- A well carved port lets its ports flow freely and has
its walls tapered to increase port velocity. It does a
wonderful job and does not
- decrease the crankcase volume (however small) due to
the nature of (any) modification. Crankcase compression
takes place for such a few degrees of crankshaft rotation
(perhaps as few as 20 degrees), it is important to
maintain a high port velocity and maintain the original
crankcase compression (in most cases). We know that at
high RPM the reeds don't close until about 90 or 100
degrees ATDC. That leaves only about 15 to 20 degrees of
crankshaft rotation to compress what's there. The tuned
pipe provides the signal to pull the mixture out of the
cases and is much more a factor of mixture delivery than
crankcase pressure, still with such a short compression
session - improper modification can lead to inconsistent
results.
-
- Knife edge dividers look like they flow better but
they must be made properly in order to improve port flow
velocity. More often than not, knife edge ports do
nothing more than look sharp. Splitting an incoming
mixture - the result of cutting the wall to its sharp
shape is probably not good as the mixture will separate
as it enters the cylinder. Mixture attachment - to the
rear cylinder wall and piston top, and the direction of
the flow is much more desirable feat to accomplish.
-
- When calculating the transfer port area needed for an
application it is possible that the tunnel area will have
to be modified to accomplish the objective. Port walls
must be carefully modified since they provide the lateral
support for the cylinder liner and its piston. Some
barrels, like the late model TRX parts with the bridged
intake port can be knife edged along with a small
decrease in their width and an increase in the overall
area without too much difficulty. To properly knife edge
the transfer ports of the same barrel a lot of things
must be considered.
-
- Many if not most port dividers are parallel, creating
pairs of air streams. These parallel streams flow with
the velocity direction. The air between the two streams
is confined - it can't cross the divider. Therefore, the
two streams act like to a single channel or tunnel. When
air flows through a tunnel, it will move faster when the
cross sectional area is made increasingly smaller,
conversely it will move slower when the cross sectional
area is made increasingly larger - Bernoulli's
principle. Because we are dealing with compressed
air, and that contains particles of fuel and that by
nature has viscosity the Navier-Stokes equation must be
considered. It is the primary equation of computational
fluid dynamics, relating pressure and external forces
acting on a fluid to the response of the fluid flow.
Click - here
or here
for more about this.
-
- Additional gains may be achieved by following laws of
aerodynamics as they pertain to airfoils. Vertical port
dividers can be thought of as though they were airplane
wings. The top edge of an airplane wing, with its longer
travel path creates a pull toward that direction because
air flow over a wing which has one curved surface and one
flat surface will flow faster across the curved surface
than across the flatter one. Air flowing under the wing
maintains its ambient pressure while air flowing over the
curved upper surface accelerates and drops in pressure.
The difference between the relatively high pressure
(flatter side) side of the wing and the relatively low
pressure (curved side) creates a force. The force
surrounding a wing shaped divider results in lift. Though
I'm not concerned with obtaining lift from the wing-like
shape inside the engine, I am interested in the shape to
provide an enhanced, revised air flow delivery on one
side of the wall.
-
- Knife edge port dividers shouldn't be cut at the
leading edge only and they shouldn't be actually knife
edged (sharp), rather they should start thin at the feed
side, be well rounded, gradually taper and blend in well.
The leading edge should be cut to 25% to 30% of the wall
length - with 30% being favored do to its pressure
reducing/port velocity increasing nature. This would
include adding material to the wall (epoxy) after the
taper has been blended to create a wing shape (for the
next 25%) then tapered it off again at the entry to the
cylinder. This will also give back some crankcase volume
that was created by the knife edge operation.
-
- These wall shapes can provide a "favor" if you will,
toward a port or toward a set of ports. This phenomenon
can be used to an advantage. Wing shaped dividers provide
a longer path from the entry to the exit and can enhance
cylinder filling and mixing. This is an alteration of the
mixtures delivery timing, strength and velocity. Inherent
in this design is increased drag which comes from the
increased pressure gradients (the tunnel walls converge)
which tends to promote turbulence as a side affect.
-
- If performed ideally, the exit path of the mixture
through a port tunnel may be revised very well. Its
direction may be aimed more
- precisely, controlled more thoroughly - regardless of
operating RPM and can aid in cylinder scavenging by
allowing the incoming charge to more closely follow the
law of aerodynamics as it applies to wings.
-
- When the air passes the trailing edge of the well
shaped port divider the air on the high pressure side
(flatter surface) tends to move toward the low pressure
side. This action, combined with the air that is rushing
by it, causes a trailing edge vortex (spiral) right at
the edge of the port - just inside the cylinder. The left
port wall trailing edge vortex rotates counter-clockwise,
and the right port wall trailing edge vortex rotates
clockwise - crawling up the rear of the cylinder. This
(rich) condition is more pronounced under low flow
situations. Enhanced mixing is a result. By letting the
streams coming off (and very close to) the port divider
collide with each other as the flow continues, the mixing
continues. The reunion of the mixture then slows as its
air pressure increases - it is gathering inside the
cylinder.
-
- Additionally, providing an increased arc to the port
tunnels by adding epoxy to the inner most wall (closest
to iron liner) and removing metal from the outer most
wall will create tunnels somewhat longer and more round -
so the mixture can enter the cylinder at a flatter level
with less obstruction from changing direction and more
time/area due to the better entry angle. This condition
can be preferred in many situations.
-
- Both of these modifications can consume serious
amounts of time. Any time a port wall divider is shaped
like a wing or a teardrop, streamlining is enhanced and
drag is reduced. Make the port tunnels tapered and
turbulence goes up as does the port flow velocity.
Straight port wall dividers may do the job of letting the
mixture into the cylinder, but there are better ways to
direct this flow.
-
- Rick
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