balance or dont balance????

[nm9stheham]

What really gets me is how a piston with a taller compression height and a .140 dome could be 50 grams lighter then a factory short piston?!?!? Kb399.

More modern design and hypereutectic material, I presume.

 

[Meep-Meep]

I balance stuff for a living that spins in the 40 KRPM range so I'm pretty picky about balancing. But I deal in milligrams. The formula to calculate the bob weight on a 90 deg V8 is the weight of the rod bearing and big end of the rod plus one half the weight of the piston, small end of the rod, ring pack, pin. The theory is that the reciprocating mass is part of the rotational mass about 50% of the time. I would have the engine balanced, or get pistons that are weight matched to what was in there and last and known to run smooth.

 

[zyzzyx]

I'm saying that a 318 engine with a 50g lighter piston won't pose a vibration problem because even according to the balancing article posted here earlier,

the reciprocating ( I got it right this time!) weight is only factored in @50% for a V-8. That's 25g. The bob-weights are mainly reproducing the weight of

the bottom half of the connecting rod, and bearing inserts, plus 3g of oil. If you want to balance it that's fine, but you'll be surprised at how little material

will be drilled from the counterweights! Not "buzzing" the motor into the danger zone will help keep this thing together. If you want to add a set of H-beam

Eagle rods, you can take it to 8500 with the right valve train. (Oh, and a balance!)

 

[rev.ronnie]

From what I've seen after balancing several thousand cranks, (even two today) is that whatever the change in piston weight seems to be how far out the crankshaft is.

99% of the time, when I'm doing a re-balance for lighter pistons, I can almost point at the exact spot on the counterweights and call out the amout of correction within 5-10 grams on each side before even test spinning the crankshaft and getting a reading on the computer. If there is another guy in the shop, they freak out like I'm a psychic.

I've just seen it hundreds of times. You're almost 2 whole ounces off...you need to rebalance that thing if you want smooth running and longevity.

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I'm saying that a 318 engine with a 50g lighter piston won't pose a vibration problem because even according to the balancing article posted here earlier,

the reciprocating ( I got it right this time!) weight is only factored in @50% for a V-8. That's 25g. The bob-weights are mainly reproducing the weight of

the bottom half of the connecting rod, and bearing inserts, plus 3g of oil. If you want to balance it that's fine, but you'll be surprised at how little material

will be drilled from the counterweights! Not "buzzing" the motor into the danger zone will help keep this thing together. If you want to add a set of H-beam

Eagle rods, you can take it to 8500 with the right valve train. (Oh, and a balance!)

The reciprocating factor is indeed 50% per piston. The bobweight total will be affected by 50 grams total, not 25, there are two pistons represented on that bobweight.

This guy needs to have his crankshaft re-balanced. Don't confuse him with conjecture and internet hogwash. 2 ounces of imbalance is not acceptable. If it were, we wouldn't use 1/2 gram as our tolerance for balancing an assembly.

Here is what's represented on a bobweight for a 90° V8 using a 100% rotating and 50% reciprocating calculation. There are 4 bobweights on the crank.
-------------------------

The weight of both big ends of the rods.
The pair of rod bearings
4 grams for oil
The weight of one piston
Weight of one pin
Weight of one ring set
Weight of one small end of a connecting rod
Weight of a pair of circlips (if used)

Obviously, this is for a assembly with all parts that are weight balanced to each other.

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This is a typical bobweight sheet. Sorry, it's a Ford i just finished

 

[IQ52]

Whew, finally a voice of reason and experience...........