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Is 50 Grams a Lot ?

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Someassemblyrqd View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Someassemblyrqd Quote  Post ReplyReply Direct Link To This Post Topic: Is 50 Grams a Lot ?
    Posted: Jan/16/2019 at 9:41pm
So, my AMX goes in for final paint (Glen Green) in 2weeks.  

I’ve been chasing a vibration issue since I first started the in September.  Same level of vibration whether in motion or at a standstill.  Dropped the tranny and took the torque converter back to the shop that built to 2500 stall.  When they spun it it was 50 grams out - almost 2 ounces, or a cooked McDonald’s hamburger patty..  Not sure how the shop missed that,  but after 4 spin-ups with each showing a 50 gram variance, I walked away with a TC at ‘zero’.

My next test will be to run the engine without the tranny/TC and check for vibration.

At what weight value variance will cause these AMC engines to vibrate?   Or another way of saying it is: what is an acceptable weight variance before they will start to exhibit a vibration?

Thanks. Greg
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1948kaiser Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2019 at 7:41am
try it with the washer and see what a difference it makes. my other half still does not get the idea of a balanced load
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2019 at 11:31am
50 grams will make a heck of a lot of difference depending on where the (undesirable) weight is placed.

IE: where two pennies = 1 gram, imagine a stack of 100 pennies attached somewhere to the side of the torque converter...

The greater the distance, from the weight to the centerline of the crank, the more effect it will have.

This can be easily conceptualized by imagining two boys on a see-saw, one heavier and one lighter.

Move the boys individually nearer or further away from the fulcrum and notice what happens to the overall balance...

We can balance the fat boy by moving the skinny boy further away from the fulcrum -where the skinny boy's weight has more balancing effect by reason of the geometry.

Understanding this concept is key to understanding why external balanced rotating assembly can be made lighter. -Done right, a lighter external balanced rotating assembly can be used to make more power by reason of reduced parasitic loss/reduced inertia. Done wrong, the force of the external weight increases torsional twist effect on the crank to decrease durability.

IE: the further away from the fulcrum the boy is placed, the more flexing load is placed on the beam of the see-saw.

Most desired is to have the see-saw beam balanced by itself with no weights at all so the see-saw beam itself can be moved easier by an external force (combustion)

(similar concepts apply to selecting and balancing wheels for the vehicle; you want lighter wheels and tires for reduced inertia & balancing weights are undesirable but if you must use them, they are typically placed on the outer peripheral areas of the wheel rim where they will have greater effect)

There's an excellent article on this topic here:

http://performancedevelopments.com/porsche6cylharmonicenginefailure/

and here:

http://www.epi-eng.com/piston_engine_technology/crankshaft_torsional_absorbers.htm


Edited by amcenthusiast - Jan/17/2019 at 11:40am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2019 at 12:00pm
What is perceived as an engine balance problem is symptomatic to improper weight attempting to shear off it's surrounding metal components. They retain the offending weight by 'centripetal force':

https://www.khanacademy.org/science/ap-physics-1/ap-centripetal-force-and-gravitation/centripetal-forces-ap/v/introduction-to-centripetal-force
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: Jan/17/2019 at 6:45pm
I assume it was 50 grams-inch.  That is a huge imbalance.   Factory balance spec for the rotating assy was less than 0.5 oz-inch.  Or 14 grams-inch.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote maximus7001 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/16/2019 at 4:50am
Some have chased vibrations that turned out to be the ring on the harmonic balancer slipped a bit. Just saying. Hope its not a bit of both.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Feb/16/2019 at 4:08pm
I want to know how a torque converter gets that far out of balance on it's own.

It probably doesn't.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Someassemblyrqd Quote  Post ReplyReply Direct Link To This Post Posted: Feb/16/2019 at 11:35pm
the TC was rebuilt to a 2500 stall. Beyond that, I could only speculate. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Feb/16/2019 at 11:58pm
Yeah it sounds like they may have screwed it up when they changed the stall speed. How does one even change the stall speed of a converter? It's a welded assembly.

I don't build high performance stuff. Maybe this is common for all I know but it sounds plain weird.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1970390amx Quote  Post ReplyReply Direct Link To This Post Posted: Feb/17/2019 at 12:41am
Originally posted by FSJunkie FSJunkie wrote:

Yeah it sounds like they may have screwed it up when they changed the stall speed. How does one even change the stall speed of a converter? It's a welded assembly.

I don't build high performance stuff. Maybe this is common for all I know but it sounds plain weird.

You can google rebuild torque converter and find videos of the process. Changing the pitch of the fins and clearances inside the converter will change the stall. Most rebuilt converters are balanced when done. I would guess it was missed on this converter.
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