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    The M50 Crank 2.8L M20

    Ok so i know im repeating a very covered topic but my question is, what combo/is the normal 2.8 cranks swap good for forced induction? When i say the normal 2.8 crank i mean the combination of... m50 crank, crank spacer, eta rods, and 325i pistons. I saw on a thread that i have since lost that this combo will yield a 9.7.1cr. What i want to know is, is this the best combo i can get for a turbo build? 9.7.1 seems a little high compression. If there a setup that will yield a lower compression ratio?
    Any input is appreciated unless you want to say i should m50 swap.
    The car already is getting a ported and polished head with bimmerheads dual pattern turbo cam, upgraded valve springs and HD rockers.

    #2
    *M52b28 crank, the biggest M50 crank has the same stroke as the M20 so pointless to swap.

    The compression ratio with this setup is 9.3:1 unless you skim the block down 0.5mm, that's the 9.7:1 you would've seen. If you figure the M52 crank has an 84mm stroke, stock is 75mm. That adds up to 9mm shorter stroke, or 4.5mm per side of the crank. This means that the piston will now travel 4.5mm further up in the block. ETA rods are 130mm long, or 5mm shorter than b25 rods. Therefore, the piston sits 0.5mm below the top of the block. It's when you skim the block down that 0.5mm that you bump the compression.

    If you leave the block alone, 9.3:1 compression is still definitely turbo friendly. The biggest gripe people have with this setup is the rod:stroke ratio. I'm no expert, so give this article a read to determine whether or not I'm right http://www.superstreetonline.com/how...-stroke-ratio/

    The factory ratio is 135:75 or 1.8, which is pretty much the accepted ideal ratio. A longer stroke and shorter rod will obviously make this ratio worse, as it's now 130:84 or 1.55:1. It's still okay in my books, but it is a trade off.

    I actually have all the parts for this build myself, so go for it if you're on a budget. A b27 block with an 885 head is (of course) a 2.7L with an 8.5:1 compression ratio. IMHO the compression ratio and displacement of the M52 crank setup are better for a turbo, 9.3:1 is still plenty low enough for boost but between that slightly higher compression and larger displacement you'll spool faster. Whether it would actually be noticeable is another discussion, but on paper it is better.

    Hope this helps, and anyone more knowledgable than me feel free to correct me.
    1990 325i Touring - The Bonsai Bimmer - Patiently Waiting
    1990 325i Sedan - The Blue Turd - Crushed Feb 2, 2016
    1984 316 Coupe - The Base Model - Pain in the Ass
    1986 325e Coupe - 2.7i Project Car - Sold July 18, 2017
    1986 325e Coupe - Currently Fixing
    1985 325e Coupe - Awaiting Resto
    1990 Subaru Sambar - The Daily Driver
    ...And a couple others...

    Comment


      #3
      Yeah that's my current setup im running a bimmerheads built dual pattern cam 885 head and 325e block with new rod bearings and rings. I was wondering if any other crank would produce a different size stroke.

      Comment


        #4
        Samsies

        Dude, I am in the same boat. I have parts for the 2.8 stroker using the M52 crank. Also thanks for the heads up that the block needs a 0.5mm deck for the higher compression. Never caught that before.

        I am torn between building the stroker and then turbo that one day, or doing just a turbo on the stock bottom end.

        I am going to do a rebuild on the engine and convert to Megasquirt regardless along with a AFM to temperature sensor conversion and coil pack conversion. They sound like great projects to wrench on an engine and learn some tuning as that is the last thing I want to do. I also want to do a really hot cam.

        Also I really want to keep it all at 93 octane. I think E85 will eventually go away entirely... And it isn't that prevalent.

        So what should I do "while I'm in there" is the real question.

        If my car is NOT a daily driver will the non-ideal stroke ratio be as big an issue?

        And if I want to turbo proof either build I just need a MLS gasket and ARP studs right?

        Comment


          #5
          Turbo E30 if you already have a strong 2.7i I wouldn’t mess with success and take apart a running engine for only a marginal increase. If your engine is apart and you’re rebuilding it anyway that’s one thing, but I personally wouldn’t redo it.

          One other thing I forgot to mention also is the crank seal spacer required for an M/S5X crank in an M20 block, and you’ll need an S52 crank bolt as an M20 is too short. Metric Mechanic makes a spacer that you can find here: http://metricmechanic.com/engine-parts/ or PPF in Sweden makes one.

          LovelessAndroid if you are planning on rebuilding your engine then absolutely go for it, but it won’t mesh overly well with the “super hot cam” you mention. The stroke:rod ratio problem gets worse the higher you want a motor to rev, so keep that in mind. An MLS is an okay idea, new cut-ring gaskets/cooper ring gaskets/pyramid rings (they have a lot of different names) is a much stronger HG than the Cometic; both are okay though. Let me know if you have any kore questions I can help you with man, and best of luck!
          1990 325i Touring - The Bonsai Bimmer - Patiently Waiting
          1990 325i Sedan - The Blue Turd - Crushed Feb 2, 2016
          1984 316 Coupe - The Base Model - Pain in the Ass
          1986 325e Coupe - 2.7i Project Car - Sold July 18, 2017
          1986 325e Coupe - Currently Fixing
          1985 325e Coupe - Awaiting Resto
          1990 Subaru Sambar - The Daily Driver
          ...And a couple others...

          Comment


            #6
            Thanks

            Stan, thanks for the encouragement and enthusiasm.

            As far as the cam... what in your opinion is the hottest cam I'd want to do with a 2.8 stroker that is also turbo friendly?

            EDIT: I have a lead on a rebuilt M20 head with a Bimmerheads 272 regrind cam already installed... wondering if the true love I have been looking for is right in front of my eyes this whole time.

