Balancing
By nature, a 90–degree V–8 isn’t the smoothest–running engine configuration, so balancing a performance rotating
assembly requires some extra precision. As long as reciprocating mass is equal to the rotating mass, a motor will
run smoothly without unwanted vibrations that reduce bearing life. With today’s lightweight aftermarket pistons
and rods, weight is removed from the counterweights the majority of the time. Only in applications where extremely
heavy–duty nitrous or blower parts are used, or where the crank is extraordinarily light, is weight added to the
counterweights.
It’s a common misconception that bobweights simply duplicate the weight of a pair of pistons and rods. Bobweights
are actually equal in mass to 100 percent of the rotating weight (big rod ends and rod bearings), and 50 percent of
the reciprocating weight (pistons, pins, locks, and small rod end). In a 90–degree V–8, each piston has a
"companion" it travels together with to TDC at the same time. So, to equalize rotating and reciprocating weights,
bobweights are selected to match that formula.
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The Sunnen DCB–750 digital balancer spins the assembly to 750 rpm and features an encoder that very accurately
measures crank position. The stands on each side detect how much imbalance exists, and calculates how much
weight must be removed or added to correct it.
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Think of rotating assembly balancers as glorified tire balancers. When rotating the crank by hand, the digital
readout tells the operator the exact amount and location from which weight must be added or removed. Balancing
to "x" number of grams simply means how much imbalance, in grams, exists 1–inch from the crank centerline.
Drill charts from crank manufacturers specify how much weight a given drill size and drill depth will remove.
Since the crank acts as a lever, the farther away from the centerline weight is removed or added, the greater
effect it has on balance. If an unusually large sum of weight must be removed, the counterweights can either
be Swiss–cheesed with a bunch of holes or be turned down in a lathe. Typically, balancing takes an hour to an
hour and a half.
Align–Boring, Honing, and Four–Bolt Mains
The basic principles of align–boring and honing are similar to standard boring and honing, but for main
caps instead of cylinder walls. The big difference is that, while over–bearing a cylinder is a common rebuild
procedure–since the crank rides on a set of bearings–only blocks that have been beat to death or have spun
bearings require align boring. A much more likely reason for doing so is to install four–bolt main caps onto
a two–bolt block. Like file–fitting a piston ring, to account for varying tolerances, aftermarket caps require
some end–user trimmings. Because they arrive undersized, they must be bored to the within a few thousandths
of the proper inside diameter spec, then honed the rest of the way. Healthy blocks that aren’t going to be
fitted with aftermarket main caps typically only require align–honing.
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