensor on the second bung on the header? Slightly further down stream to reduce temps ever so slightly?

After a bit of searching, I believe I found what you're talking about:
https://apexi-usa.com/exhaust-manifo...-20-subaru-brz

It's a Header, Overpipe, and Front Pipe combo from Apexi. (Ofcourse, you can buy their catback as well for the full system that you were referencing)
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Old 02-19-2019, 01:43 PM   #6
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Mm, probably i thought about this, and thought about +catback due memory not being clear on it since when i saw first mentions about it in this forum. Still, i wouldn't be 100% sure on this not throwing CELs (unless unlike most of other aftermarket vendors they had lot more info available on cat readiness check algorithms in stock ecu tune from toyobaru and use that to fine tune sensor spacer size/shape/location and such), and also wouldn't be 100% sure on it passing emission tests at pipe (while at pipe most owners pass MOT with secondary stock cat in stock frontpipe left, there were few that didn't, and high-flow cat in this particular set of four times less cell count is sure to reduce real gas cleanup efficiency vs stock cats, just as with many other catted w. high-flow cat aftermarket frontpipes & catless header). There is also bit of it's posted low performance gains relative to stock. One will need ecu tune to rise gains anyway .. and if non-stock ecu tune, why not one with cat readiness checks disabled too?
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Old 02-19-2019, 02:52 PM   #7
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Depending on the manifold, the 2nd O2 sensor position may actually be better at reading all 4 cylinders.....
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Old 02-19-2019, 02:59 PM   #8
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Better for what? For adjusting fueling? But are readings of 2nd sensor used for that at all?
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Old 02-19-2019, 03:44 PM   #9
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if you position the 1st sensor a bit more far, it has more possibility of catching all cylinder exhaust mixed together, but with the stock manifold with cat removed i doubt there is much difference
since the 2 bungs are basically in line of each other

2nd does and collects small amount of corrections, totally removable from ecu tune (I do remove it), it s much better to just fine tune using just the first sensor
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Old 02-19-2019, 11:20 PM   #10
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Isn't the upstream 02 sensor pre-cat on the stock manifold? I haven't seen one in a while. Pretty sure the ECU is looking at the delta between them so that would not work. If you are getting a catless header you should tune anyway and disabling the check engine light for cat efficiency is standard practice.
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Old 02-19-2019, 11:51 PM   #11
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HKS does it too.


https://www.ft86club.com/forums/showthread.php?t=41108
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Old 02-20-2019, 01:15 AM   #12
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Just get a catted header and tune it with oft and you'll pass emissions and no cel.
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Old 02-20-2019, 02:18 AM   #13
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www.ft86speedfactory.com/avo-catted-front-pipe-1172.html
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Old 02-20-2019, 10:29 AM   #14
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Quote:
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Depending on the manifold, the 2nd O2 sensor position may actually be better at reading all 4 cylinders.....
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Better for what? For adjusting fueling? But are readings of 2nd sensor used for that at all?

He was suggesting putting the primary O2 sensor in the secondary bung. Most aftermarket headers have the primary O2 only reading from 1 or 2 cylinders, the secondary bung will catch a mix of all 4.
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