When to re-balance a motor?

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dibbons

dibbons

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I think I may have overestimated the ring set weight when I ordered my custom billet crankshaft for a small block Mopar. I weighed a used set of rings I had on hand for a 340 with stock pistons and 5/64" top rings and they came out to about 57 grams.

However, come to find out the NOS TRW pistons I have to install (L2322F) use smaller 1/16" top rings, which I assume are going to weigh less. Anyone know the difference in weight between the two ring sets? And how much of a weight difference can I get away with? (will be mostly a street motor with yearly drag runs just for fun). I am willing to re-balance the rotating assembly, but would rather not ($). Thank you.

ring set.jpg
 
My concern is based more on what I have been reading, and that is the fact that although 1 gram is not much, due to the laws of physics, its effective weight at a high RPM speed is multiplied many times over. 28 grams = 1 ounce

From engine builder mag:
Maintaining Your Balance: Engine Building Tips to Reduce NVH and Increase Life - Engine Builder Magazine

The centripetal forces generated by any imbalance increases exponentially in proportion to speed. Double the rpms and you quadruple the force. A 2 oz.-in. (ounce-inch) imbalance on a crankshaft will generate only about 14 pounds of centripetal force at 2,000 rpm. But when the engine revs to 8,000 rpm, that slight imbalance now becomes a pounding force of 227 pounds!
 
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Another interesting quote:
Little Tolerance for Imbalance
The “old school” tolerances for balancing used to be plus or minus 2 ounces for stock engines (56 grams), half an ounce (0.5 oz. or 14 grams) for street performance and two-tenths ounce (0.2 oz. or 5 to 6 grams) for racing engines.

These numbers provided relatively good results with the engines and speeds that were common a couple of decades ago, but they are not even in the ballpark with today’s stock and racing requirements.
 
dibbons said:
Another interesting quote:
Little Tolerance for Imbalance
The “old school” tolerances for balancing used to be plus or minus 2 ounces for stock engines (56 grams), half an ounce (0.5 oz. or 14 grams) for street performance and two-tenths ounce (0.2 oz. or 5 to 6 grams) for racing engines.

These numbers provided relatively good results with the engines and speeds that were common a couple of decades ago, but they are not even in the ballpark with today’s stock and racing requirements.
Click to expand...
I would say you answered your own question.
 
I always have the rotating assembly balanced when I replace pistons. Being a new crank too, it needs to be balanced as a rotating assembly. You should also include damper (If not a fluid type) and flexplate/flywheel. 65'
 
The rings are lighter. I would let it go. It isn't worth it.

I balanced all my race stuff to under a gram but that's just because I have mental issues about stuff like that. I know other builders on race engines that when they were under 6 grams they stopped.

There is a lot going on with oil slinging around, crank deflection and other things that make for a bunch of mental masturbation.
 
Reciprocating weight is less of an impact than rotating weight. If the rings are lighter, you're fine.
 
You re-balance one when it become out of balance. Rings probably not enough to matter.
 
Install them and forget it.... stock engines aren't balanced even close to that standard.
 
dibbons said:
Another interesting quote:
Little Tolerance for Imbalance
The “old school” tolerances for balancing used to be plus or minus 2 ounces for stock engines (56 grams), half an ounce (0.5 oz. or 14 grams) for street performance and two-tenths ounce (0.2 oz. or 5 to 6 grams) for racing engines.

These numbers provided relatively good results with the engines and speeds that were common a couple of decades ago, but they are not even in the ballpark with today’s stock and racing requirements.
Click to expand...
So... everybody is turning their engines a lot faster than a couple decades ago? I really doubt that is true, in general. Especially on the "street".
 
69_340_GTS said:
So... everybody is turning their engines a lot faster than a couple decades ago? I really doubt that is true, in general. Especially on the "street".
Click to expand...
I would tend to agree... On the dirt tracks 7500 was almost unheard of before the 80's... Now many classes have an 8500 chip rule.

I have a friend who really leans on his sbc to the tune of 8800 in 25 lap features... not uncommon.

Now my plan is 7200 this season but it used to be 6500.


I'm skeered...
 
After sending my machine shop a new harmonic balancer and flexplate, now they say the motor does not need re-balancing and the short block is finished and ready to pick up.
 
dibbons said:
After sending my machine shop a new harmonic balancer and flexplate, now they say the motor does not need re-balancing and the short block is finished and ready to pick up.
Click to expand...


At least you KNOW you don't need to rebalance. If you didn't have it checked, you'd be guessing.
 
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