difference between s50 and s52 downpipe?

[hugh jass]

i'm trying to figure out what downpipe i need to run my obd1 s52. is there a preferred one becasue of o2 sensor ports, or does it matter? is a custom downpipe the best way to go?

[Mike B.]

The E36/M3 OBD-1 mid-pipes work well, and don't require much fabbing to get them to clear the sub-frame to mate up with the S52 headers.

Or, you can have something fabbed up with an x- or y-pipe and have an O2 sensor bung welded into the cross-over area.

As long as the O2 sensor is monitoring both banks you'll be fine.

[FredK]

What if you only monitor one bank? Will the world end? Just asking because the exhaust is still on my to-do list, and I'd rather not have to weld in an x-pipe if I can avoid it.

[Mike B.]

It don't think it would be the end of the world, but monitoring both banks mimics the stock setup.

[hugh jass]

So are the standard E36 325 and s50 M3 downpipes the same?

[Mike B.]

No, they have different part #'s. The look almost identical, but i'm sure the ID's are different.

[1990m3]

you will get a more accurate reading if you monitor both banks and not just one of them.

FYI - if it's important to you or not, but i have an OBDI m3 midpipe and muffler and I just passed emissions last week without an issue. It's required in my area so I went after an exhaust that is quiet, efficient, and heavy. That's right, it's heavy but what can you do. It works and I don't have to worry about the state kicking my ass for not passing emissions.

[C ///M]

Some clarification:

I used to run OBDII exhaust manifolds on my OBDI s52, mated to the stock OBDII center section. My o2 sensor only monitored half the exhaust flow (as it was inserted into one exhaust manifold.

I now run headers mated to an x-pipe with an o2 bung and then the OBDII center section. No real difference from an o2 sensor standpoint.

Having the o2 sensor only measuring 1 bank isn't a big deal. It basically gets the same readings as it would monitoring all the exhaust, just lower resolution.

[grib]

This is the best comparison I have seen of the two down pipe/cat sections. It's from Rogue Engineering.

http://www.rogueengineering.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=CATS

Both have dual cats (one per bank) and I believe they will both mate up to either OBD-1/2 manifolds. The obvious difference is the merge point in the OBD-1 manifolds for the single O2 sensor. As the analysis on Rogue's page indicates, the bottleneck in the OBD-1 cat is this merge point. I have read in several places that an exhaust shop can open this section up and it will flow much better - eliminating the bottle neck. Additionally the OBD-2 pipes are double-walled, so they will be more difficult to work if you want custom exhaust work done.

I am also doing a full OBD swap on an S52. I went with the OBD-1 cat.