M50 block/head compatibility and OBDII

[Beej '86 325es]

Hate to beat a dead horse with another M50 question, but I searched and cannot find anything on this issue.

Is there anything physical about an M50 engine that makes it OBDII, or is it all software and wiring? I ask because I was wondering if an aluminum 2.8 block (from an OBDII car right?) would work with a 2.5 head. I'd like a vanos motor that is all OBDI to start with and not have to worry about converting it at all, but didn't know if the block had anything to do with that. I just thought if I could get ahold of an aluminum block, without converting from OBDII, then that would be pretty nice.

Am I clear on what I'm asking? Because I'm confused.

[Jon325i]

Re: M50 block/head compatibility and OBDII

Hate to beat a dead horse with another M50 question, but I searched and cannot find anything on this issue.

Is there anything physical about an M50 engine that makes it OBDII, or is it all software and wiring? I ask because I was wondering if an aluminum 2.8 block (from an OBDII car right?) would work with a 2.5 head. I'd like a vanos motor that is all OBDI to start with and not have to worry about converting it at all, but didn't know if the block had anything to do with that. I just thought if I could get ahold of an aluminum block, without converting from OBDII, then that would be pretty nice.

Am I clear on what I'm asking? Because I'm confused.

It is my understanding that you can take an OBD II block (the 2.8L M52 for example) and top it off with a OBD I head.....either an M50 head or S50 head. The head bolt pattern, the coolant passages, etc are identical on OBD I and OBD II motors, so heads and blocks should be interchangable.

For your E30, it would be more cost-effective to go with the OBD I head since you won't be required to convert anything over to make it work with your existing wiring harness. Another plus with this option is being able to comply with local smog regulations.....something not very easy to do out here in Cali. If maintaining vehicle balance is your goal, getting the aluminum block M52 from a Z3 and top it off with an OBD I head would give you a very impressive motor while leaving your E30s handling characteristics largely unaffected.

Hope this clear things up.

Jon

[Beej '86 325es]

Thank you that is very helpful. Since the aluminum block is 2.8 liter, would that throw off the compression ratio if I used a 2.5 head? I found a cheap aluminum bottom end and though I would take advantage for when I do the swap in the distant future.

So all of the OBD II stuff happens in the head, I assume with all of the sensors and intake/fuel wiring, right?

Thanks.

[Stu]

You should go for an M3 head if you're going to start with a 2.8 bottomend. Try to get a '95 M3 head (bolts right on the 2.8 block). You will need custom software for that setup though. It would be weaksauce to slap a 325 head on the 84x84 'square' bottomend; the power potential with the M3 head, large MAF sensor, etc will be so great.

Use a full 2.8 aluminum bottomend and the compression will be around 10.5:1 (stock).

[JasonH]

So all of the OBD II stuff happens in the head, I assume with all of the sensors and intake/fuel wiring, right?
Thanks.

This is close, but not entirely correct. There are a few block mounted sensors, but that is beside the point.

If you have an M/S52 bottom end and a M50 head readily available, go ahead and use them. It will save you the trouble of having to drill and tap the head for the second temp sensor (unless you solve this issue with wiring). Otherwise, if that alum. block you referred to already has the M52 head on it, just leave it alone. Swapping everything over to OBD-1 was not very difficult at all.

[Beej '86 325es]

It would just be the 2.8 block only that I would buy. I thought switching from OBD II to I was supposed to be really difficult. I mean Zionsville charges like $1500 for the parts to switch it.

You should go for an M3 head if you're going to start with a 2.8 bottomend. Try to get a '95 M3 head (bolts right on the 2.8 block). You will need custom software for that setup though. It would be weaksauce to slap a 325 head on the 84x84 'square' bottomend; the power potential with the M3 head, large MAF sensor, etc will be so great.

I currently have a TMS chip in my eta, I assume I could have this one modified by someone? Or does the ECU need to be replaced with an M50 unit and modify the stock chip?

Sorry for the stupid newbie-type questions. I do almost all of the work on my car, but have never done any major engine work before on any car. Having limited space to store parts and knowing only a few other people who work on cars, I like to know exactly what I'm getting into before I start buying tons of parts. :) Thanks.

[arnib]

Or does the ECU need to be replaced with an M50 unit

Yes

[Beej '86 325es]

Is an S50 head, an S50 head, or are there and different variations? To my understanding the S50 is the single vanos, OBD I.
Thanks.

[Beej '86 325es]

Bump. Is there an S50 OBD II head?