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Discussion Starter #1
I'm sure it's been done a zillion times so I am calling on the collective knowledge.
I am putting the two piece exhaust manifold on my 1983 S3 spider with the early style down pipe also. Where is the best place to put in the O2 sensor bung? Just where the twin pipes meet is my guess but I just want to make sure.
My plan is to put the manifold on and hold the down pipe in place just to make sure there is clearance for the O2 sensor on the top w/o hitting anything, (floor, chassis stiffener cross brace etc.)
Ok, plan?
Thank you,
Chris
ps with those brass factory nuts the exhaust manifold came off like a dream, they had me worried I had been spraying them with penetrating lube for a week!
 

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But Mad North-Northwest
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Generally you want it as close to the engine as possible where it can see the exhaust from all four cylinders. So where the twin pipes meet or a little beyond that is fine.

It doesn't need to be exactly in the top center of the pipe but it should be in the top half.
 

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The S4 2pc manifold setup is similar to the pre-74 system you are installing, and its O2 sensor is side mounted (facing the center of the car) at the spot you are talking about. Unlike the original S3 exhaust that has the Y at the firewall which provides the clearance above to have the O2 there, because the Y is further back on the S4 and pre-74 systems, I don't think you have the clearance on the top. If you are able fit it on top, it would require the sensor to be pre-installed before putting the exhaust on the car.
 

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There is no reason the sensor needs to be top mounted. I did the same conversion on the '88 Spider and the bung is installed on the side, facing the center of the car. No issues whatsoever in over three years.
 

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Discussion Starter #6 (Edited)
@65 Sprint,
Thank you! That was my next question as there isn't much room on top for the sensor. So I am assuming I can go in from the side or even the bottom as long as the wire is protected and the sensor is in the exhaust flow?
Chris
 

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But Mad North-Northwest
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Like I said, you should mount it in the top half of the pipe. So 90 degrees out the side is okay, but don't go lower than that.

Two reasons for this. The first is that it makes it less likely it'll get smacked by road debris. The second is that moisture can condense in the exhaust and puddle at the bottom of the tube, and if water gets into the sensor it'll damage it.
 

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Chris, I would mount it on its side. As Tom points out, more than 90 degrees from the top and the chance of damage increases substantially.
 
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