I have my doubts that you will find such a thing. Lighter flywheels are for racing.
A flywheel is an inertia device, meaning that it stores energy. The heavier it is the more energy it stores, the lighter the less. The energy storage makes it harder to accelerate or decelerate the flywheel and anything attached to it. More difficult to accelerate is a bad thing if you are running the quarter mile where you want maximum acceleration.
In a Jeep if you were to lose traction the engine rpm will increase. The heavier flywheel gives you a bit longer to let up on the go pedal before the engine reaches red line. Alternatively, the heavier flywheel makes decelerating more difficult so if you misjudge how much power you need for a particular situation, you will have a bit more time to give the engine a little more gas before it bogs down and stalls.
not sure how it is done w/ the 4.2, but i ran into BIG probs on an '87 firebird 2.8 5 speed, bought it with a "rebuilt" engine, problem is, they installed an '88 long block and the thing shook itself to death....literally....had to redo the whole bottom end at 23,000 miles...turned out that the '88 2.8 was balanced on the crankshaft, while the '87s were balanced on the flywheel...mix the two together, not good
moral of the story, sometimes those "engineer" types who design factory setups actually DO know what they are doing/ubbthreads/images/graemlins/grin.gif
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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