Balancing Act

[Dibbons]

Francisco Javier, the local "Mr. Mopar" mechanic in this town of La Paz, BCS, Mexico now owns a 1969 383 auto Road Runner, a 1969 383 four-speed Charger R/T (he built a truck Dana 60 for the Charger), a 1971 Plymouth Satellite Sebring Plus, and a 1972 Barracuda (auto-not sure about motor).

Today we discussed engine balancing and he had two interesting comments which, in the Christmas spirit, I took at face value:

1) After machining my small block crankshaft .020 under, the motor is no longer balanced as well as it was before the clean-up (it was already .010 when I purchased the motor).

2) Although a tire tech may mark the position of the tire in relationship to the valve stem, after he patches the tire and mounts it in the same position as before, the tire will still need re-balancing (because of the addition of the patch).

 

[hangn0ut]

This is under the assumption that the balance was perfect in the first place. Factory balance of older engines might be off by 5 grams. For normal commuter driving no imbalance would be noticed. Depending on how things were out of balance the machine work on the crank might just improve the balance of the assembly.

 

[PurpleBeeper]

I believe Francisco is correct about both things.

 

[steve340]

Technically if you split hairs he is right. However I believe in both cases if they were right before you started you would be hard pressed to notice any difference.
Using your crankshaft example a machinist has ground 0.010" of metal of 4 big end journals and 0.010" of the main journals. You can forget the mains as the are so close to the centreline of the crank. The material ground off each big end would be a gram or so I guess.
Provide you changed nothing else definitely not enough to notice.

 

[hangn0ut]

Technically if you split hairs he is right. However I believe in both cases if they were right before you started you would be hard pressed to notice any difference.
Using your crankshaft example a machinist has ground 0.010" of metal of 4 big end journals and 0.010" of the main journals. You can forget the mains as the are so close to the centreline of the crank. The material ground off each big end would be a gram or so I guess.
Provide you changed nothing else definitely not enough to notice.

This right here is on target. Material off the mains does not affect the balance. And like was said material off the rod journals has minimal effect. 5 grams at 5000 rpm maybe you feel it. Thats a generalization as stroke, total weight and other factors come into play. For example, how much does the oil in the big end weigh? 3 grams is a common estimate i believe. In a high perf job you want the balance to less than a gram. Sometimes its a bit of a struggle ,for example, when weight needs to be removed and there is no counterweight to remove it from. Balancing is a lot of work!

 

[furryfrog]

My balance job on my rotating assembly was expensive, close to half the cost of a new 4340 crank. Prices are high up here in canuckastan though, and the steel crank had shards of metal hanging off it from the balance job in the factory years ago. Ditching the heavy six pack rods and pistons for much lighter forged stuff saved nearly a pound a cylinder in rotating weight. Crank looked nice when I got it back, they cleaned it up and must of turned some material off the counter weights as well. My engine is a happy smooth, high reving, gas guzzling machine.
I have since watched on youtube how it's done, it is a lot of work, especially more I think because of the amount of weight i took out of the rotating assembly.

 

[plum70chrgr]

One other thing to keep in mind is that the material removed from the crank is replaced by the thicker oversized bearing.
Albeit the density of the bearing material is less than that of the material being ground off the crank, the difference I think would be negligible.

 

[Kern Dog]

I think that the guy is exaggerating. The amounts he refers to are so small, they are not enough to be noticed.
For example:
In 2015 I took our new Challenger to Willow Springs to run it on their road course. I started with a half tank of gas, thinking it would handle better with less weight from gasoline. The instructor stated that the difference in 9 gallons of gas weighing 6 lbs per gallon is inconsequential. Tire pressure variances of 3 psi would make more of a difference.
Come on.. A tire patch that weighs one quarter of an ounce is going to imbalance a 45 lb tire and wheel ? Really ???

 

[hangn0ut]

Actually I would think its up to the driver. Cause if the driver can feel it its there. Some drivers really have a better feel for the car than others. And some just dont care! LOL.