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M42 dual-mass flywheel replacement

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    #1

    M42 dual-mass flywheel replacement

    Hey guys,

    I have a 91 318i that mainly needs to be a reliable DD. As part of the clutch refresh, I have a couple of questions:
    1. Dual-mass vs. conversion to single-mass - I'm leaning towards sticking with the dual-mass since the weight savings are approximately 5lbs (this thread by Simon was very helpful), and with this being my DD and not a track car, I don't think the conversion is as critical. Thoughts?
    2. Resurface or replace - Assuming I stick with the dual-mass, how can I tell how much life is left in the flywheel and if resurfacing will be good enough or if I should replace? As an aside, I'm planning an s52 swap down the line so a new flywheel will probably be replaced as part of the swap anyways.

    Any other considerations?
    Thanks!
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    https://www.facebook.com/Lego.e30
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    #2
    I went from a ~17 lb stock i flywheel to a ~13 lb euro i flywheel on a m20 and it was a very noticeable difference. You could pick up one of the RHD flywheels...they work on M42 and M50.

    AFAIK, you can't resurface dual mass flywheels.

    Comment

      #3
      As e30sh said, you can't resurface dual mass flywheels.

      Also, an M20 flywheel can be lightened down to the 15-17lb range, making the weight savings near the 10lb mark, which is significant.

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        #4
        As said get a rhd lightweight flywheel m20 clutch set up, and good to go. Im planning on doing that in a couple of months. its cheaper than a dualmass stock set up.

        Comment

          #5
          ... and nearly cheaper than acquiring, surfacing and balancing a stock M20 flywheel. If you want save money, look for someone selling a used clutch. I was patient and found a perfect stock clutch for an M20 for $50 shipped. It still had the machine marks on the pressure plate.
          You say "Where are your other two cylinders?"
          I say "Where's your other camshaft?"
          Frankenmotor: if an M42, M44, M20, S50, and S52 were to have a kid.

          Comment

            #6
            I am curious how much the flywheel changes the dynamics. Since my 318is is a track car it would be great to increase its acceleration. I've got a ~10 pounds m20 flywheel and I'm wondering if it would be worth the trouble to swap it.

            Any comments from those who have done the swap?
            318iS Track Rat :nice: www.drive4corners.com
            '86 325iX 3.1 Stroker Turbo '86 S38B36 325

            No one makes this car anymore. The government won't allow them, normal people won't buy them. So it's up to us: the freaks, the weirdos, the informed. To buy them, to appreciate them, and most importantly, to drive them.

            Comment

              #7
              It won't make the car much faster, but will allow the engine to get in to the power band more readily. The downside is that you can have some lurching issues when running A/C (not an issue for you probably), and that the engine spins down from RPM more readily as well (not always a detriment).

              Does it make it feel like an entirely new car, no, but it certainly changes the feel, especially on a stripped out track car.

              Comment

                #8
                The lighter flywheel makes down shifting much easier. It's ironic, I suppose, that this mod, designed to help the car accelerate more quickly, is best experinced while slowing down. That's the M42 for ya.
                You say "Where are your other two cylinders?"
                I say "Where's your other camshaft?"
                Frankenmotor: if an M42, M44, M20, S50, and S52 were to have a kid.

                Comment

                  #9
                  Totally agreed- the blip for shifting is so much quicker and smoother.
                  That applies to pretty much any race engine, btw.

                  It does make flat- shifting less effective, though, as the engine
                  jumps straight to redline, and you don't get the extra energy
                  out of the heavy flywheel.

                  t
                  now, sometimes I just mess with people. It's more entertaining that way. george graves

                  Comment

                    #10
                    Originally posted by roguetoaster View Post
                    ...The downside is that you can have some lurching issues when running A/C...
                    My m42 lurches at low RPM when AC is on. But this is normal? Why does it do this?

                    Comment

                      #11
                      There are trade-offs with the light flywheels. The heavy one was used in the first place to provide a lot of damping for engine vibrations since a 4 cylinder without balance shafts will run a little rough. Also, the lighter flywheel has a smaller moment of inertia, storing less rotational energy, so any kind of impulse or step-increase in torque demand (such as is common from an AC compressor) will eat a much higher percentage of that stored rotational energy before the ECU can adapt to the increased load. I'd imagine that this could be addressed with some tuning, although I am not sure if it would be worth it. A combination of changing the idle speed and some of the response times when AC is on would probably be needed.

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