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Infact some make there flywheels heavier.

link

link 2

I am thinking of doing this on my 4.0 if it works for the 2.5 then it will work on an 4.0.

Only in racing where speeding up is hte issue is low inertia in the engine and tranmission a good thing. At the speeds most jeeping is done inertia in the drive train only helps.
 

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What you are refering to is a big problem. Internally balanced engines need a neutrally balanced flywheel. The 258 and 4.0 and 2.5 are interally balanced. However, Externally balanced engines need a flywheel that is out of ballance with the extra mass at an exact position relative to the crank. The AMC V8's are externally ballanced. So even though a 258 flywheel will bolt to a 304 they will not work together. This has caused problems with poorly researched AMC V8 retrofits to CJ's. However, 258 flywheels can be rebalanced to 304 specs. Other engines such as your 2.8 example switched ballance methods over time.

When I say balanced I mean balanced about the center of Crank rotation. So if you balance the ring separate from the flywheel the ring center must fit exactly to the crank and must have a center hole for indexing when banancing that will be true to the crank when combined with the flywheel.

Adding mass to a flywheel if the added mass is neutrally balanced about the rotation of the crank or if the fly wheel is rebalcaned to original specs will not cause a problem. Suppose you have a properly ballanced engine and flywheel, all is well. Now add a neutrally ballance ring to the engine. The ring will not cause problems because it is neutrally ballanced. So if you add a neutrally balanced ring there is no change. No mater if your engine is externally or inernally balance.

If you add a unballanced ring to a flywheel you will have problems. However, if you ballance the two together too original specs they will work fine. To prove this work through the previous argument in reverse. Removeing a neutrally balance mass from the modifyed flywheel, this leaves you with a the original flywheel; plus a neutrally balanced ring which will have no unbancing effect on the crank.

Do not miss understand me, the Ring either needs to be independently, neutrally ballanced about the center of the rotation of the crank. Or the two need to be matched together in a perminate alignment and balanced to original factory specs for the engine to which they are connected. However, I must take some exception to the second link I previously posted. I do not see the need for Dynamic Ballancing. In my opinion static balanceing should work fine.
 
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