How many of you guys spinning 7000+ have your crank over-balanced?

 

First I've heard of "over-balancing"....usually a machine shop wants to know what you're going to spin it to and then balance it accordingly. I guess you could tell them 8K RPM and only spin to 7K, but that would be fairly safe IMO....

 

i run a 49/51% over balance on my motor...........works out fantastic for stroked motors planning on spending most of their time above 5000rpm+.........that little extra weight really helps to pull the rod/piston back down the bore espically at very high rpm.....

 

Did you mean to say you've NEVER heard of "over-balancing" in the above quote???

The "over-balancing" I have heard about has nothing to do with what RPM you're spinning the motor to.

Over balancing is just adding an additional 1/2% to 1% to the "exact" calculated bobweight, and then balancing the rotating assembly for that "new" bobweight.

For example, say all the rotating assembly componets you're going to use in an engine build, require an "exact" 1,500 gram "calculated" bobweight. To "over balance" this rotating assembly by 1%, you would add, 1% times 1,500 grams = 15 (additional) grams, for a total of a 1,515 gram bobweight, and then "balance" the assembly using this new "over-balanced" 1,515 gram bobweight.

Using a 1,515 gram bobweight, instead of the "exactly" called for 1,500 gram bobweight, (in this example) ........ would give you an engine "over-balanced" by 1%. Got it???

And, just for the OP, yes most all NASCAR engines are "over-balanced" by between 1/2% to 1%.

 

correct in the procedure but not the usage..........a high rpm engine will benefit from an overbalance.......espically one with a larger stroke.....

 

I guess the 9,000 to 9,500 RPM those NASCAR engines turn all day ........ doesn't qualify as "high RPM" in your book???

 

u stated that it has nothing to do with rpm...........I'm saying a high rpm engine will benefit from an overbalance..........

 

You know what, you learn something new everyday... That being said I can see the benefit on a race engine, but to me, it seems it would add extra wear and tear on a DD street engine. But then again, I don't look for RPM over 6500 for a street engine. Not in an SBC anyway. But again this is just me...

 

At least we agree on the "High RPM" part now!

My comment regarding "having nothing to do with RPM" was aimed at poster duh, who seemed to imply to the OP that an engine builder will balance an engine differently if you tell them to balance it for 7,000 RPM vs 8,000 RPM; and that is definitely ...... WRONG!

The "usage" example I cited, namely NASCAR engines ........ "pionerred" the concept of "over-balancing" engines for high RPM racing, because, unlike in drag racing ........... their motors run for hours at 9,000+ RPM versus just seconds ...... like they do in 1/4 mile racing.

In truth, ANY engine can benefit from "over-balancing", it will just be MORE beneficial at higher RPM.

Hope this clears things up.

 

Now it has been a while since I've had to have a rotator balanced, but the last time I was asked what RPM I planned on spinning to. Now maybe this had to do with the over-balancing you are discussing. So there is no reason to say I'm wrong. Just because I was asked a question without an explanation doesn't mean I am incorrect. I'm sorry you seemed to take it as I was implying that it was RPM specific. That doesn't make much sense if the piston, ring package, pin and rod are all finite weights....The only difference would be the amount of force exerted on a specific journal as the RPM increases due to centripetal forces. So I again can now understand why over balancing would help a high RPM engine. It's using the cranks' counterweight to accelerate the piston either to TDC or BDC.

But again this seems it would add unnecessary wear on the bearings due to more force being applied to them. Granted that could be accounted for by loosening up the bearing tolerances a bit to allow more of an oil cushion...

(d*mn engine theory)

 

agreed lol

 

HaHa you guys are funny!!! I know exactly what over-balancing is... I was just wondering if there a few that have done it to their exact setup. No need for a theory discussion!!

 

yea i know u know lol........like i said i have a 49/51 on the current 385 in my car now.....