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Thread: Euro M50B25 with B28 Crank und B25TU Pistons, please confirm I don't **** this up

  1. #1
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    Euro M50B25 with B28 Crank und B25TU Pistons, please confirm I don't **** this up

    Hey guys,

    I THINK I have all my ducks in a row, but I would like to have confirmation, that my pile of parts is good to go for big boost (low compression).

    As some of you know, we have alloy blocks in our M52's, so we have to source M50 iron blocks and put the M52 crank in there etc.

    So, what I currently have - and I know its not ideal, but there must be a way to figure this out:

    M50B25 Vanos Block, bored to 84,5mm.
    M50B25 TU (!) Wiseco 8.8:1 pistons 84,5mm (33,2mm compression height)

    Complete M52B28 minus the useless alloy block.

    I now need better rods, because the M52 rods are somewhat "weak" compared to H beams or even the stronger M50 rods with ARP bolts.

    So, if my pistons are originally made for 140mm rods (which are 5mm longer than the stock B28 rods) and 75mm stroke, BUT I now have a crank with ~5mm MORE stroke, I should be good to go with 135mm rods, right?
    84mm stroke / 2 = 42mm + 135mm rods + 33,2mm CH = 210,2mm height.

    Or easier:
    "Stock"--37,5mm crank, 140mm rods, 33,2mm pistons = 8.8 CR
    Now-----42,0mm crank, 135mm rods, 33,2mm pistons = 8.3-8.5 CR (? just guessing, what does this 0,5mm do? The Block is decked 1/10mm and I can easily shave the head 2 or 3 tenths)

    Thank you for your help!

  2. #2
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    Iíve run the same pistons with 135mm rods and M52 crank. Compression ratio will be like 10.3:1 with a stock headgasket. The compression ratio is increased because of the larger swept volume.
    Last edited by someguy2800; 05-12-2022 at 06:09 PM.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  3. #3
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    If the pistons are designed for 140mm rods, 75mm crank, the piston dish volume should be around 13.1cc @8.8:1 CR and should be .7mm above the deck.

    with the 135 rods and 84mm crank, the CR is now approximately 9.3:1 and .2mm above deck from my calculations.
    WOT

  4. #4
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    @Someguy:
    I haven't thought about the fact, that there will be more air being compressed, because of the longer stroke, so that makes sense.

    Now I have two completely different answers and don't know what to believe, haha.

    Problem is, we "only" have 102+ octane fuel in Germany, it's very knock resistant, no problem running 30psi on a 66mm turbo, but I would say I need to stay ~9:1 CR. No E85 here.

    I'm shooting for 6-700whp.

    Some other guys want to chime in?

  5. #5
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    Quote Originally Posted by PiepPiep View Post
    Hey guys,

    I THINK I have all my ducks in a row, but I would like to have confirmation, that my pile of parts is good to go for big boost (low compression).

    As some of you know, we have alloy blocks in our M52's, so we have to source M50 iron blocks and put the M52 crank in there etc.

    So, what I currently have - and I know its not ideal, but there must be a way to figure this out:

    M50B25 Vanos Block, bored to 84,5mm.
    M50B25 TU (!) Wiseco 8.8:1 pistons 84,5mm (33,2mm compression height)

    Complete M52B28 minus the useless alloy block.

    I now need better rods, because the M52 rods are somewhat "weak" compared to H beams or even the stronger M50 rods with ARP bolts.

    So, if my pistons are originally made for 140mm rods (which are 5mm longer than the stock B28 rods) and 75mm stroke, BUT I now have a crank with ~5mm MORE stroke, I should be good to go with 135mm rods, right?
    84mm stroke / 2 = 42mm + 135mm rods + 33,2mm CH = 210,2mm height.

