The most enlightening article I have read on the subject:
Opinions vary on the ideal rod length-to-stroke ratio, but it is generally
accepted that the closer this number gets to 2:1, the better. When a
cylinder is in its power stroke, the piston is thrust against the bore wall
due to the angularity of the connecting rod. The greater the angle, the
greater this thrust. This increases the stress on the piston and cylinder
bore, increases the friction between them, and robs power from the engine.
If an engine was designed using short rods with a given stroke, this thrust
would be greater than if it had been designed to use longer rods with the
same stroke.
Aside from reducing the loading of the piston against the cylinder wall
during the power stroke, a greater rod length-to-stroke ratio offers the
benefit of increased piston dwell time. The dwell time of the piston can be
thought of as the number of degrees of crankshaft rotation during which the
piston remains at or near top dead center. This is more of a concept than an
actual number. In any piston engine, when a piston is at mid-stroke, it is
moving much faster than when it is near top dead center or bottom dead
center. Once it passes top or bottom dead center, it accelerates until
mid-stroke, then decelerates through the rest of that stroke. In an engine
with short rods, at a given RPM each piston is moving faster as it nears the
top or bottom of the bore, spends less time there, and accelerates in the
other direction sooner than an engine with comparatively longer rods. In an
engine with long rods, each piston approaches the extremes of its travel
sooner in terms of degrees of crankshaft rotation, and doesn't begin rapid
acceleration in the opposite direction for several more degrees of
crankshaft rotation.
This additional piston dwell time can go a long way toward taming the
undesirable characteristics of a long-duration/long-overlap camshaft. As the
piston descends on its intake stroke, the intake valve is open. It may
remain open for several degrees after bottom dead center, but in a long-rod
engine, with a longer piston dwell time, the piston remains near bottom dead
center longer than in a short-rod engine. Therefore, the loss in low-speed
cylinder pressure associated with long-duration camshafts will not be as
great in a long-rod engine.
As the piston travels upward on its compression stroke, it nears top dead
center sooner in a long-rod engine, and stays near top dead center longer.
After the spark plug fires, it begins to accelerate downward on its power
stroke a little later, so not as much timing advance is needed, and the
engine won't be as sensitive to timing errors as an engine with a smaller
rod length-to-stroke ratio. As the piston nears bottom dead center, the
exhaust valve begins to open.
In a long-rod engine the piston will be lower in its bore sooner than in a
short-rod engine, so not as much cylinder pressure is lost at the end of the
power stroke. After the exhaust valve opens and the piston passes bottom
dead center, it begins its ascent through the exhaust stroke. Near the top
of its exhaust stroke the intake valve opens, and for the next several
degrees of crankshaft rotation both valves will be open. The reason for this
is to promote exhaust scavenging at high speeds. The exhaust leaving the
cylinder flows past the exhaust valve, through the exhaust port, and into
the primary header tube.
As the cylinder is emptied, the inertia of the exhaust gases causes a
low-pressure area in the cylinder. By opening the intake valve sooner and
leaving the exhaust valve open longer, this low pressure will help to
initiate the flow of the incoming fuel/air mixture. At low engine speeds,
however, the velocity of the exhaust gases leaving the cylinder is too low
to create the inertia needed for good exhaust scavenging. Because the
intake valve is opened so soon and the exhaust valve is closed so late, the
incoming fuel/air mixture becomes diluted with inert exhaust gases.
In a short-rod engine, the piston reaches top dead center later than in a
long-rod engine. Therefore, it is likely that more of the exhaust gases will
be pushed past the open intake valve at low speeds in a short-rod engine.
Because the piston leaves top dead center on its intake stroke sooner in a
short-rod engine, it will likely draw more exhaust gases past the open
exhaust valve, further diluting the incoming fuel/air charge. Because the
pistons in a long-rod engine spend more time at or near top dead center
during valve overlap, the negative side effects of low-speed driveability
will be less severe than on an otherwise identical engine with short rods.
Excerpt from "How to Build High Performance Chrysler Engines" by Frank
Adkins, S-A Design 2001
Bob
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