M52 11.25CR Is this real or what?

[jrdeamicis]

So I have this 2.8 Block, 2.8 crank and 2.8 rods. Also I have a set of 2.5 pistons (Vanos)

Some word on the street has been it can be a high compression 2.8. And some say it will drop the CR. Has anybody on here done it or have any evidence to prove me wrong?

The setup (supposedly)

2.8 Crank
2.8 Rods
2.5 (Vanos) Pistons
2.8 Block

Achieved CR 11.25

I do not have enough measurements to really check the math on this.

[Mossman]

Bore is the same on both motors, but m50b25's have 75mm stroke vs. 84 on m52b28's. I believe they both have 135mm rods, so all other things being equal the wrist pin height on m50 pistons should be about 4.5mm lower, right? It would definitely yield higher compression I don't know how many points higher or if it's safe, or if you'll have piston interference with the valves or edge of combustion chamber. Maybe the 4.5mm extra deck height + 12 thou over head gasket would be a better combo? 12 thou is what... .3mm?

[Wh33lhop]

10.5CR of M50TU vs. 10.2CR of M52B28

75/10.5 = 7.14mm effective remainder "stroke" from top of cylinder
84/10.2 = 8.24mm effective remainder "stroke" from top of cylinder

Rods are both 135mm therefore with the same head volume the WP height of M50TU should be (84-75)/2 + (8.24 - 7.14) = 5.60mm lower. That would make your effective remainder stroke 2.64mm, and 84/2.64 = 31.8:1 compression ratio for maaad tyte powahh!

NV pistons would yield a tiny bit more compression than M52 pistons as the 140mm rods move the WP height up 5mm plus 75/10 = 7.50mm for another .36mm (7.14-7.50) higher than the M50TU pistons, so they're 5.60-5.32 = .28mm lower than the M52 pistons.

8.24-.28 = 7.96mm; 84/7.96 = 10.55 CR if you used NV pistons. This is assuming identical head volume and HG thickness between all the motors.

[jrdeamicis]

10.5CR of M50TU vs. 10.2CR of M52B28

75/10.5 = 7.14mm effective remainder "stroke" from top of cylinder
84/10.2 = 8.24mm effective remainder "stroke" from top of cylinder

Rods are both 135mm therefore with the same head volume the WP height of M50TU should be (84-75)/2 + (8.24 - 7.14) = 5.60mm lower. That would make your effective remainder stroke 2.64mm, and 84/2.64 = 31.8:1 compression ratio for maaad tyte powahh!

NV pistons would yield a tiny bit more compression than M52 pistons as the 140mm rods move the WP height up 5mm plus 75/10 = 7.50mm for another .36mm (7.14-7.50) higher than the M50TU pistons, so they're 5.60-5.32 = .28mm lower than the M52 pistons.

8.24-.28 = 7.96mm; 84/7.96 = 10.55 CR if you used NV pistons. This is assuming identical head volume and HG thickness between all the motors.

I have been working too hard lately. I will come back to this tomorrow morning.

[Wh33lhop]

Cliffs:
1) M50TU pistons are a no-go
2) NV pistons will bump compression a bit but probably nothing you'll notice
3) Wait for someone else who knows what they're talking about to chime in before you scrap the plan, there is always the chance I missed something... but probably not ;)

[lennon]

The Non-Vanos m50, s50, s52, and m52 all use 135mm rods. the M50 Vanos uses 140mm rods.

[BenM]

10.5CR of M50TU vs. 10.2CR of M52B28

75/10.5 = 7.14mm effective remainder "stroke" from top of cylinder
84/10.2 = 8.24mm effective remainder "stroke" from top of cylinder

Rods are both 135mm therefore with the same head volume the WP height of M50TU should be (84-75)/2 + (8.24 - 7.14) = 5.60mm lower. That would make your effective remainder stroke 2.64mm, and 84/2.64 = 31.8:1 compression ratio for maaad tyte powahh!

NV pistons would yield a tiny bit more compression than M52 pistons as the 140mm rods move the WP height up 5mm plus 75/10 = 7.50mm for another .36mm (7.14-7.50) higher than the M50TU pistons, so they're 5.60-5.32 = .28mm lower than the M52 pistons.

8.24-.28 = 7.96mm; 84/7.96 = 10.55 CR if you used NV pistons. This is assuming identical head volume and HG thickness between all the motors.

wat.




No, but seriously, like lennon stated M50TU rods are 140mm. The rest of the US motors (M50NV, S50, M52, S52) all use 135mm rods. Its also rumored that the M50NV rods are beefier than the other 135mm rods, but I haven't seen it confirmed.

I know M50TU pistons are flat topped and S52 pistons are dished. Fidhle has also said that M52 pistons are flat topped as well.

The easiest way to find the CR would be to measure the compression height (center of the wrist pin to top of the piston) of both an M50TU and M52 piston and calculate where the piston would sit at TDC with the various crank/rod combinations.

I also believe the M5X/S5X head volume is 33cc.

HTH.

[dave.a.cote]

Ben is correct. The NV rods are huge. NV pistons are dished and M52 pistons are flat.

I am building a 2.9 from a NV block/head combined with a S50US crank, M52B28 pistons and rods. I faced 0.015" off the pistons to get me down to 11:1 CR. If I did not face them I would have been at 11.4:1.

I had a spreadsheet that I made to do all my calcs but my computer took a dump and I am currently trying to retrieve my data.

[Wh33lhop]

Forgive me, I just did a quick google and assumed Mossman was correct re: rod length. With 5mm longer rods, the M50TU wrist pin height would be 5mm higher than I previously thought, so .60mm lower. 8.24-.60 = 7.64mm, and 84/7.64 = 10.99CR with M52 rotating assembly and M50TU pistons. Not bad.

[jrdeamicis]

Well I am getting a set of pistons for 50 bucks. So I will try putting one into the block and check it out.

[jrdeamicis]

Thanks for all the info to. I appreciate it.