325i O2 sensor placement in 328i (or M3) cats?

[Morgion]

I'm thinking of swapping the whole exhaust system sans manifold of my 325i to a 328i system, purely because mine is so rusty and I have a used but pristine 328i system readily available. Not expecting gains or anything.

According to my research, all parts of the 325i and 328i exhaust systems fit each other without modification, so you can mix and match as you want, the only difference is the amount of O2 sensors (and the vacuum flap in the 328i muffler).

325i is OBDI, so only has one O2 sensor for both pipes in the X-junction of the front pipe, while the OBDII 328i has two O2 sensors, one for each pipe located in the exhaust manifolds, and two post-cat heat sensors. The heat sensor holes will need to be plugged, but where would I attach the one and only O2 sensor of my 325i? Would I need to weld a section connecting the two pipes together and install the O2 sensor in that, essentially simulating the X-junction of the 325i front pipe?

I'm sure I'm not the first one slapping 328i exhausts on a 325i, so could anyone more savvy give me a hand?

If I'm not mistaken, M3 also uses a dual-pipe, dual-sensor system, so guys with M3 cats in your 325i, where did you plug the one and only O2 sensor?

[Eric93se]

You would only need to use one of the pre-cat sensor ports, close up the other three.

Remember that the sensors for OBDI and OBDII are not compatible (different technology).

[Morgion]

The 328i pre-cat O2 sensors are located in the exhaust manifolds, which I'm not swapping, so I would have no pre-cat sensor ports whatsoever. Would fabricating a sensor port in one of the pipes be sufficient, even though it'd be getting readings from only three cylinders?

[JDStrickland]

The problem with yoru plan is that the OBD I pipes have the O2 Sensor after the union where the headers come together so that the fuel ratio of the entire engine is monitored from one location, the OBD II pipes monitor the front three and back three cylinders (Bank 1 & Bank 2) separately.

The OBD II pipes need the O2 Sensor moved else the sensor will only sample half of the exhaust. The unused sensor ports must be blocked and a new port must be created.

Another problem with your plan is that the M52 motors have the CATs mounted to the exhaust manifold, so the pipe you are getting should only be the muffler.

Beyond the holes needed or not needed for the O2 sensor, are you sure the pipe from the 328 even fits under the 325?

[Morgion]

Knew about the port-blocking, thanks, though.

Would the best course of action be to weld a section connecting the pipes with each other just before the cats, fabricate an OBDI O2 sensor port in the junction and call it a day? The pipes would be interconnected, which would mean that the sensor would get readings from all 6 cylinders.

Cats on the exhaust manifold? In the exhaust system I'm thinking about (fresh out of a T-boned 328I) the cats were on the front pipe, just like on 325i. The only difference is the X-junction (OBDI O2 sensor location) in the 325i front pipe, while the 328i has dual pipes from manifolds to muffler.

328i muffler bolts straight on to 325i front pipes and vice versa, and unless I'm horrifically mistaken, 328i front pipes bolt straight on to 325i exhaust manifold and vice versa.

[Eric93se]

I think you can get away with only sampling half the stream.

[Morgion]

328i front pipes & cats:






328i exhaust manifold (no cats):






325i front pipes & cats:

I think you can get away with only sampling half the stream.

The car's OBDI anyway, so it wouldn't warn me anyway if something's wrong with the exhaust like OBDII cars do, so it doesn't matter if one of the pipes is left unattended as long as the sensor works as intended, I guess.

Input from somebody with an 328i (or M3, if I'm right) cats in their 325i would be highly appreciated! I seriously can't be the first one to do this.

[JDStrickland]

Okay, the port for the OBD I and OBD II sensors is physically the same port, although in different places. So, your question to weld in an OBD II port is a bit nonsensical, what you are really asking is to weld in an O2 Sensor port, and the answer is yes, weld one in where all six cylinders will be sampled, and your computer should be happier than a pig in a mud pit.

I have it in my head that the CATs of the M52 are on the manifolds, but you're the guy with a pile of car parts in your living room beside the motorcycle, so I'll defer to you on that one.

I'm not on board with Eric, I think you need to put the O2 sensor in a location where all six cylinders will be sampled. It sounds like you have the skill set to do this, so why bother not doing it?

I see what's happening by looking at realoem. On the OBD I system, the pipes come together in front of the CATs to support the O2 Sensor port, on the OBD II system the pipes don't come together until after the CATs, otherwise they look the same.

I guess the 64-thousand dollar question is if the OBD I O2 sensor will work with only sampling three cylinders. I don't know, but my instinct is no.

[Morgion]

Got this fancy picture here:

Red are the post-cat OBDII heat sensor ports on the 328i front pipes that need to be plugged unless I want a leaky exhaust.

Green is the OBDI O2 sensor location in the X-junction on the 325i front pipe.

Blue is the location where I should weld the H-junction on the 328i front pipes and fabricate the port for the O2 sensor for.

Does that seem like it would work?

I guess the 64-thousand dollar question is if the OBD I O2 sensor will work with only sampling three cylinders. I don't know, but my instinct is no.

Yeah, I don't know anything about O2 sensors (apart from what they do and why), that's why I made this thread in the first place. I know there's got to be an O2 sensor guru here, c'mon in, why don't ya?
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EDIT:
From Wikipedia, the free encyclopedia and savior of men:

The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.

According to that and some common sense, as long as each cylinder is working the exact same way (e.g. cyl #5 isn't running lean for some reason), it doesn't matter whether or not the sensor is measuring exhaust from 1 cylinder, 3 cylinders or all 6. Keeping in mind this is an aging car where the cylinders might not be working in perfect unison (the engine seems okay, but you never know), it should be the best course of action to get the O2 sensor to read all 6 cylinders so that it can measure the average output of the cylinders instead of just half. I mean, that's the way the guys in Bavaria designed it 20 years ago, so let's do what they deemed best. I'll weld a H-junction before the cats and plant the O2 sensor in that. Wish me luck!

[JDStrickland]

I got all of that already. I was thinking of something different, perhaps the exhaust on the E46, I just don't know. I thought the CATs were higher on the pipe in my head, although I don't find the picture to support the image in my brain.

So, yes, you need to find a way to put the O2 sensor from the green circle in the pipe with the blue circle. I think you need to sample both banks else the computer will not be very happy, which means you have to fabricate a green circle where the blue one is.

[Morgion]

Yeah, the H-junction idea with a port for the sensor (I just might go easy mode on this and cut the port from the old exhaust and transplant it on the new one so I don't have to fabricate a new one from scratch). My friend does stuff like this for a living, so he can make sure that there are no leaks and that the welds hold even after the exhaust pipe has heated up. Or maybe I'll just buy him a couple of beers and let him do the welding altogether, I'm not a pro welder by any means and I'd rather only do this once.

[JDStrickland]

I'm all in with the beer idea, buy some beer for somebody that knows how to weld for a living.

Cut the union out of the old pipe and splice it into the new one.