            EDIT 2: Your thoughts on used ARP head studs? Someone was selling a set. I've heard some builders prefer used studs...
            Last edited by LovelessAndroid; 11-16-2018, 05:13 PM.

            Comment


              #7
              I don’t have any idea about the used vs new studs, I have new ARP head studs in my M20 turbo build. That head sounds pretty good, if it’s a lot more affordable I’d do that for sure. The dual pattern turbo cam mentioned by Turbo E30 is probably the best you’ll find, and it’s still relatively agressive without going too overboard (off the top of my head it’s a 284/280 but correct me if I’m wrong). I know people have seen massive gains on turbo cars with a Bimmerheads ported and rebuilt head with that cam, but if the head you mentioned with the 272 is cheaper and you’re on a budget then that’s still a good upgrade.
              1990 325i Touring - The Bonsai Bimmer - Patiently Waiting
              1990 325i Sedan - The Blue Turd - Crushed Feb 2, 2016
              1984 316 Coupe - The Base Model - Pain in the Ass
              1986 325e Coupe - 2.7i Project Car - Sold July 18, 2017
              1986 325e Coupe - Currently Fixing
              1985 325e Coupe - Awaiting Resto
              1990 Subaru Sambar - The Daily Driver
              ...And a couple others...

              Comment


                #8
                Originally posted by LovelessAndroid View Post
                Stan, thanks for the encouragement and enthusiasm.

                As far as the cam... what in your opinion is the hottest cam I'd want to do with a 2.8 stroker that is also turbo friendly?

                EDIT: I have a lead on a rebuilt M20 head with a Bimmerheads 272 regrind cam already installed... wondering if the true love I have been looking for is right in front of my eyes this whole time.

                EDIT 2: Your thoughts on used ARP head studs? Someone was selling a set. I've heard some builders prefer used studs...
                No competant builder would ever recommend used headstuds. They're torque to yield pieces, meaning they are torqued to the point of no return and deform. I know ARP bolts are not TTY bolts, but they do have a tolerance check to re-use, though the extra few bucks for new shouldn't be a big deal especially if you're committing to turboing.

                Originally posted by stanhayward View Post
                *M52b28 crank, the biggest M50 crank has the same stroke as the M20 so pointless to swap.

                The compression ratio with this setup is 9.3:1 unless you skim the block down 0.5mm, that's the 9.7:1 you would've seen. If you figure the M52 crank has an 84mm stroke, stock is 75mm. That adds up to 9mm shorter stroke, or 4.5mm per side of the crank. This means that the piston will now travel 4.5mm further up in the block. ETA rods are 130mm long, or 5mm shorter than b25 rods. Therefore, the piston sits 0.5mm below the top of the block. It's when you skim the block down that 0.5mm that you bump the compression.

                If you leave the block alone, 9.3:1 compression is still definitely turbo friendly. The biggest gripe people have with this setup is the rod:stroke ratio. I'm no expert, so give this article a read to determine whether or not I'm right http://www.superstreetonline.com/how...-stroke-ratio/

                The factory ratio is 135:75 or 1.8, which is pretty much the accepted ideal ratio. A longer stroke and shorter rod will obviously make this ratio worse, as it's now 130:84 or 1.55:1. It's still okay in my books, but it is a trade off.

                I actually have all the parts for this build myself, so go for it if you're on a budget. A b27 block with an 885 head is (of course) a 2.7L with an 8.5:1 compression ratio. IMHO the compression ratio and displacement of the M52 crank setup are better for a turbo, 9.3:1 is still plenty low enough for boost but between that slightly higher compression and larger displacement you'll spool faster. Whether it would actually be noticeable is another discussion, but on paper it is better.

                Hope this helps, and anyone more knowledgable than me feel free to correct me.
                Thanks for the information. Trying to understand this and wondering if a 135mm rod in this equation is possible with an M52 crank?

                1991 325iS turbo

                Comment


                  #9
                  actually the main gripe with the m52b28 crank is the piston skirts and pin bosses dont clear the counterweight and its borderline only being able to modifying the skirts its a fair bit off them, so most get the counterweights modified. the rod ratio is much ado about nothing in this case.
                  89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                  new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

                  Comment


                    #10
                    I'm with Digger about rod/stroke ratio. BMW didn't seem to have an issue approaching 1.5:1...

                    s54 139:91 = 1.52
                    s52 135:89.6 = 1.51
                    s50 135:86 = 1.57
                    s62 141:89 = 1.58

                    Anything less than 10:1 is fine for turbo IMO.
                    john@m20guru.com
                    Links:
                    Transaction feedback: Here, here and here. Thanks :D

                    Comment


                      #11
                      Most of the writeups say that as long as you have an early Single VANOS crank it should clear without any machining... I really wasn't expecting that to be an issue based on my research.

                      Edit: WITH late model 325i pistons.
                      Last edited by LovelessAndroid; 11-20-2018, 11:26 AM.

                      Comment


                        #12
                        Yeah I don't think those cranks exists in the US and if so they are probably rare.
                        89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                        new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

                        Comment


                          #13
                          Thanks stanhayward, well unfortunately i didn't decide to go with a different crank i did end up doing a lot of engine work... AKA I blew it up. I got the bimmerheads supersport head with the 284/280 cam hd rockers, polished and ported head. Unfortunately I'm having trouble with head gaskets now sealing and now the car is at my local shop that tunes the car. I put in new rod bearings and rings and I will be running AEM's meth injection kit. Hopefully when she's all figured out It will be a 400hp car, wish me luck...

                          Comment


                            #14
                            Any updates on your build TurboE30? I'm currently on a cheap set up, 325is stock everything but with VR headgasket. I'm beginning to think my piston rings are going and am interested in doing a build similar to yours. Just curious as to how much power you ended up making with all the work you had done.

                            Comment

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