M50b25TU head volume

[RetroBeamerBoy]

Hi, I am building M50b32 stroker and want to calculate my compression ratio. Has anyone volume tested m50tu head? I saw numbers varying from 32 to 35cc, so I would appreciate if someone knows this for certain. Thanks in advance.

[dominator293]

Check out this champs video. Very helpful if you have the head off the car: https://www.facebook.com/49667501710...41128802594365

Edit: Also important to note that every M50B25TU heads might have difference cc's dependant on how many times the head has been decked.

[RetroBeamerBoy]

Thanks, the head was never decked yet, sitting at machine shop now while I wait for new pistons. I will have to measure it, I guess. From what I could find, undecked head is around 35cc. Weird how some people get numbers like 32cc on undecked head, I cant see such variance as logical.

[importbanana]

Can't it be calculated? You know how much displacement the engine has per cylinder, and if the compression ratio is known for the motor, you should be able to calculate the combustion chamber volume.

[RetroBeamerBoy]

It can, but it is well known that actual compression ratio advertised by BMW is higher than it actually is. For example M54b30 has advertised CR of 10.2, but in reality it is only around 9.8. It appears that BMW includes combustion chamber carboning up in their calculations, thus using smaller combustion chamber volume than actually is.

[spyderg0d]

My tub30 came to roughly 10.9 to 1.

Maybe the m50 head has a larger dome than m54?

[RetroBeamerBoy]

Was that tub30 using m50tu block and head with m54 internals? With no machining it should give 10-10.3 cr. Variance is due to no one knowing exact m50tu combustion chamber cc

[spyderg0d]

34cc of tu head. I've confirmed myself on several heads I had laying around at the time. But m54 pistons have large reliefs. account for crushed headgasket as well.

[RetroBeamerBoy]

Nice, so I will take the number as 34cc in calculating my cr. M54 have large valve reliefs? From what I saw they are quite small, less than 1mm depth if using oem pistons. I will use forged oversize m54 86.5 bore pistons with deeper valve pockets of around 2mm.
I think you are mistaken about m54 valve pockets being large, here is a comparison I found:
https://cliosport.net/attachments/im...6-jpg.1169182/

[spyderg0d]

B30 had larger reliefs than 2.5 pistons I believe after a certain production point.
Maybe I got lucky and got a set of high comp on accident. My reliefs measures 3mm and caves into the edge of the piston. Just like these.
https://images.app.*******/TiH4YqXm9miKpDWq5

- - - Updated - - -

Here's my actual. I'm pretty sure they are direct replacements tho. As you an see.. they are much more than 1mm deep. 20201007_191634.jpg

[spyderg0d]

It's weird at the comparison pic. The m54 should have the chamfered edge like the m52 pistons in that pic. They may have them miscategorized. But then again. I do remember reading on piston changes in the 3l as production went on, also along with piston ring and oil ring design changes.

[Greenday694]

This Thread had me curious about the differences. These Pistons are from 2003 M54 B30

Sent from my VS995 using Tapatalk

[spyderg0d]

Well.. it would seem I did luck out and got high comp spec pistons or a forged set on accident from the supplier.. another blessing I suppose as I only paid I think $350 for the set. .. huh.

[RetroBeamerBoy]

Yeah, I think you got non standard pistons. 2003 m54b30 pistons I have laying around are similar to what Greenday has shown.

[Greenday694]

These had the beveled crown, and coated side skirts. With the 140 head gasket, in the M50 block, CR should be in the low 9s.

Sent from my VS995 using Tapatalk

[RetroBeamerBoy]

As we are on CR subject I want to ask if anyone knows or ever wondered why M50 has 10:1 CR and M50Tu has 10.5:1 (at least according to BMW) M50 pistons clear 0.7mm above deck and m50tu is flush with deck. Also it is generally accepted that m50 has smaller head chamber than vanos heads. That would make non vanos higher cr, but BMW states othervise. So is there something I'm missing, or are advertised CR's total bs?

[spyderg0d]

Tu has longer rods to specifically up the compression ratio.

[RetroBeamerBoy]

non vanos m50 - 135 rod+ 38.2 compression height = 173.2. Vanos m50- 140rod +32.55 comp height = 172.55. same crank. so even with longer rods the tu piston sits lower.

[spyderg0d]

You didnt add in the different piston height difference from the pin. I think the piston was higher with a longer skirt as well.. not that skirt matters here.

- - - Updated - - -

Oh.. sry its early.

- - - Updated - - -

Screenshot_20201203-083454_Chrome.jpg

[spyderg0d]

Well.. this turned out good for my learning. Lol

- - - Updated - - -

It's got to be the dome in the nv head is larger or to do with the piston sitting higher at bottom dead center.

[RetroBeamerBoy]

By the way, I retrieved from machine shop and measured M50B25TU head. It is 34cc.