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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