Which block and head?

[e30tom]

I'm planning on building a motor that uses m52 crank and rods and m50 pistons to get a high compression ration.

I've got a nearly complete m50 and m52. Both motors have about the same mileage.

Which is the better block/head combo?

[PiercedE30]

If you go that route, you won't have any high compression. You will have a 9.5:1 compression ratio, using the m50 pistons and m52 crank actually adds about 3.5cc's per cylinder and the pistons sit 0.653 mm inside the block. This is all using the 135mm rods, M50 vanos pistons, and M52 2.8L crankshaft. Now what you could do is mill the block down by 1 mm, leaving you with the pistons sticking out of the block by 0.347 mm and then just take up some of the slack by using a thicker headgasket and have a high compression motor.
Or, you could take the M52, do a OBD-1 conversion (intake, sensors, etc) and have a killer M52.

[triggrhaapi]

I'd do the M52 stuff with the OBD I sensors and call it a day. There's no high comp situation with your stuff unless you have an S50 or S52 crank.

[e30tom]

http://www.e30tech.com/forum/viewtopic.php?t=25243

This guy talks about M52 crank and rods and M50 pistons getting 11.25:1.

Anyway...here's what I've got on hand:

M52, no cams, needs rebuilt, no wiring.
M50, good shape, complete, ready to run.
S50 cams, lifters, and springs.

What's the best combo for what I have?

[triggrhaapi]

S50 top end, M52 block, rods, pistons. Get software and call it a day. M52 pistons are the same height as the M50 pistons, so choose whichever. The compression ratio stays roughly the same (actually lower) because the rods are 5mm shorter, and the stroke is only increasing by 9mm, resulting in .5mm LESS piston thrust than with the M50. You're the same way off either way. I'm sorry but that guy in the other thread is dumb.

Stock M50 comp ratio: 10.5:1
Stock M52 comp ratio 10.2:1

Sorry.

[PiercedE30]

If you reuse a set of NV M50 pistons, then your pistons will stick out of the block by 5 mm! If you use the M50TU (vanos) pistons, then they sit inside the block, same with the M52 pistons. Bottom line. I have a M50 NV and tried out a M52 crank, and M50TU pistons....blah blah blah. I work at a BMW shop, I wanted to see what worked. None of this did.

[BimmerToad]

Stock M50 comp ratio: 10.5:1
Stock M52 comp ratio 10.2:1

Sorry.

IIRC

Stock M50 = 10:1
Stock M52 = 10.2:1
Stock S50/52US = 10.5:1

[e30tom]

Thanks for all the good info guys, I probably should have started asking questions awhile ago.

So here's my plan:

• Refresh my M50 head and install S50 cams, lifters, and valve springs
• Clean and inspect M52 block
• Clean and inspect M52 crank and rods, polish bearing journals
• New gaskets and seals all around
• New waterpump
• New timing chain tensioner
• Weld air injection port on M52 exhaust manifold

How's that sound so far?

[triggrhaapi]

If you reuse a set of NV M50 pistons, then your pistons will stick out of the block by 5 mm! If you use the M50TU (vanos) pistons, then they sit inside the block, same with the M52 pistons. Bottom line. I have a M50 NV and tried out a M52 crank, and M50TU pistons....blah blah blah. I work at a BMW shop, I wanted to see what worked. None of this did.

Oh, you didn't say they were NV pistons. That may be too high, since the NV uses the same 135mm rods as the M52, so the pistons are like 4 and some change mm taller. That could get ugly, alot of work to get them not to stick up too much.

[e30tom]

Oh, you didn't say they were NV pistons. That may be too high, since the NV uses the same 135mm rods as the M52, so the pistons are like 4 and some change mm taller. That could get ugly, alot of work to get them not to stick up too much.

I've got TU pistons.

[e30tom]

If you go that route, you won't have any high compression. You will have a 9.5:1 compression ratio, using the m50 pistons and m52 crank actually adds about 3.5cc's per cylinder and the pistons sit 0.653 mm inside the block. This is all using the 135mm rods, M50 vanos pistons, and M52 2.8L crankshaft. Now what you could do is mill the block down by 1 mm, leaving you with the pistons sticking out of the block by 0.347 mm and then just take up some of the slack by using a thicker headgasket and have a high compression motor.
Or, you could take the M52, do a OBD-1 conversion (intake, sensors, etc) and have a killer M52.

Can you share where you got these measurements from? I'm not trying to pick a fight, I'm trying to learn.

I found this on Bf.c:

KH Compression height (wrist pin to top)
VT Valve recess depth 1
VT Valve recess depth 1
GL Total length
M52 cast iron:
KH 31,82
VT 0,5
VT 3,8
GL 51,82

M50 vanos pistons:
KH 32,55
VT 0,87
VT 2,24
GL 50,05

The way I understand it, this means that the M50tu pistons have a taller compression height and less valve recess depth. When mounting M50tu pistons on M52 crankshaft and rods, this should increase the compression, correct?:D

This past weekend, I broke down the M50 motor and mounted a m50 piston to the m52 crank and rods, then put it in the m50 block. I measured a deck height of .37mm, not .65mm.

I'm getting conflicting information on this project from a variety of sources. I want to get this sorted out before I spend another dollar.

This project isn't about going uber cheap. I want to build something unique....most importantly...I want to build the engine.

I guess if this doesn't work out, I can massage the head, have the rotating mass lightened up, and bump the compression slightly with a thinner headgasket or decking the block.

And anyway, back to my original question....is there any significant difference between the m50 and m52 blocks that would make one superior?
:D

[PiercedE30]

Actually what I did was to throw stuff together and use a caliper to measure everything.
Now, if you look at the valve recesses and compare the two, which one do you think would hold more water (as in CC'ing the pistons)? The M52 cast pistons. The M50TU pistons have less material cut away, total, for valve recesses than the M52. This leads to higher compression since there is less volume to them.
There is not that much of a difference between the M52 and M50 block. My first block was actually an M50, took it to the machine shop and found out it was cracked, walked across the street to work and grabbed an M52 block and had it machined and I am using it now.

[e30tom]

Actually what I did was to throw stuff together and use a caliper to measure everything.
Now, if you look at the valve recesses and compare the two, which one do you think would hold more water (as in CC'ing the pistons)? The M52 cast pistons. The M50TU pistons have less material cut away, total, for valve recesses than the M52. This leads to higher compression since there is less volume to them.
There is not that much of a difference between the M52 and M50 block. My first block was actually an M50, took it to the machine shop and found out it was cracked, walked across the street to work and grabbed an M52 block and had it machined and I am using it now.

Just so I'm clear: you're saying that all things equal, the m50tu piston would generate a higher c/r due to less valve recess. And I agree with that. Now...given that the m50tu also has a higher compression height (wrist pin to piston face), wouldn't that raise the c/r also?

[PiercedE30]

It will raise it higher than the M52 pistons, yes.

[e30tom]

It will raise it higher than the M52 pistons, yes.

Cool thanks. I think I might assemble everything this weekend and torque it down, then get some proper measurements.