The ratio between the connecting rod
length and the stroke length of a motor greatly affects the way it performs,
and how long it lasts. This ratio (normally represented by “n”) can be
calculated as follows:
Ratio “n” = Rod Length ÷ Stroke
The rodís length is measured (for this purpose) from
the center of the piston-pin opening to the center of the big-end bore,
not overall. There is a small range of ratios for most conventional piston
engines: the rod is between roughly 1.4 and 2.2 times the stroke length.
Itís not possible for the rod to be the same length as the stroke, and
rods much longer than twice the stroke make the motor very tall, and are
not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction
at the cylinder wall and piston skirt. A greater angle (smaller value of
"n") will occur by installing a shorter rod or by increasing the stroke.
A reduced angle (larger value of “n") will occur with a longer rod or a
If the rod length is decreased, or the stroke is
increased, the “n” ratio value becomes smaller. This has several effects.
The most obvious is the mechanical effect. Motors with low values of “n"
(proportionately short rods or long strokes) typically exhibit the following
characteristics (compared to high “n” motors):
Physically shorter top-to-bottom & front-to-rear (more
Higher level of vibration
Shorter pistons, measured from the pin center to the bottom of the skirt
Greater wear on piston skirts and cylinder walls
Slightly higher operating temperature & oil temperature due to
There are also differences in how the motor breathes:
Intake vacuum rises sooner ATDC, allowing bigger carburetors or intake
port runner & plenum volumes to be used without loss of response
On the negative side, a small or badly designed port will “run out
of breath” sooner
Piston motion away from BDC is slower, trapping a higher percentage
of cylinder volume, making the motor less sensitive to late intake valve
closing (hot cams)
Spark advance is also affected:
Earlier timing (more advance) is required, as the chamber volume is
larger (piston is farther from TDC) at the same point of rotation
The motor may also be less knock-sensitive, as the chamber volume increases
more rapidly ATDC, lowering combustion pressure (this is useful for nitrous
& supercharged motors)
Effects of Long Rods
Provides longer piston dwell time at & near TDC, which maintains
a longer state of compression by keeping the chamber volume small. This
has obvious benefits: better combustion, higher cylinder pressure after
the first few degrees of rotation past TDC, and higher temperatures within
the combustion chamber. This type of rod will produce very good mid to
upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced
angle which will place less stress at the thrust surface of the piston
during combustion. These rods work well with numerically high gear ratios
and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and
therefore lighter) piston, and generally have a safer maximum RPM.
They do not promote good cylinder filling (volumetric efficiency) at
low to moderate engine speeds due to reduced air flow velocity. After the
first few degrees beyond TDC piston speed will increase in proportion to
crank rotation, but will be biased by the connecting rod length. The piston
will descend at a reduced rate and gain its maximum speed at a later point
in the crankshaft's rotation.
Longer rods have greater interference with the cylinder bottom &
water jacket area, pan rails, pan, and camshaft - some combinations of
stroke length & rod choice are not practical.
To take advantage of the energy that
occurs within the movement of a column of air, it is important to select
manifold and port dimensions that will promote high velocity within both
the intake and exhaust passages. Long runners and reduced inside diameter
air passages work well with long rods.
Camshaft selection must be carefully considered.
Long duration cams will reduce the cylinder pressure dramatically during
the closing period of the intake cycle.
Effects of Short Rods
Provides very good intake and exhaust velocities at low to moderate
engine speeds causing the engine to produce good low end torque, mostly
due to the higher vacuum at the beginning of the intake cycle. High intake
velocities also create a more homogenous (uniform) air/fuel mixture within
the combustion chamber. This will produce greater power output due to this
The increase in piston speed away from TDC on the power stroke causes
the chamber volume to increase more rapidly than in a long-rod motor -
this delays the point of maximum cylinder pressure for best effect with
supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than
in a long-rod motor.
Causes an increase in piston speed away from TDC which, at very high
RPM, will out-run the flame front, causing a decrease in total cylinder
pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will
descend at a faster rate with a reduction in cylinder pressure and temperature
as compared to a long-rod motor. This will reduce total combustion.