Adding balancer and flexplate

[Quicksilver]

In an attempt to meet the NHRA rules for SFI approved equipment...
Can a BHJ harmonic balancer and JW flexplate ( both for External balance )be bolted on to a stock crankshaft without pulling the crank out and visiting my local balance shop :fragez:
I have been reving my motor to 6000 rpm but not more.

[Race Jace]

these two items are what the weight is added to and subtracted from when externally ballancing the motor. So just because they say they are externally ballanced style units doesnot mean that they are ballanced for your particular rotating assy.

[gnsrule]

How much of a differance is there between SFI equipment and stock as far as the weight offset ? I would like to think their fairly close. Im vaguly familer with balanceing procedures in that the crank is balanced to a percentage of the rotateing weights. I think that means that if you bolted up all the rods assemblys to the crank and layed the crank on V blocks, it would NOT be a true static balance. The offset weight is used instead of a chunk off crank weight. If the offset weight is the same,wouldnt that work ?
Im thinking that if a different balancer/flywheel is used, as long as the weight offset is in the same relation to the keyway, that mass weight differance of the balancer/flyheel would just move the balanceing rpm up or down accordingly.
Im certainly not saying that this is procedure to be used, just asking how much does it change things and how.
Im in the same boat being that i built my 86 motor w/ stock pieces and really wanna up to SFI stuff w/o the rebalanceing shit.
Just looking for the ins and outs of balanceing, not a pissing match.
Anyone up to teaching Balanceing 101 ?? :beerglass

[KendallF]

How much of a differance is there between SFI equipment and stock as far as the weight offset ? I would like to think their fairly close. Im vaguly familer with balanceing procedures in that the crank is balanced to a percentage of the rotateing weights. I think that means that if you bolted up all the rods assemblys to the crank and layed the crank on V blocks, it would NOT be a true static balance. The offset weight is used instead of a chunk off crank weight. If the offset weight is the same,wouldnt that work ?
Im thinking that if a different balancer/flywheel is used, as long as the weight offset is in the same relation to the keyway, that mass weight differance of the balancer/flyheel would just move the balanceing rpm up or down accordingly.
Im certainly not saying that this is procedure to be used, just asking how much does it change things and how.
Im in the same boat being that i built my 86 motor w/ stock pieces and really wanna up to SFI stuff w/o the rebalanceing shit.
Just looking for the ins and outs of balanceing, not a pissing match.
Anyone up to teaching Balanceing 101 ?? :beerglass

Craig,changing the flexplate and balancer weights will move the total imbalance percentage around (away from 36.6 or 50% or wherever the motor was balanced). This changes the rotating couple magnitude and direction..you'd have to ask somebody with a clue exactly how. :)

Whether you can swap them and be close will depend on two things. Look for drill marks on the inside of the balancer hub and on the flexplate.

1: If your balance guy took weight off of the original balancer and flexplate when he did the initial balance, the new stuff would have to be done the same. In theory, I suppose you could take him the old balancer and flexplate and he could spin them and match the new stuff to them. In practice, I don't think most of these guys have any practical way to attach compensating weights to them without having the crank to spin as well.

2: If he did all of his balance work on the crank itself (our local guy tries to do that, for just this reason) you are better off. Now it's down to whether the new stuff precisely matches the imbalance of the stockers. If you had a nice precise +/- 1g. balance job, I would bet they're still not quite that close..just from manufacturing variances.

I doubt bolting them on will cause your motor to self-destruct, in any case..plenty of very roughly balanced factory motors have run big numbers. It depends on how picky you are, I guess. :)

[gnsrule]

So it sounds like were both on the same tract...cept for a thing or 2
The first thing that hit me was "if its balanced to +\- 1g". If things are balanced to a percentage, how much is 1g ?? Is that really a good number or just a balancers hype word to say a good job ? I would think the 1g figure stands for all the rod \piston combos to be w/in 1 g, but the total balance cant be that close being that its on a percentage. Is it 1g at 5k or 1g w/ the matematical equation for X percentage at whatever givein rpm u wanted ?
If the previously balanced stock balancer was mounted in a v block and the offset weight was measured, would makeing the new balancer the same be the same or does the mass weight change things ?? I dont know about a sfi balancer, but a flywheel damm near doubles in mass wgt if not more.
No offence Kendall, but i think we need an actual balance guy to step in here, this is the shit i like. :) aside from beer of corse. :beerglass

[KendallF]

So it sounds like were both on the same tract...cept for a thing or 2
The first thing that hit me was "if its balanced to +\- 1g". If things are balanced to a percentage, how much is 1g ?? Is that really a good number or just a balancers hype word to say a good job ? I would think the 1g figure stands for all the rod \piston combos to be w/in 1 g, but the total balance cant be that close being that its on a percentage. Is it 1g at 5k or 1g w/ the matematical equation for X percentage at whatever givein rpm u wanted ?
If the previously balanced stock balancer was mounted in a v block and the offset weight was measured, would makeing the new balancer the same be the same or does the mass weight change things ?? I dont know about a sfi balancer, but a flywheel damm near doubles in mass wgt if not more.
No offence Kendall, but i think we need an actual balance guy to step in here, this is the shit i like. :) aside from beer of corse. :beerglass

I will answer once more, even if I fail to be on the same "tract". Perhaps an "actual balance guy" will step in and edumacate us both. :marchmell

The weight match to +/- some tolerance, and the balance percentage, are two different things.

