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Thread: Group 5 E21 with M50b25 turbo - forged crank question

  1. #1
    Join Date
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    2012 1M, 1977 633, E21

    Group 5 E21 with M50b25 turbo - forged crank question

    Hi Guys and Gals

    I'm building a slightly mad E21 Group 5 homage and have based the powerplant on an E34 M50b25 engine which has done a million kilometers. Engine has yet to be built, but is dry sumped and will have all of the usual upgraded internals. Turbo is a custom Holset 35 from my mate Steve Murch and the setup should provide around 600 flywheel hp and 500ft/lbs of torque (circa 700nm). We have a locally built 5 speed sequential etc... etc... Car is a pure track car and with the alloy body I'm making, it should be only about 1000kg.

    I want the thing to sound nasty and to rev. We have a Rama RHD 2.6kg flywheel and a twin plate Tilton clutch with an all up weight of just under 6kgs.

    I'm hopeful of getting the thing to rev and breath up to 8000 rpm. Head will receive whatever it needs to support this.

    Somebody has been in my ear about possible harmonic issues with running such a light clutch and flywheel and has suggested as we all know, that running a forged crank would be a better thing.

    Now I know that the M52b28 has a forged crank, but moving from a 75mm stroke and a 140 rod takes me away from the ideal 1.7 or greater rod stoke ratio. I'm told that piston dwell at TDC is a good thing for turbo engines. The 75x140 yields a ratio of 1.86.

    So my learned BMW chums, is there a forged option for a 75mm stoke crank? Research so far seems to indicate that it's only the 2.8, 3.0 and the 2.4 diesel.

    Thanks in anticipation.

    Full build thread to date at https://www.facebook.com/Elan-Motors...2026323873874/

    20190216_171626.jpg20190219_193304.jpg20150425_123917.jpg

    Cheers, Richard

  2. #2
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    I believe the m54b25 crank is forged. Might be worth looking into.

  3. #3
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    As far as I am aware all the 75mm stroke cranks are cast. I don't know anything about the diesel cranks. For what its worth I use an M54B28 crank and shift at 8300 rpms with a ported non vanos M50 head with completely stock valvetrain. Its hard on the oil pump and damper hub but have not had any issue with anything else. I just got an ATI damper which hopefully will take the stress off the front of the crank.

    Also just want to add that the dwell difference with different rod stroke ratios is complete bullshit. Someone made that up without doing the math and its been falsely tossed around on the internet ever since. I'm copying the below of my response to a discussion we had on the BMW facebook group last week.

    You can prove this mathematically if you actually draw it out in solidworks or whatever. The truth is there is almost no difference in piston position between a
    long and short rod near TDC at all. The biggest difference is actually in the middle of the stroke and the differential tapers off at TDC and BDC. For example if you take an 84mm stroke and compare a 135 and 140mm rod, the difference in piston position at 10 degrees ATDC is 7 thousands of a MM or .00027", (yes the decimal places are correct, that is 3 ten-thousandths of an inch). At 20 degrees it is 1.1 thousands of an inch difference in piston position, and at 90 it is 9.8 thousands of an inch. I have seen 2 different test at PRI presentation about this and in both cases there was no measurable difference when tested on an actual engine on a dyno.



    The other thing that gets brought up on rod stroke ratio is the piston thrust. Since I had this drawn up I decided to check the rod angularity relative to the bore to see how much change in thrust load there would be. The maximum angularity would be at 90 deg ATDC and would be 18.13 degrees for the 135mm rod and 17.46 degrees for a 140mm rod. At 20 degrees ATDC which is around where peak cylinder pressure is the thrust angle is 6.11 degrees for the 135 rod and 5.89 for the 140.







    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

  4. #4
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    Many thanks E36 - will do, although not sure what you need to do (if anything) regarding the internal crank position wheel on the later cranks.

  5. #5
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    Thanks for the detailed response Someguy. Presume you are running 135 rods in your 2.8 and that you have your M54 crank in an M50 block? Did you do anything special about the internal crank position wheel on the M54 crank?

  6. #6
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    Quote Originally Posted by Skintkarter View Post
    Thanks for the detailed response Someguy. Presume you are running 135 rods in your 2.8 and that you have your M54 crank in an M50 block? Did you do anything special about the internal crank position wheel on the M54 crank?
    Yes 135mm rods and m50 block. I ran it for years with a set of non vanos m50 rods but now have a set of eagles in it. I took the timing wheel off the crank and run it with an m50 balancer and crank sensor. The internal timing wheel setup on the m52 is a superior setup but I just run what I have since my blocks don’t have the provision for it. For my next motor I’m considering just drilling and tapping the block for a thread in sensor to use the internal timing wheel.

    Also just an FYI the m54b28 is identical fitment as the m52b28 crank. I just use the m54 version as it’s a bit heavier of a forging with more material in the rod throws. The m52 cranks work fine and are easier to find.


    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

  7. #7
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    Thanks for that Someguy. I'll try to find a junked M52b28 engine and pillage the crank. Presume you are running quite large horsepower with your setup? We are only looking for around 600 flywheel, so an easy task I'm told.

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