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Engine vibration

Discussion in 'Performance' started by kzooman83, May 28, 2016.

  1. kzooman83

    kzooman83 Active Member

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    Hello, everyone.

    I am having an issue with engine vibration in my big block duster and I am trying to find the cause. The specs are:
    -'77 400 big block bored .030" over with Keith Black hypereutectic pistons
    -9:1 compression
    -Summit 282/292 cam (224/234 @ .050) with .465/.488" lift and 114 lsa
    -Edelbrock Performer RPM intake manifold
    -Holley 750 vacuum secondary carb
    -Mopar performance ignition
    -windage tray
    -Harmonic balancer and torque converter/flexplate are stock

    I guess the vibration has been with me for a little while, but it got much worse when I put the new polyurethane trans mount in last week. I thought it was driveline at first, but it does it when the car is in park or neutral also. The vibration is present at all engine speeds, although it varies in intensity. It is particularly bad from about 1200 rpm to about 1800 rpm. I have checked my engine mounts and they are tight and not missing any bolts, and I took the accessory belt off to rule out the water pump and alternator and the vibration is still there. I also checked my ignition timing and the timing is still where I set it so the harmonic balancer does not appear to have slipped.

    Could it be the torque converter/flexplate? Or could the damper be faulty even though the timing marks have not moved? It feels like the time the water pump bearing went out on my truck, but like I said, it still does it when I remove the belt. Any thoughts? Is there any way to rule out either the torque converter or the harmonic balancer? Thanks!
     
  2. pt006

    pt006 Active Member

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    kzoo; your engine came thru with a cast iron crank which is externally balanced. The harmonic balancer will have an offset weight either added or removed. It might say 'cast crank only'. This is visible from the front of the engine. The weight of the factory pistons is 768.5 gms. Factory pins are 1.0936 Dia. These are 1976 specs. Probably the same as for 1977. I'm not sure about this, but your torque converter should have 2 weights welded to it.

    I like the combination you put together.
     
  3. ImperialCrown

    Level III Supporter

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    I have diagnosed engine/driveline/wheel vibration sources by using an analyzer in class and in the field.
    For engines, it looks at the speed of the crankshaft and the frequency of the vibration. By looking at the various diameters of the spinning masses and ratios along with some simple math, one can usually find what the possibilities are:
    https://mopar.snapon.com/item-detail.aspx?itemid=83930390&type=equipment
    If you removed the belts, you have eliminated the accessories as a cause. A good place to start.
    I would begin suspecting imbalance in the crank/flexplate/torque converter area as these are fairly heavy masses with 1:1 ratios.
    Do the piston/connecting rod weights match what the crankshaft counterweights are 'tuned' to cancel out?
    Flexplates can crack and get bent and counterweights can fall off. Diagnose first.
     
  4. kzooman83

    kzooman83 Active Member

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    Thanks for the replies. The Keith Black pistons I went with are supposed to weigh 660 grams each and the pins weigh 190 grams. I'm not sure if the 768 grams listed above is for just the piston or the piston and pin, but that would be either a difference of 108 grams or 82 grams. Is that enough to cause a balance issue? Also, I am running polyurethane motor mounts and trans mount; I know these can transmit more vibration, but this still seems excessive. Can I rule out the torque converter by removing the inspection cover and unbolting it from the flexplate and then running the engine to see if the vibration is still present?
     
  5. AC TC

    AC TC Well-Known Member

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    If you can slide it back enough to unseat its guide from the crank- yes.
    But it still leaves you with a flexplate.
    Another source of vibes is the connection between engine and tranny, is it straight and true?
    Are the dowelpins in place?
    Getting the engine to line up correctly to the tranny is one key thing when it comes to a smooth and vibration free driveline, its also very important
    for the tranny inputshaft bearings.
     
  6. pt006

    pt006 Active Member

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    kzoo; your engine is way out of balance. I'd call keith black and see if they have a tech line. They might say it is easier to make your engine into an internally balanced engine??? [by using neutral balance harmonic balancer and/or converter]. Or, going back to factory weight pistons.

    As far as unbolting the torque converter from the flex plate to test for vibration, sure. But finding a heavy duty rope pull starter would be the tricky part!
     
  7. ImperialCrown

    Level III Supporter

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    Are both of your bellhousing-to-block locating dowels in place?
     
