O2 sensor location

[mcdougal]

My S52 obd1 swap is up and running so now I'm working on the exhaust. The plan is to run the eBay long tube headers into a single 3" pipe via a custom y-pipe.

Here is the question: should the O2 sensor be placed before (reading off 3 cylinders) or after (in the 3" pipe reading off all 6 cylinders) the y-pipe transition?

All the oem obd1 exhaust pictures I've found show the sensor reading off 3 cylinders.

Any advice is appreciated.


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[nando]

after is better IMO

[mcdougal]

after is better IMO

In the 3" pipe reading off all 6 cylinders?


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[nando]

yes

[M3PO]

I put mine after. Back near the guibo.

edit: You want it to read all cylinders with as much mixing as possible for a good average. But the zirconia sensor needs to be hot so don't mount it near the muffler lol

[GDA]

I have read that most tuning for OBD 1 cars is based on O2 sensors being placed well back after the exhaust headers. IIRC the match of the tune will work best if placed further back.

[woc]

I have mine way back by where the end of the transmission is. Near the transmission mount there is even a little hook to pass the o2 sensor wires though it.

[nando]

you don't want it too far back as the sensor needs to be hot. It just needs to be past the collectors so it sees both cylinder banks.

also the further back it is the more lag time there will be between the sensor reading and when the actual combustion happened. OBD1 is ancient but on newer OBDII cars they account for this and the tune would need adjusted if it moved too far.

[woc]

OBDII usually will have 4banks. 2 in front 2 further back, another reason it is recommended to have the o2 in the back because it reads clear air from the bottom of the car, and not hot condensed air form the headers. The sensor has a small hole where the wires are, it breathes through that hole and where most scavenging happens inside the exhaust, now if you have a turbo will probably best to be in the down pipe.

[nando]

the second set on an OBDII car is only for emissions purposes - they don't do anything for controlling the engine. they read after the cats to make sure they're working, and trigger a CEL if the reading is off. that's it.

[ian e30 318is]

I have mine on the x-pipe directly after the long tube headers so it reads both banks. Works great! I had a check engine and it ran fairly rough when it only read 1 bank.

[mcdougal]

I have mine on the x-pipe directly after the long tube headers so it reads both banks. Works great! I had a check engine and it ran fairly rough when it only read 1 bank.

I ended up doing the same thing but I've got a y-pipe instead of an x.


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