Balancing can and will take tons of stress out not hundreds
of pounds.. And the lighter the parts the less the stress.
If I was racing an engine with 3 mains i would own the
lightest stuff i could find..
1 gram is common place work now and has been for 20 years if
the shop is any good at all.
The trick with the 2 ends and error is to balance 1 end in
the fixture usually the big end if memory serves me right..
get them all the same the do a total weight to get the small
end.. swapping parts around ought to be a last resort for
me.. the best way to do it is pair up all the big ends for a
2 big end weight that is the same. then use the small end
weight difference to mix and match the different pistons..
then you can come up with a correct bob weight.
On the subject of bob weight it does not have to be a 50% of
reciprocating weight deal and depending on your rpm you may
want to go to 51 or 52%.. the real beauty of a 51% job is
later on if you have some new bob weight and it falls within
50% to 52% you are good to go with no rebalance on the
crank.
The real thing to remember is that the balance job is only
valid over an rpm range not from 0 to 9000 rpm. The higher
the RPM the more you have to look at more than 50% recip..
many slower turning engines are balanced at less than 50%
BTW.. On some of the small stuff i have worked on that
really turns up (10 to 13k rpm) i have used 54%.. I spent a
lot of time with the engineers and field guys at Stewart
Warner years ago on this very subject for Winston Cup
stuff.. if you are mathematically inclined there is a bunch
of good stuff that is written on this subject. I would
suggest Stewart Warner a good place to start for a suggested
reading list.
Hope this helps a bit
Dave Dahlgren
ardunbill@webtv.net wrote:
>
> Hi Folks, recently delved into this subject, one I've always found of
> interest, because I had to consider it for my 290 Ardun. After talking
> to various experts and reviewing the literature, I realized it is
> necessary, at least this time.
>
> First thing I found out is that a lot of people believe in balancing, it
> is traditional hot-rod lore, but consider it a black art, so they just
> hand their crank and parts in to a speed shop that "undertakes" this
> work, and trust/hope they will do it right. This is not a bad approach,
> if the shop is trustworthy. This reminds me of the time I asked a
> grizzled mechanic if retread tires were okay, and he said, "Bill, a
> retread tire is as good as the man who makes it."
>
> Balancing is of interest not because a poorly-balanced V-8 will
> necessarily cause vibration in the car(although it may), after all, the
> crank has to vibrate the massive inertia of the block, heads,
> transmission, etc, through the rubber engine mounts if used, before the
> vibration gets to the chassis and thereby the driver. What is more at
> stake here is that poor balance increases the internal stresses in the
> various parts of the crankshaft, and more as the rpm increases. This
> may add a couple hundred pounds of stress on a crankpin that already has
> very high stresses on it from the simple matter of snatching the heavy
> piston back down from top dead center at high rpm. The extra stress
> might be just enough to crack or break your crank.
>
> Reference to P.E. Irving's authoritative text, "Automobile Engine
> Tuning" brings forth the info that for balancing a V-8 crankshaft(of the
> conventional type where the four crankpins are at 90 degrees to each
> other) the formula for the bobweight to be used on each crankpin is 100%
> of the rotating weight + 50% of the reciprocating weight. When this
> bobweight is fastened symmetrically about each of the four crankpins the
> crank is rotated on a balancing machine and weight added or subtracted
> to the counterweights by drilling, etc. until the crank is in dynamic
> and static balance.
>
> Here the "rotating weight" is the weight of two con-rods' big-ends, plus
> the bearing inserts, weighed when the rods are horizontal and supported
> by frictionless bearings at the centers of the ends. The "reciprocating
> weight" is the weight of the rods' small ends weighed horizontally as
> above, plus the weight of both pistons with rings, pins and locks.
>
> Irving goes on to say that all the parts should be matched to each other
> to a tolerance of one gram, "for the best work." Now he is setting up a
> very high standard here, that is difficult to reach in practice. I
> daresay that it rarely is reached in commercial practice. But the
> closer to the ideal, the better.
>
> With flathead V-8s of the type I am dealing with, original con-rods of
> the same pattern are found to vary commonly by 20 grams or more, because
> they were made in several different plants. I imagine the makers of
> today's aftermarket flathead racing rods supply them closely matched in
> weight, as do the makers of racing pistons. If we use original rods of
> various weights, it seems reasonable that we can match light rods with
> heavy ones, and perhaps add a few grams with hardened ARP washers under
> the nuts, to finish up with four pairs of rods that are very close to
> the same weight. With today's production rods, they usually have "pads"
> of metal at both the big and small ends which are intended to be ground
> off to match the weights. Original Ford flathead V-8 rods do not have
> such pads, instead they have four discreet little places that can be
> lightened by drilling, to a small extent.
>
> The business of weighing the rods horizontally is a whole game in
> itself. Accurate electronic scales with a 2000 gram capacity, reading
> in 1 gram increments, are readily available everywhere for $125 or so.
> But it's not just a matter of supporting the rod horizontally with one
> end or the other on the scale. Both ends must be supported from their
> centers on frictionless (ball-bearing) stands, because the slightest
> error in friction or distance will throw the measurement way off. If
> you try it, you will quickly see that the weights you get for each end
> do not add up to the complete weight of the con-rod, because of the
> errors. So there is a ready means to check the truth of your process.
> And this is one job not worth doing unless it is done right.
>
> The approach I am presently taking is to work up my own bobweight
> figures by matching the parts I have to work with (domed Ardun pistons
> are much heavier than original Ford pistons, for example), then giving a
> bobweight figure to the speedshop to use on my crank in his balancing
> machine. Just for my own satisfaction, and to save a few bucks maybe.
>
> Besides your crankshaft assembly as described above, needless to say all
> your rotating parts, flywheel, pressure plate, torque converter,
> pulleys, etc, have to be in dynamic balance, and your speedshop will be
> equipped to deal with them.
>
> The above is merely what this amateur has been able to learn about V-8
> balancing up to this point, so I hope the real experts and veterans on
> this List will point out any errors they see. Cheers from Ardun Bill in
> the Great Dismal Swamp, Chesapeake, VA
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