  8. kzooman83

    kzooman83 Active Member

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    The trans is straight and the dowel pins are in place; we bolted the engine and trans together while they were still out of the car and dropped them in already bolted together. I will have to see if I can find the machine shop receipt from '07 when I built the engine, but I don't believe I had the machine shop balance the rotating assembly. I know now that I should have requested it, but this was the first engine I ever built and I was learning. The only way to balance it at this point would be to pull and disassemble the engine, correct?
     
  9. kzooman83

    kzooman83 Active Member

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    The 400 is an external balance engine; my understanding is that with this type of engine the counterweights on the crank are not heavy enough to totally balance the rods and pistons, so weight is added to the balancer and torque converter to compensate. Am I correct?
     
  10. dana44

    Ad-Free Member

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    Well, being a 400, you can make it an internally balanced engine easy enough because it is the same crank as the 383, most of which were steel cranks, a matching harmonic balancer and removal of the counterweights on the torque converter, then using the bobweight of the rod/piston/pin/bearings/rings and everything is fixed in this case. You can also go with a stroker kit which balances everything as a unit, but that also means new pistons and a bunch more money. Either way it is not going to be an inexpensive fix.
     
  11. kzooman83

    kzooman83 Active Member

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    I have actually had my eye on the stroker kits from 440source for a few years, but I don't have the $2500 right now to do that. I think I am going to put a rubber trans mount back on the car to see if that kills some of the vibration; the engine has never been buttery smooth, but it has never vibrated excessively until now. Maybe I broke something, or maybe it is just out of balance enough that the stiffer poly trans mount is a problem.

    Also, I contacted the machine shop and the rotating assembly was not balanced, due to the fact that all the original pieces were used except for the pistons, which are "stock replacement type."
     
  12. dana44

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    If it is something that has just started, yeah, I've had mounts break and do that and not really know there was a broken mount to begin with. Might be worth a try to swap the mounts out or at least check that they aren't broken, a prybar to see if the engine can be moved.
     
  13. kzooman83

    kzooman83 Active Member

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    I actually had one of the bolts on the passenger side mount loosen up and exit the premises last summer and it did cause a vibration similar to what I am feeling now; I visually inspected all of them and everything appears to be tight with no missing bolts this time around. Like I said, I think I will go back to a rubber trans mount and see if that helps. If not, I will know for sure that something is wrong. This really started right after I installed the polyurethane mount.
     
  14. dana44

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    So there is actually a good chance you stiffened things up and simply feel vibration more, something the polyurethane mounts are known to do. If breaking mounts is the main issue to change them, I always prefer a chain to go around the mount and bolt on each end, or, since there is clearance, drill a half inch hole and a large washer on each side of the bolt, nylock type nut to limit movement so the two metal shells can still allow what the rubber in between the shells do their job, but they don't come apart.
     
  15. hemirunner426

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    awesome
     
    AC TC likes this.
  16. kzooman83

    kzooman83 Active Member

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    I got over to the garage and put a new rubber trans mount on the car today. The excessive vibration is gone and it feels like it did before again. So, lesson learned I guess; if you're going to use a poly trans mount, make sure your engine and driveline are VERY smooth. BTW, does anyone know who makes a high quality replacement trans mount? The one I purchased works fine, but it is really cheap: the metal is much thinner than on the factory mount and I had to use a thinner bolt because the original bolt was too fat.
     
  17. AC TC

    AC TC Well-Known Member

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    Try a dealer, im sure they still carry em.
     
  18. Meester Beeg

    Meester Beeg Active Member

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    Since the harmonic balancer is stock-I assume original it may be bad. The rubber ring inside may be dried out and not dampening as when new, or the outer ring could have rotated some causing the entire rotating mass to be unbalanced.
     
  19. dana44

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    I did see a really cool way to verify where the TDC mark should be in order to verify the balancer is still good, or intact, not slipped. Use your basic TDC bolt though the sparkplug hole, roll the engine over (by hand) until the piston touches the TDC bolt, mark where the zero point is on the harmonic balancer, then roll the engine backwards until the engine touches the TDC bolt again and mark the balancer at the zero point again. The center of these two marks will be true TDC, and if the balancer is good, should be right on top of that mark (even works if the timing chain is worn, which has nothing to do with TDC anyway, and the depth of the TD bolt doesn't matter as long as it touches the top of the piston, which is why you split the difference). And rolling the engine backwards once isn't going to do any harm to the rings or bearings, too slow of a motion to cause damage.
     
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