Balancing Experts....

[Procharged 434]

I have a question for the engine balancing experts out there. I have a SBC 350 that I'm putting fresh rod bearings in. It was balanced with a Federal Mogul "Tri-Metal" bearing. New bearings going in it are King Alecular "Bi-Metal" bearings. Although I'm not really worried about it at all, my curiosity got the best of me, and I just wanted to ask if there is much of a weight difference between a Bi-Metal and Tri-Metal rod bearing, enough to affect engine balance?

Thanks

[turbo camino]

Balancing, except for very specific applications, is cargo cult science. You'll be fine. The oil clinging, or not, to the internal bits, and flying around every which way, and in different ways from one moment to the next, will make more of an impact on the 'balancing' than any rod bearing ever will (that's why I run all my engines with no oil - don't want to upset that ultra-precise balance job I spent so much money on!).

If you run super speedway type stuff where the engine spends hours on end stuck in the same 300-500 RPM window, then yes, balancing can be beneficial. Because of the combination of rotating and reciprocating parts, and that the weight added or removed will not likely ever be the same distance from center of rotation as the imbalance you're trying to correct for, an engine can only be truly 'in balance' at one specific speed. Above and below that speed it is out of balance.

[rebelyell]

I'm neither expert or journeyman; but have some balancing experience. I suggest we hear from those with far more direct experience.
Don't know, but can imagine mass diff between the two bearing types is so small that it would be significant to only true pro race motors.
AFAIK, balance is important enough, that all OE serial production motors are processed with a Static balance; the mfg process accounts for and controls the weights of the various parts. The result is a Static balance. No, I don't know what that production Static tolerance is; but imagine it's small. I do KNOW this; Vibration wastes energy, reduces efficiency, fatigues parts and shortens MTBF / TBO. Auto & truck makers Must keep NVH to a minimum. There are standards for NVH; moreover, industry pundits & end-users won't accept it.

Also, (until advent of GPS) consider the very high rpm gyro compass on board ships; can you imagine the resulting calamity / catastrophe when & if it were not Dynamically balanced to the highest order of accuracy & precision ?

[turbo camino]

A gyro compass, or another high-speed marine device, the gyro stabilizer, neither are applicable here for two reasons. One, they do not have a combination of rotating and reciprocating parts inside, and two, they run at, and are balanced for, only one very specific speed. That's not even an apples-and-oranges comparison, that's more like apples-and-fruitbats.

[Mark O'Neal]

I have a question for the engine balancing experts out there. I have a SBC 350 that I'm putting fresh rod bearings in. It was balanced with a Federal Mogul "Tri-Metal" bearing. New bearings going in it are King Alecular "Bi-Metal" bearings. Although I'm not really worried about it at all, my curiosity got the best of me, and I just wanted to ask if there is much of a weight difference between a Bi-Metal and Tri-Metal rod bearing, enough to affect engine balance?

Thanks

No.

[Procharged 434]

Thanks guys!

[Dave Koehler]

I agree, no problem. I doubt you would find a gram or two difference.

[n2omike]

Plus, keep in mind that if your rod throws are all light... they will be opposite of each other... which makes things a lot less 'bad' than you might think.

There are four pairs of rod journals... all at 90 degree intervals to each other... so it kind of cancels out, as for each rod journal, there will be another 180 degrees from it.

I realize it's not 100% because of some harmonics, but it's not as significant as one might think.

[piston guy]

Balancing an engine as if it were a Swiss watch is a joke. If you "equalized" the force generated by all eight cylinders ( virtual impossibility) through out the engine's full rpm range , it might be worth it. You would also have to eliminate the torsional harmonics generated ( a watch doesn't have them). In short small changes don't bother anything like Mark said.

[Mark O'Neal]

You guys could complicate buttered bread.

[houser45]

The difference will be very small Mabie a gram or two. It won’t matter. The most important thing is that the crankshaft is balanced end to end fairly close. With you changing the rotating weight a little will not change that. Now if you were putting a set of rods that weighed 30 grams a piece heavier and it was a race engine you need to put the brakes on

[modok]

it is true the aluminum bearing should be lighter,
But the layer of bearing material is only...maybe .015" thick in most cases.
Other factors like the exact width of the shells may be a bigger variable. Fractions of a percent difference.

[Baprace]

The difference will be very small Mabie a gram or two. It won’t matter. The most important thing is that the crankshaft is balanced end to end fairly close. With you changing the rotating weight a little will not change that. Now if you were putting a set of rods that weighed 30 grams a piece heavier and it was a race engine you need to put the brakes on

houseer-45 has it correct on the balance question, When I balance or check an assembly, if the crank was balanced correctly
in the beginning you will not feel any vibration.

Any difference in the bob weight by 10 grams +/- Will just make the over/under a little difference in formula, a little higher or a little lower in the formula will not make a difference , IF THE ORIGINAL DYNAMIC BALANCE WAS CORRECT, will not be a problem