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ChumpCar-spec KA lightweight crank pulley

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  • ChumpCar-spec KA lightweight crank pulley

    I cut the pulleys off the back section, leaving only the front pulley and the damper. I'll shave what remains of the spokes down when I can get the thing spinning on a drill press. Then I'll balance it on a bubble-balancer. I'll need to add some new timing marks when we degree the cams.



    Obviously this requires the one belt mod. This under-drives the alternator a bit, which I like as alternators have been our most common powertrain failure.

    Thoughts? Might I not be able to get it balanced well enough? The modified bit is pretty small in diameter now, so I'm not sure it'd matter.
    Attached Files
    Last edited by gbeaty; 11-30-2013, 02:30 PM.
    Flatout Racing
    #23 Z32 Chumpcar

  • #2
    If you can put the thing on a lathe then I wouldn't worry about the balance too much, but it is cast so there's always the density variable.

    That's why the factory balances them with holes. But honestly, I don't think a few grams per cm out of balance is going to kill a KA when the half-counterweight crank is shaking the rest of it apart pretty well by design.
    '18 Chevrolet Volt - Electric fun hatch for DD duty!


    DefSport Koni Sleeve and Spring Perch Buy!!!
    http://www.nissanroadracing.com/showthread.php?t=5902

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    • #3
      ASP makes an Alu under drive pulley for like $120.. cut off the a/c grooves, paint it black, and run it.
      Originally posted by SoSideways
      I don't care what color they are as long as they are LONG AND HARD.
      '04 G35 Sedan 6MT- The DD
      '96 240SX- The Track Toy

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      • #4
        Originally posted by Matt93SE View Post
        ASP makes an Alu under drive pulley for like $120.. cut off the a/c grooves, paint it black, and run it.
        Yeah, but it has no damper (you can see the stock damper in the above pic, the big thing sitting between the cut up piece and the pulley). Its also illegal, and definitely not worth the chance of getting disqualified.
        Flatout Racing
        #23 Z32 Chumpcar

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        • #5
          For stock RPM range, you don't really NEED a damper. When you start revving above 7000, then you need one. I've been running said ASP pulley for.. 8 years? ~ over 30k track miles and that engine still goes. I don't know of many engines that have lasted that long given the beating I give it regularly.

          ..They can't be THAT bad...

          But then again legal for the class is a different issue.
          Originally posted by SoSideways
          I don't care what color they are as long as they are LONG AND HARD.
          '04 G35 Sedan 6MT- The DD
          '96 240SX- The Track Toy

          Comment


          • #6
            The bad 2nd bending mode vibrations occur above where the crank damper can even damp them. Every crank damper I've seen has been designed more around the 1st bending mode, which is what you encounter much more often in daily driving, and likely rarely encounter on track (it's probably around 3.5-3.7k RPM on a KA).

            Hence why a solid damper doesn't kill an engine after being wrung out on track for 30k mi like above.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!


            DefSport Koni Sleeve and Spring Perch Buy!!!
            http://www.nissanroadracing.com/showthread.php?t=5902

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            • #7
              Maybe attach my scope to a gyro sensor or accelerometer, and smack a piston with a hammer? Run a FFT and see at what frequency it vibrates? Replace the crank pulley and try it again? People do something similar to measure changes in chassis torsional stiffness, except with potholes instead of hammers. The crank may be too heavily damped when sitting still though, due to static friction and no oil pressure.

              Just got through reading this:
              http://www.bhjdynamics.com/downloads...amper_Info.pdf

              As expected, it sounds like a lighter flywheel/clutch assembly raises the first mode frequency a bit. They claim the second mode frequency is usually 1.5-1.8 times the first though.

              Matt, have you ever changed or looked at your bearings?

              I could remove the damper to reduce weight further, but I think I'd only do this if I was very sure it was not helping
              Flatout Racing
              #23 Z32 Chumpcar

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              • #8
                I've never bothered to even pull the pan in this car. My plan was to beat on this engine till it blew, then build up the spare I had in my garage.

                I change the oil and flush the coolant once a season or so (I only get about 40hrs/yr on the car now-- used to get 40hrs/month). but otherwise don't mess with the engine. I check the oil before a race weekend and top it off as needed, and just drive.

                The front main seal is leaking a fair amount at this point, but these ASP pulleys are known for it. it could be due to the crank vibes, but also could just be the aluminum pulley wears faster around the sealing surface and doesn't get as good a seal as it used to. nonetheless, the oil pan is a bit greasy but I just hose it off with engine degreaser whenever I have the belly pan off to change the oil.

                As for reliability though, I've been running this ASP pulley since I bought the car at 90k miles and it's now at about 135k. I put the cage in it at 105k and haven't driven it on the street since. I run the engine near/up to the rev limit on every shift (the S13exh cams in both sides move the power band from ~4500-6700rpm and it's dog slow below 4000), and the damn thing just won't die.

                The car is also getting faster every weekend too.. not sure how or why, but I'm using the same worn out tires and the damn thing just keeps getting faster. It surely can't be my driving since I only get to race once a month anymore- I used to be at the track 4-6hrs/weekend. now I'm in the seat about 1.5hrs/month. _I_ can't be getting faster with that kind of seat time!
                Originally posted by SoSideways
                I don't care what color they are as long as they are LONG AND HARD.
                '04 G35 Sedan 6MT- The DD
                '96 240SX- The Track Toy

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                • #9
                  I'm pretty sure you have to have the engine running to get an accurate torsional vibration profile for it - and like the article mentioned, it's $$$. Keep in mind that not only do you get a power pulse that's not exactly instantaneous, but there's other mass rotating in the engine which reacts to the vibrations as well.

                  IMO, it's pretty clear the OEM damper deals more with first order modes down in the lower RPM range, since people have fitted solid dampers to the engine and they are happy after many many hours on track.

                  There's also the issue of changing the natural frequency of the system with different stuff we do (different inertia clutch/flywheels, different weight accessory pulleys, different weight pistons/rods etc.) - such that an OEM damper might actually be causing a torsional vibration response on its own, and not doing a very good job of damping the actual new natural frequency. There is no free lunch here, the damper puts its own torsional vibration back into the assembly, and if that drifts over time and age (I know most 90's Nissan dampers are in fairly poor shape at this point) or doesn't match up to the new frequency of the assembly - you COULD be shaking things more with it there than without it.
                  '18 Chevrolet Volt - Electric fun hatch for DD duty!


                  DefSport Koni Sleeve and Spring Perch Buy!!!
                  http://www.nissanroadracing.com/showthread.php?t=5902

                  Comment


                  • #10
                    it may not get you the exact same results, but you can do some vibration testing pretty easily with a basic accelerometer and o-scope.

                    strap an accelerometer to the engine block or one of the mounts and stick the car on a steady-state dyno (or even just street driving). record the spectra at various RPM increments throughout the range.

                    You may not be getting readings from the crank itself, but remember that anything the crank does other than perfectly rotate is 'contained' within the block and transferred through the block to the engine mounts/chassis.

                    it's at least worth a look to see where the engine is buzzing in the RPM range. you can also play with adding mass to the crank pulley with just lead weights/duct tape or pouring urethane into the center and letting it harden. it's certainly not going to be as precise as other methods, but it will give you a good idea of the direction you need to go.
                    Originally posted by SoSideways
                    I don't care what color they are as long as they are LONG AND HARD.
                    '04 G35 Sedan 6MT- The DD
                    '96 240SX- The Track Toy

                    Comment

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