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Once
upon a time, a 454-cubic-inch engine was considered a
big motor, and anything over 500 inches was referred to
in awe as a “Mountain Motor”. But like Big Gulps and
the budget deficit, everything is bigger in the 21st
century – especially drag racing engines. While
we’ve run 500-cubic-inch engines in Pro Stock since
1982, the rest of the drag racing world has adopted
600ci, 700ci, and even 800ci motors.
The one essential ingredient for a big-inch engine is a
long-stroke crankshaft. The availability of reasonably
priced aftermarket cranks has fueled the displacement
inflation. When the only alternatives were factory
forgings and high-dollar billets, racers had few
choices. It’s a different world now with high quality
crankshafts available with strokes that range from 4.250
inches up to 5.750 inches. And with the growing
popularity of Top Sportsman and Pro Mod classes, the
trend toward bigger and bigger engines is gaining
momentum.
A
long-stroke crankshaft is a highly specialized
component, and not all cranks are created equal. When
racers pulled crankshafts out of junkyards or bought
over-the-counter factory forgings, they took what they
could get. Now with the advent of affordable aftermarket
cranks, racers face a bewildering assortment of
materials, counterweight styles, oiling systems, and
options. These choices have serious consequences when a
75-pound chunk of machined steel is spinning in the
heart of an engine.
Balancing
is a serious issue with long-stroke crankshafts. As the
stroke increases, it takes larger and heavier
counterweights to offset the weight of the rotating and
reciprocating assemblies. I’m not an advocate of
external balancing, which puts a portion of the required
counterweight on the flywheel and balancer. I’m also
not a fan of counterweighted flywheel flanges on
crankshafts. While external weights do balance the
crankshaft assembly overall, they also introduce
torsional forces into the crankshaft by positioning some
of the weight at the extreme ends of the crankshaft.
Although a steel crankshaft seems quite rigid, in fact
it twists and bends in response to power pulses and
torsional forces. A crankshaft doesn’t have to fail to
make the effects of this stress apparent: Worn bearings
and cap walk (fretting of the main caps against the
block) are signs of torsional bending.
A
production V-8 crankshaft typically has six
counterweights positioned at the front and rear of the
crank. Many aftermarket crankshaft manufacturers offer
“eight weight” designs with two additional
counterweights adjacent to the middle main bearing
journal. These center counterweights simplify balancing
and significantly reduce the torsional loads on the
crank.
While
external balancing is less expensive than internal
balancing, I believe it’s better to balance the
crankshaft internally even if it’s necessary to
install heavy metal in the counterweights. These heavy
metal plugs must be installed parallel to the crankshaft
axis by drilling holes through the counterweights and
pressing the plugs in place. Plugs installed in the
counterweights perpendicular to the crank axis can be
dislodged by centrifugal force, turning them into heavy
metal projectiles.
When
we install heavy metal in a crank at Reher-Morrison
Racing Engines, we don’t put all of it in the end
counterweights. We drill through the first counterweight
and into the second counterweight; in some instances,
it’s necessary to drill through to the third
counterweight as well. We then repeat this procedure on
the other end of the crank. Each hole is progressively
smaller, and the corresponding plugs of heavy metal are
turned on a lathe to produce the correct interference
fit. Distributing the weight throughout the crank in
this manner also reduces the torsional loads on the
crankshaft.
Hollow
rod journals are a real asset for a long-stroke
crankshaft. Drilling the crank pins to lighten the
throws has the same effect on balancing as adding mass
to the counterweights, but it produces a lighter overall
rotating assembly. The longer the stroke, the more
important it is to drill the crank pins. Most
manufacturers offer drilled crank pins as an option, and
it’s money well spent. Don’t be tempted to buy an
“economy” crank when building a big motor; the cost
of balancing with heavy metal can more than offset the
low initial cost of an undrilled crank.
The
position of the counterweights is also important to
proper balancing. Unfortunately it’s difficult to
determine whether the counterweights are in the right
positions unless the crank is mounted on a balancing
machine. If a crank needs a lot of material to be
removed from one side of a counterweight and then a plug
of heavy metal inserted at the opposite end, it’s
likely that the entire counterweight is in the wrong
place. It’s possible to balance a crank with this
problem, but the fact that the counterweights aren’t
indexed properly means that more weight is required to
balance it than if the counterweights were in the
correct positions.
A
crude example of this is an out-of-balance tire. If a
tire is mounted on a wheel and it’s way out of
balance, often the easiest solution is to rotate the
tire to a different place on the rim. Perhaps the wheel
and tire are both heavy in one place; if both heavy
sides happen to be together, it takes a lot of lead to
balance them. But if the heavy sides of the wheel and
rim are opposite each other, the overall balance is
better and less lead is required. Applying this
principle to crankshafts, if the counterweights aren’t
in the right place, the balancing job is much more
difficult and ultimately requires more weight to achieve
a balanced state. An engineer could analyze the moments
and angles of force that are involved, but I just know
what I see on the crankshaft balancer.
Crankshafts
are a complex subject, and I’ve already filled the
available space for this column. Next month I’ll get
into knife-edged counterweights and why you should never
use a cross-drilled crankshaft.
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