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There
are very few "SECRETS" to discover about connecting rods,
but there is one thing every engine builder must understand:
IT IS ESSENTIAL
TO TRY AND USE THE
LONGEST
CONNECTING ROD
YOU CAN
POSSIBLY FIT IN THE ENGINE
Rod
length is tied to the stroke length of the crankshaft, and the ratio
between the two is a significant factor in engine design.
Historically, the designers of American V-8 combustion engines have
viewed rod ratios of about 1.65:1 to 1.75:1 as ideal for all around
engine performance. However the Rod Ratio in a racing engine should
be much higher.
The
effect of the rod ratio is more easily understood if you try a
reasonably simple experiment. Take a standard 5.70-inch Chevy rod and
assemble it in a block with a 3.48-inch crankshaft and piston. Mount
a dial indicator against the top of the piston and place a degree
wheel on the crankshaft snout. If you place the piston near
TOP-DEAD-CENTER and read the dial indicator as the piston moves from
0.001-inch before the top of the stroke to 0.001-inch after the top
of the stroke, the degree wheel will move through about 3 to 4
degrees of rotation. Then if you perform the identical experiment
with a 6.00-inch rod, you will find that the degree wheel will move
through as much as 9 to 10 degrees over the same range of piston
movement. In effect, the longer rod causes the piston to
"DWELL" longer at TOP-DEAD-CENTER.
This is
important during the crossover from compression to power stroke. If
the piston dwells longer near TOP-DEAD-CENTER and ignition is
initiated properly, there will actually be a longer period of time
for the pressure created during combustion to press against the top
of the piston. Increasing the piston dwell allows the pressure to
build higher while the chamber is at minium size, this translates
into more effort against the top of the piston during the early
portion of the power stroke.
There is
also a secondary mechanical advantage from a longer rod ratio. Since
the piston dwells longer near the top of the stroke, the crankshaft
arm swings over further before the combustion cavity begins to open.
This allows the pressure of combustion to be more effectively
transmitted to the crankshaft arm during the period when the pressure
is highest. This increased leverage exists throughout the power phase
and the result is a smoother engine that produces more effective work
during the power stroke.
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