What's up with my rot assy balance???? ...need answers!!

[turbonut85]

So I'm having a 4.1 build done currently. The machinist has done several TR motors thus far....Here is the call I got today:
I am being told in order to balance the rotating assy...he would need to remove upwards of 260g from each end of my crank... Is this right?? or even possible?? That's WAY too much to remove IMO. What would cause it to be that far out? He spun 3 separate flex plates and 3 separate balancers...all results were within 6-8g of each other, and the balance machine said to remove near the same amount. Would i be better off doing an internal balance and going with a zero balance flex plate and balancer??

Components are these:
Stock turbo crank 10/10
.040 hyper pistons
stock resized 2 dot rods
Stock balancer
stock flex plate
Any input is greatly appreciated!!

 

[bison]

something is wrong. You shouldnt need to remove much at all from the crank. The rods may need a little off them

 

[turbonut85]

Could it be in the balancing machine setup?? Is there a certain way they need to be setup in the machine vs say, a SBC?? Maybe the bob weights are set wrong? They can't be that much different than setting up a V8, right?

 

[turbodave231]

Do you have the Balance Card? What is the Bobweight he calculated? Ask your machinist what balance factor he is using to calculate the bobweight. Buick engines can be done at 36.6% (.366) or 50%(.50). The formula for the calculation is:

100% of Rotating Weight + (.366 x Reciprocating weight) = Bobweight.

The .366 is the 36.6% mentioned above. This has a big effect on the final balance solution. If 50% works out better for a correction, then go with it. You might notice a bit more shake in the engine, and you might not. I've balanced plenty of engines both ways and I honestly can't tell the difference.

You shouldn't have to take any more metal off the rods than necessary to weight match them.

 

[Nick Micale]

What we have done in many similar cases is instead of using the stock external balancer and flexplate, is to change one of them to an internal balance piece.

 

[longball]

260G? That's over 1/2 lb. each end!!!

 

[bison]

Do you have the Balance Card? What is the Bobweight he calculated? Ask your machinist what balance factor he is using to calculate the bobweight. Buick engines can be done at 36.6% (.366) or 50%(.50). The formula for the calculation is:

100% of Rotating Weight + (.366 x Reciprocating weight) = Bobweight.

The .366 is the 36.6% mentioned above. This has a big effect on the final balance solution. If 50% works out better for a correction, then go with it. You might notice a bit more shake in the engine, and you might not. I've balanced plenty of engines both ways and I honestly can't tell the difference.

You shouldn't have to take any more metal off the rods than necessary to weight match them.

The underbalance of 36.6% throws the vibration into the horizontal plane which is absorbed by the factory style engine mounts. This is how it was done from 1978-1989 with all the rear drive non balance shaft v6 engines.

 

[turbodave231]

Brian,

True, the only difference between 36.6% and 50% balance factor(s) is the plane where the couple happens. However, using the 50% BF increases the Bobweight and therefore requires more weight on the counterweights.......sounds like that might work for this particular situation.

 

[750H.P.V6]

Make sure he's got the balance machine set up so it will read how much material needs to be removed on the actual radius. It sounds like he has it set to a 1 inch radius.

Neal

 

[turbonut85]

Ok...so this is where it's at.....Basically what it boiled down to, is the machinist had his calculations off...apparently it was late in the day, and he mixed up the 100% and 36.6% factors and came up with the goofy numbers. He called me yesterday morning and said he had it figured out. He came in and took everything apart and put it all away, and then started over....then it came out right. However, he still had to take 60ish grams from one end and 90some grams from the other...seems still a touch high...but I'm no machinist, so I dunno? The crank WAS originally out of this 4.1 block, so maybe it was that far off because of the difference in weight between the stock cast 4.1 pistons, and the hypers?? I greatly appreciate everyone's input and help!!!

 

[turbodave231]

I know you are using a 3.8 balancer. Question: Are you using a flexplate from a 3.8? The engineered imbalance between a 3.8 flexplate/balancer will be different than a 4.1. That may account for the larger correction.

Glad you got it figured out!

 

[turbonut85]

I know you are using a 3.8 balancer. Question: Are you using a flexplate from a 3.8? The engineered imbalance between a 3.8 flexplate/balancer will be different than a 4.1. That may account for the larger correction.

Glad you got it figured out!

We used a 3.8 turbo flex plate and balancer...with the 4.1 crank...which, I suppose could cause the larger correction.