    Or easier:
    "Stock"--37,5mm crank, 140mm rods, 33,2mm pistons = 8.8 CR
    Now-----42,0mm crank, 135mm rods, 33,2mm pistons = 8.3-8.5 CR (? just guessing, what does this 0,5mm do? The Block is decked 1/10mm and I can easily shave the head 2 or 3 tenths)

    Thank you for your help!
    Keep on coming with the "useless alloy block" comments!
    Maybe that will keep the cost of them down so I can boost a few more

  6. #6
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    Quote Originally Posted by et89 View Post
    Keep on coming with the "useless alloy block" comments!
    Maybe that will keep the cost of them down so I can boost a few more
    Well my block had a broken tab, where the gearbox bolts to and in Europe (you should also know) we have a lot of information about them and if you absolutely don't want to think about your 800hp 5000€ engine, you use an iron block.
    On a street car, no problem with the weight, different story on a race car, you will love the 50lbs less on the front axle.
    I know a guy who is DAMN quick every weekend on the Nordschleife with his E36 M3 and he says, he carries 2-4kmh more in every corner with an alloy block. (When his iron block engine is being rebuilt, he puts in an M52B28 alloy engine)

    I also know you can even bore out the sleeves in the alloy blocks and use regular pistons, but I could get an iron block bored out to spec with the matching wiseco pistons and machined flat for the price of the pistons alone.
    That's why I'm in that dilemma and need your help.

    If I really end up with 10.3 compression, I could use a spacer, which would be fine.
    If it's 9.3, I THINK I would run it as is.

    But who is right?
    Last edited by PiepPiep; 05-13-2022 at 04:47 AM.

  7. #7
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    Assuming these are KE118M845 right? Note that the specs in the wiseco catalog are wrong, the combustion chamber volume is actually 34cc and the deck height is actually 210mm



    Last edited by someguy2800; 05-13-2022 at 10:37 AM.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  8. #8
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    Hey Perry, thank you for that!

    My pistons are actually WKE315M845, I suspect Wiseco is using a different part number system over here?

    See yourself:
    pistons.jpeg
    piston.jpeg

    1,3070" compression height = 33,2mm

    So, if I put in the numbers, I also end up with ~9.3:1, am I using this right?

    CR.png

    EDIT:
    I could lower the CR with a 2.0mm cutring gasket instead of the 1.75mm and would be ending up at like 9.1 CR, if I put in everything right and I THINK that would be totally fine for my goals and type of fuel.
    Last edited by PiepPiep; 05-14-2022 at 07:02 AM.

  9. #9
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    Quote Originally Posted by PiepPiep View Post
    Hey Perry, thank you for that!

    My pistons are actually WKE315M845, I suspect Wiseco is using a different part number system over here?

    See yourself:
    pistons.jpeg
    piston.jpeg

    1,3070" compression height = 33,2mm

    So, if I put in the numbers, I also end up with ~9.3:1, am I using this right?

    CR.png

    EDIT:
    I could lower the CR with a 2.0mm cutring gasket instead of the 1.75mm and would be ending up at like 9.1 CR, if I put in everything right and I THINK that would be totally fine for my goals and type of fuel.
    Yes that looks correct.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  10. #10
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    Last question:
    I'm pretty much sold on M50 NV rods with ARP bolts (max 700hp/700tq for me), as these are 135mm.

    Now I'm somewhat confused about buying rods, because per example: K1 sells M50B25 and M52B28 rods individually, different parts. BUT same weight, same specs, same width etc.
    Is there something I don't know about? Are the M50 rods 100% compatible with the M52B28 crank? I'm using M50TU Pistons, so I'm just not sure what to buy now, even if I decide on new H-Beams.

    Thank you in advance
    Last edited by PiepPiep; 05-17-2022 at 06:49 AM.

  11. #11
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    All M20, M50, M52, and M54 rods are identical in terms of fitment. The only difference is lengths. There are 130, 135, 140, and 145mm lengths depending on application.

    M50nv do work perfectly with an M52B28 crank and are the same length as M52B28 rods. I have a couple sets of M50nv rods if you need a set.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  12. #12
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    Thank you very much.

    I'm located in Germany, so the heavy rods will propably break the deal with expensive shipping, but thanks for your offer =)

  13. #13
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    Quote Originally Posted by PiepPiep View Post
    Thank you very much.

    I'm located in Germany, so the heavy rods will propably break the deal with expensive shipping, but thanks for your offer =)
    It would be about $65 USD to ship a set to germany.


    86 325es, 2.8L m50, S476sxe, ProEFI 128 ecu, e85, solid rear axle, TH400 trans, 28x10.5w slicks, zip ties, popsicle sticks, tape
    best time 9.06 @ 151.8 mph, best 60 foot 1.30

  14. #14
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    I'll PM you in a bit =)

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