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  • crankshaft balancing?

    hi

    have a question that I have not really got a good answer

    I am a new evo 3 crankshaft
    and other pistons (lighter + - 430 g)
    connecting rods and lighter (600 grams)
    8000 rpm and would make

    I must now again to balance my crank
    on the weight of what the pistons and connecting rods are ?




    regards dave

  • #2
    Hi Dave. It's normally unnecessary for a street engine, or even a track engine that sees street time. Adjusting bob weights to match rods and pistons is usually reserved for race engines with a narrow RPM range in mind. This is because specific balance to the nth degree focuses vibrations, in other words it will be better in one area while worse in another. An engine which will see its full RPM range on a regular basis needs a more bland balance to maintain acceptable vibration levels throughout the entire RPM range.

    FWIW, I will not go adjusting bob weights on my street 2.5. Stock crank with Arrow rods, JE FSR pistons and a red line around 8200.

    Jake

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    • #3
      I have a differing opinion on balancing. The single best thing you can do to a motor is to balance it. Balance both the reciprocating and rotating assemblies.. the cost is minimal and well worth it. Think of it this way... an out of balance engine continuosly fights it's own rotating mass. Balancing affects the engine in a positive way in ALL rpm ranges. Harmonics are handled by the active balancer on the end of the crankshaft.

      FWIW I balance EVERY engine I build regardless if it is one of our championship winning race motors or my lowly street car. The cost isn;t even a noticeable bump in price. My S52 revs can quickly and smoothly to 8500 rpm but it is good to 9k. Currently it is rev limited to 8k rpm...

      The S54 that we are running in my E46 touring car will run 9500 but we'll keep it around 8500 for longevity. I prefer headroom with respect to revability etc....
      Last edited by vmwerks; 06-24-2012, 08:11 PM.
      ________________
      2010 335d
      1999 E46 U.S. Touring Car
      1988 E30M3 S52

      sigpic

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      • #4
        We're talking about rotating/reciprocating balancing. Rotating assembly only balancing is the norm (crank, flywheel, damper & clutch).

        Jake
        Last edited by Jake; 06-25-2012, 12:18 AM.

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        • #5
          We balance the reciprocating assembly on all engines. This goes for street or track motors. The crank is balanced from the factory using the stock bearings, rings, rods, and pistons. When you change the rotating mass, you should rebalance the crank to match the new mass. The engine runs smoother at all rpms since the counterweights on the crank match the rotating mass of the rods and pistons. Also, it's one of the cheapest mods you can do on a new build.
          Several E30 M3's.

          More than I need but not as many as I want....

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          • #6
            hi
            oke thanks


            thats what want to no

            greetings dave

            Comment


            • #7
              Can ya'll ellaborate on the process you use to balance the bottom end?

              Jake

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              • #8
                This explained a few things to me I was curious about.

                1988 E30 M3 S38
                1970 E10 2002 S14

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                • #9
                  Good article on engine balancing............

                  http://www.eatonbalancing.com/blog/category/balancing/

                  Comment


                  • #10
                    I think what Jake is getting at is,

                    How does one alter the crankshaft counter weights to take in to account the change in Rod/Piston assembly mass?

                    Correct me if I'm wrong Jake.

                    This is different to dynamically balancing a crank rotating assembly or Rod/Piston reciprocating assembly as it deals with the interaction of both.

                    Steve
                    Sport Evo No.47

                    My Sport Evo Restoration

                    Comment


                    • #11
                      Originally posted by xworks View Post
                      Good article on engine balancing............

                      http://www.eatonbalancing.com/blog/category/balancing/

                      Very good article.
                      Rich!

                      Comment


                      • #12
                        Correct Steve.

                        Usually people balance the crank, flywheel and damper pulley only. Sometimes the clutch housing is included. Simple, cheap, effective. Rarely are cylinder reciprocating assembly weights determined, calculated to whatever percentage you use (100% big end weight plus 46 - 54% of the small end/piston weight), bob weights added and THEN balanced. That's why I asked for some elaboration. If folks say they always do the whole kit-and-caboodle for all their engines, street or track, well then I stand corrected.

                        Great video Josh showing a full rotating and reciprocating balance. The one below is just the crank.

                        http://www.youtube.com/watch?v=y60dTiuJv24


                        Jake

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                        • #13
                          This explains it better than I can. Some engines are externally balanced, meaning the external parts are used to balance the crank and rotating assembly, think harmonic balancer. BMW and other engine builders use internally balanced assemblies which don't need an external balancer. The pulley is just that, a pulley. This is why you don't have to rebalance the engine when using a different front pulley. As long as the rotating mass is balanced, you can just put it on and go.

                          http://www.fordmuscle.com/archives/2...atingAssembly/


                          If the crank and reciprocating masses are balanced, then you don't really need to balance the clutch and pulley assemblies since they are balanced by themselves. As long as all the individual external parts are balanced, you only need to be concerned about the crank assembly.
                          Several E30 M3's.

                          More than I need but not as many as I want....

                          Comment


                          • #14
                            Jake,

                            From the Eaton article:
                            "Most engines with opposed or inline cylinders (i.e.. 4 & 6 cylinders) normally do not require a bobweight to be attached to the crankshaft rod journals prior to spin balancing in order to simulate any of the component pieces (rods, pistons, etc.) that are normally attached to the crankshaft assembly. This is due to the nature of the physical forces being applied at equally spaced intervals on these engines and subsequently being equally opposed or counteracting."

                            Peter

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                            • #15
                              vmwerks/inastrangeland,

                              Do you guys go all the way when you balance? I'm under the impression that for our engine most do not weigh each component and add bobweights as descibed above, just spin balance the crank/flywheel/damper.

                              Jake

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