bobweight rod upgrade

HI EVERYONE,fist time post,enjoy this site a lot! My question is when you upgrade to a heavier rod (80-90grams)should the bobweight be changed? if the weight was dead on before.How does an overbalance effect things? We have a multi engine pull tractor w/venney hemis, just did not want to spend time and expense on older cranks if not much difference.Note 4 are crank to crank engines 45deg apart. others have their own clutch.thanks alot for any input!!

 

What your doing is under balancing, you definitely have to balance the cranks to the new rods, if the rods were lighter it would be over balance, and with that much weight you would still have to balance the cranks, or your going to shake the rods and blow the motor. chose your poison.

 

lance just sent me a crank that had a bob of 2800 and now is 2500..I sent him a rod piston wrist pin and buttons to have it ballanced and did but he told me it not necessary...the fuel guys swap combo's all the time..rod length/pistons/ect...what do u guys think..Dave
www.myspace.com/daveloweswr

 

The fuel guys blow up there shit every run so they probable don't care. and you should have sent him the rings. and like Timmy Richards once told Frank Manzo, to run real good, put away your hammer and chisel, every thing has to be PERFECT. but you guys can do what ever you want!

 

2 weeks ago I had the same dilemna. I spoke with our long time friend and machinist about it. Remember their are 2 variables (educated geuss) in the bob weight formula, oil weight and % balance. 50% is normal, but he said he's seen as much as 15% variance of this.
In my particular case I was debating over about 20 grams which equated out to .6%, yea, 50.0 vs 50.6.
My belief is that the most important thing is keeping all 8 cylinders equal to each other first, then worrying about how far off of 50% you are.
Some feel their is advantage to over or under balancing but I don't know what their argument is?

 

sorry Eli, I sent the rings and bearings also...Dave
www.myspace.com/daveloweswr

 

First let me tell you that i had my own engine balancer, it wasn't like the new ones that spin the crank and it tells you were to add or remove weight, this one had a two cradles that rocked back and forth, it had two dial indicators on each end journal, so you can see with your own eyes if the crank was balanced or not, i would get the dial indicators to not move then the crank was balanced, A V8 motor can not be balanced perfectly, thats were the 50% comes in, back in the day we balanced motors with 50% cuz they didn't rev like they do today, (by the way the 50% is for the recpotating part the rotating is 100%,) any way when you use 52% the motor will stay in balance at higher RPM's. on a in-line motor you don't use bob weights, just put the crank in the balancer and spin it and balance as necessary and once it's done it is truly balanced.

 

Here is some good information about balancing

http://victorylibrary.com/mopar/crank-bal-c.htm