Question on balancing

[joe cool]

I am just a bracket racer, would like to have a class car but I do not have the talent and the knowledge to mantain one. I appreciate all the info I get by reading all of your posts and answers ,even when they are above my brain level.

I read about over balancing engine components and I would like a possible simple explanation wjy this is done, It seems to me overbalancing would be just as bad as not balancing at all.Thanks for the help

I am told I won't get much help because of not using my real name,well i'm
Joe Keightley
lost in iowa.

[FireSale]

Racing needs cars. No such thing as "Just a bracket racer". You're either in or not.

Never really heard of over balancing. Seems to me that you are either balanced or not. Maybe it applies to balancing to ridiculously close tolerances. I'm balanced to 0.5 gram. Someone with more knowledge will fill us in.

BTW: No one in Iowa can't spell...

Dale

[Alan Roehrich]

Over balancing is something done in many cases because of things learned over time. Some builders have notice that bearings look better if the balance weights are adjusted.

Realize that there is a set formula for the bob weight. It does not take into account the variations between engines, or how they're built and used. Over balance can be used to account for those variations.

[CFMCNC]

The standard V8 balancing formula is a bobweight consisting of 50% reciprocating weight and 100% of rotating weight,From expierence we will alter the reciprocating percentage,We have gone anywhere from 47% to 55%,Both underbalanced and overbalanced.Variables are stroke,rpm range-power curve, component weights ,crankshaft counterweight design and more.Bill C.

[BillK]

Joe,
Like the others said, there can be some variations mainly from experience. I think most of the time you see an "overbalanced" engine in a situation where it is run for long time periods at high rpms, like a Nascar engine. Basically you make the bobweight a little heavier than the "normal" formula.

Harley actually has three different recommendations depending on whether the engine is going to be "street", "street high performance" or "strictly race"

A lot of it is more art than science but I don't think you can go wrong with the standard formula on most engines. I have overbalanced a couple for circle track engines but really don't know if there was really any difference.

[Mike Taylor 3601]

Originally Posted by CFMCNC

The standard V8 balancing formula is a bobweight consisting of 50% reciprocating weight and 100% of rotating weight,From expierence we will alter the reciprocating percentage,We have gone anywhere from 47% to 55%,Both underbalanced and overbalanced.Variables are stroke,rpm range-power curve, component weights ,crankshaft counterweight design and more.Bill C.

Bill,
Gave good explaination,I'll add a little I think what is most important is to get a good balance job,I see balanced rotating assembly's people buy where pistons,rods never have been out of the box and unwrapped,rods still in sealed bags, they must use weight that is on box,which isn't always correct,3 grams here 2 there before you are done you end up with a bobweight that isn't very close to what it should be.

I have rebalanced several one guy just wanted to check,bobweight I would use was 6 grams lighter than they had used, spun crank rear needed 80+ grams added.
another one pistons needed fly cut so crank needed respun had to add weight to it,
which is opposite of what it should have been,anytime you lighten pistons,rods,etc and rebalance a crank you should have to remove weight from the counterweight area.

On the harley's you have to balance them to what rpm they will be ran @ % is changed
to match,they will only be balanced in certain RPM range like a single cylinder is
Mike Taylor 3601

[joe cool]

So do you over/under balance for the reliability /longivity of the components or for a performance advantage, or both ?
thanks

[ss3011]

If your parts are out of balance, the vibration you feel is a force or load. This force will increase exponentially with increase in rpm. Those forces could fail parts, depending how high the load gets, compared to how strong the parts are. Bottom line, stuff needs to be balanced !

[Mike Schwartz]

I guess I need a little schooling too. How is the process of equalizing the weights of the pistons, rods, pins, etc related to the overall balancing process? Isn't that done first before making adjustments to the crank bobweights?

[Mike Taylor 3601]

I'll go through basics first, rods,pistons are weighed you find the lightest piston ,lighten others to match,how close varies, I balance +/- 1/2 grams or within 1 gram whichever way you want to say it. Weigh big ends rod find lightest,lighten rest to match,weigh small end find lightest,lighten rest to match.
Then you start figuring bobweight, reciprocating side uses piston,pin, locks, rings,small end of rod,then that weight is totaled and mutiplied by number of rods per throw ,V8 would be two, is mutiplied by recprocating factor normal 90 degree v8 use 50 % this is factor that is changed to over or underbalanced.
rotating side uses big end of rod,bearing,usaully we add 3 grams for oil then this weight is mutiplied by number of rods per throw,V8 would be 2.
then rotating and reciprocating are added together to make up what bobweights will weigh then bobweight s are installed on rod throws and crank spun on balancer,then crank is corrected for unbalance.

Are far as over or underbalance I never have done either,I know alot of blown alky,and big inch,and ultra high rpm 10K+ change percentage.
I would'nt worry about over or undrer balancing unless I was having vibration/unbalance caused problems
Hope this answer your question if not I try to answer them.
Mike Taylor 3601