Six-Gun-Todd
Member
We agree to disagreeYou do not use Bob weight calculations or Bob weights while balancing four cylinder engines.
When balancing a four cylinder crank you first balance the crank by itself.
Then you bolt the flywheel on the crank and balance that as an assembly and only drill the flywheel to correct any unbalance.
Then you bolt the pressure plate on the flywheel and balance the pressure plate by drilling, grinding or welding wieghts around it.
You need to make sure your Pistons are all within a gram or so of each other.
For the rods you need to use a jig that supports the wrist pin end of the rod while the big end is on a gram scale to make sure the big ends all weigh the same by removing weight from the heaviest rods to make them match the lightest big end. Once you get all of the big ends the same weight you can weight the whole rod and grind on the little ends of the heavier rods until they all weigh the same.
When balancing an engine that is 90 degrees such as a V8 you have to use a percentage of the Piston, rod big end, rod wrist pin end, bearings, wrist pin, wrist pin clips, rings and a little for oil under the Piston.
From these measurements you load up Bob weights to the calculated weight and bolt them to the crank rod journals. Some V8 cranks also have external weights on the flywheel and balancer that will need to be in place while balancing the crank.
With a V8 if you change the wieght of any of the rotating or reciprocating components it changes the Bob weight so you have to rebalance the crank.
I've spun a couple hundred Honda, VW, Subaru, and the like and have always found room for improvement. The balancing machine I used was accurate enough to "see" weight as light as a dollar bill taped to a counterweight....would it impact something outside of a racing application perhaps unlikely. My background is racing engines so I'm a bit one sided....still like to chase perfection though.