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Found the limits of a stock M20B25 Bottom End

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    #46
    Originally posted by ForcedFirebird View Post
    [ATTACH]117150[/ATTACH]
    Interesting, never seen before. Why does the timing shoot to 30 at 3000 and drop to 18, back up to 22, and down to 15, up to 24 and 29. Are you sure that is correct?
    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.

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      #47
      Originally posted by downforce22 View Post
      Interesting, never seen before. Why does the timing shoot to 30 at 3000 and drop to 18, back up to 22, and down to 15, up to 24 and 29. Are you sure that is correct?
      Thats kind of what I was thinking. I can understand a dip at max torque, but not the rest. It seems like the changes in timing might be to flatten out the torque curve?
      - AXIS POWERS - 2JZ E30 Build
      Current Project - 87 325is 2JZ Swapped with M4 DCT

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        #48
        You can see it clearly on a Spec e30 dyno, and if the curve isn't there, you are suspect of cheating with a chip. This is a stock chip read. The WOT tables have 3 variants, but I have always skip traced WOT1 map with the emulator. The Part Throttle tables have 2 axis and are split into upper and lower part throttle. When the TPS is zero resistance (between the switches), the ECU will stay in the better tables, but have far less timing than WOT. This is why a car will be very down on power when the TPS WOT is faulty. Not only is it staying in PT tables with less advance, it uses o2 feedback which is ignored on the WOT map.

        So, a lot of times I will disable the WOT switch and/or change the WOT to match the upper parts of PT table when using Motronic and boost. When I get time later, I will try and post the other tables.
        john@m20guru.com
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          #49
          OK, really should have waited till home, have a race this weekend and still prepping the car, but here you go. Gives the bigger picture.

          [ATTACH]117151[/ATTACH]
          john@m20guru.com
          Links:
          Transaction feedback: Here, here and here. Thanks :D

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            #50
            Mr SleepyDub you are doing the lords work keep it up.

            Get the 2.7 in there and get some dyno numbers before you go 'experimenting' on a street tune at 6800rpm again haha!

            I did some googlin and inertial forces on your rod are going to increase exponentially with your rpm. At a peak cylinder pressure of ~1800psi? and maybe a 8.7sqinch (84.5mm?) piston you are forcing the rod with about 15,000lbs? of force, which you would then do some fancy math based on the length/angle of your stroke and section of the rod to figure out if that is where a cast rod would snap. It doesnt seem like the pin blew out but the actual rod snapped from your photo...

            If you were more handsy than math smart you could take one of your old rods, put it in a test rig at about your almost tdc angle, and then see how many lbs it takes to snap... then work your max cyl pressure backwards from there. Make a youtube video called 'will it bend'.

            Obviously a single detonation could double your cyl pressure, and all rods are not the same... so you would want some kind of safety built in...

            Sorry for nerding out.
            Fun link to some material selection questions:


            The obvious solution here is titanium H-beam rods and bearings so you can run 50psi and figure out where the crank breaks at.

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              #51
              I dont know how much this will help, but lots of guys destroy GM LS motors by doing spark cut only. apparently the reasoning is that there is still unburnt fuel in the cylinders when doing spark cut only and the next cycle that fuel preignites....Nothing destroys rods faster than flame front going down with the cylinder driving up.

              My turbo LS has spark and fuel cut....i do 5500 rpm -15 degree launch to build boost. Its a stick car so its always hard to launch on 28 slicks....I very much try to stay away from bouncing it off the revlimiter. junkyard 5.3. 17psi and 654whp

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                #52
                dont the old L-jetronic effectively use spark cut...?
                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

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                  #53
                  Originally posted by Q5Quint View Post

                  I did some googlin and inertial forces on your rod are going to increase exponentially with your rpm. At a peak cylinder pressure of ~1800psi? and maybe a 8.7sqinch (84.5mm?) piston you are forcing the rod with about 15,000lbs? of force, which you would then do some fancy math based on the length/angle of your stroke and section of the rod to figure out if that is where a cast rod would snap. It doesnt seem like the pin blew out but the actual rod snapped from your photo...
                  I have pretzel-shaped m20 rods from a hydro-lock, they definitely are not cast.
                  Last edited by ForcedFirebird; 09-21-2017, 04:55 AM.
                  john@m20guru.com
                  Links:
                  Transaction feedback: Here, here and here. Thanks :D

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                    #54
                    Cast rods pretty much don't exist. I wish that myth would go away...
                    Build thread

                    Bimmerlabs

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                      #55
                      Manufacturers used to make them cast but probably a long time ago. Still they don't snap in compression
                      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

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                        #56
                        yeah, but that was long before the time of the E30.

                        FWIW, the yield strength, shear modulus and ultimate strength of even mild steel are all way higher than 15,000 PSI.

                        I still think you had piston/valve contact - maybe you got "lucky" and the rockers didn't break (not that it mattered in the end).
                        Build thread

                        Bimmerlabs

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                          #57
                          just fyi maybe this might be interesting maybe not.

                          i'm breaking in an m30 still at 7psi on 91 octane.
                          Hardcore freeway runs at about 30*, no pinging.
                          Assuming m20/m30 is the same when it comes to timing needs.

                          like downforce222, 23 advance @ 30psi when the pistons cracked on meth injection with the old m30.
                          maybe interesting maybe not. the old m20 I used the same settings and it blue a rod. but with heavy abuse.
                          Last edited by LowR3V'in; 10-26-2017, 08:58 PM.

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                            #58
                            They are similar, but the M30 has a bigger bore, bigger combustion chamber - and thus needs more timing than an M20 would for similar conditions.
                            Build thread

                            Bimmerlabs

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                              #59
                              Goes to show you how efficient the m20 is compared to m30. MBT m20 numbers are less than 30 for the most part.
                              john@m20guru.com
                              Links:
                              Transaction feedback: Here, here and here. Thanks :D

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                                #60
                                Originally posted by digger View Post
                                Manufacturers used to make them cast but probably a long time ago. Still they don't snap in compression
                                Powder forge does, though. Seen enough carnage in my lifetime to make an accusation...
                                john@m20guru.com
                                Links:
                                Transaction feedback: Here, here and here. Thanks :D

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