The rod/piston combos are balanced to weigh the same, and the rods are additionally balanced so that they weigh the same on both the big ends and the small ends (using a jig that holds the rod on one end so that the other end can be weighed). A good balance job will hold these weights to within +/- 1 gram, or even half a gram.

Then, the bobweights are calculated using a balance factor that adds up the rotating weight (big end of the rod and bearings) + some percentage of the reciprocating weight (small end of the rod, pin, piston, rings). There are mathematical formulas that calculate the resultant force rotating couples and RPMs where a given balance factor will minimize them. In the end, though, these are approximations as there are many other forces acting on the rotating assembly (combustion pressure, etc. etc.). So...the factor is often empirically determined, a nice way of saying they try different things and see how they work. V8s are usually balanced using a 50% formula; our V6s are stock balanced at 36.6% and race motors are often balanced at 50%.

A bobweight is assembled for each rod journal and clamped on; in the case of an externally balanced motor, the flexplate and balancer are also bolted up. The whole mess is spun; the spin speed is dependent on the balancer used. The spin or "dynamic" balancing allows a more precise balance because the force due to an imbalance mass goes up with the square of the rotational speed (F=(m*rot speed^2)/R) so smaller imbalances can be detected. The rotating assembly is now balanced by adding or removing material, usually with a precision at least equal to the weight matching of rods or pistons.

So...now that I wrote a book... :arco:
duplicating the static imbalance of your current balancer and flexplate will get you very close to the dynamic balance. You can't tell where the effective radius of the imbalance mass is (see the force formula above) so it will not be exactly perfect. I think that's what you were trying to say. If you take weight off in the same locations as the stock balancer and flexplate, it will be very close.

Now I'm gonna have my 3rd :beerglass

[87natty]

Is it just me or are we all just a bunch of f-ing drunks around here? Not like I care, I tailgate at school. :beerglass

[gnsrule]

Kendall, thanks for the how to's of balanceing. Very inforfmative stuff here. But i've still got my original question unanserd. "does the mass weight change things ? ". I dont think so but im not sure.
Example, converting a SBC ( i know there internally balanced cept for the 400) from a auto w/ flexplate to a stick w/ a 20lb flywheel. I dont belive that changes the balance as long as the new flywheel is in the same balance state as the old.
Once again Kendall, I'm not looking for a pissing match nor was i trying to insult your intelligence, just looking for a little more. This is what bench raceing is, isnt it ??

[Chuck]

Craig said:
"Example, converting a SBC ( i know there internally balanced cept for the 400) from a auto w/ flexplate to a stick w/ a 20lb flywheel. I dont belive that changes the balance as long as the new flywheel is in the same balance state as the old." Correct.
As for Kendall, he's a real :fragez: when he's :beerglass, or smokin a :joint: !!!. He'll be ok in the morning!! :add_wegbr

[KendallF]

Kendall, thanks for the how to's of balanceing. Very inforfmative stuff here. But i've still got my original question unanserd. "does the mass weight change things ? ". I dont think so but im not sure.


I was trying to answer that at the end: If you match the imbalance (what you're calling the mass weight), it'll be fine.

I did some looking around to see if I could find some good material online regarding balancing; I didn't find much. It's a topic that interests me as well; the funny part is that talking to a couple of local balance guys, they don't know jack about the physics of it; the ones with nice new machines can barely read the instruction manual and take weight off where the machine tells them to. Dave Williams, who posts on the gnttype list, has a balancer and has done some weird stroker stuff; he could probably add a lot here. He has a steel crank of mine that he's going to rework right now..

As for Kendall, he's a real :fragez: when he's :beerglass, or smokin a :joint: !!!. He'll be ok in the morning!! :add_wegbr

Hey, I work for the gov't! No pot smoking jokes! :) I'm sorry about all the sheep jokes the other night...we were on a roll... :marchmell :marchmell :arco:

[Chuck]

I'm sorry about all the sheep jokes the other night...we were on a roll...

You are only saying that so I'll bring my trailer full of tools to BG!! :add_wegbr

[gnsrule]

Chuck,
Thanks for the answer. I'm glad someone stepped in that knew. :applaus:
I worked in a machine shop a few years back and did all the engine assembly. They ground there own cranks , but the balance was sent out so i wasnt able to gather more info on the subject.
Kendall,
"If you match the imbalance (what you're calling the mass weight), it'll be fine."
I dont know how imbalanced weight could be mixed up w/ mass weight since I
referanced a 10 lb. +\- sfi flexplate to stock. :fragez: :joint:
"If the previously balanced stock balancer was mounted in a v block and the offset weight was measured, would makeing the new balancer the same be the same or does the mass weight change things ?? I dont know about a sfi balancer, but a flywheel damm near doubles in mass wgt if not more."
If either of you make it to BG, stop by, I'll buy you a :beerglass if Kendall supplies the :joint: Then we'll all be :elefant: :marchmell :elefant: :marchmell

[Quicksilver]

Kendal, Craig and Chuck
Thanks for you time and effort.Tthis has been a great benefit for me.
John

[1of1547]

I was given a sheet with all of the balance weights on my rotating assy. My machinist told me to hang on to it in case something needs replaced.