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Glossary of Input Data Descriptions




Cylinder Overall Length:     Measured from base gasket surface to head gasket surface.


Deck Height:     This is the distance that the piston crown edge is above or below the top of the cylinder at TDC (Top Dead Center).  Piston above the cylinder deck is called "positive" deck height and is indicated with a + sign preceding the measurement.  Conversely, piston below the cylinder deck at TDC is called "negative" (more common of the two situations) and has a - sign preceding the measurement.  A perfectly "flush" condition is referred to as a "zero" deck height.


Rod Length:     This is the exact big end pin center to small end pin center connecting rod length.


Actual Crankcase Compression Ratio:     Sometimes this is referred to as the Primary Compression Ratio with the ratio above in the cylinder head called the Secondary Compression Ratio.  The Crankcase Compression Ratio in a two stroke engine is the ratio of the volume below the piston at TDC (including all of the transfer passages, piston underside volume and intake tract back to what would be the closing item such as reeds, rotary disc or piston skirt plus the crankcase volume with crankshaft installed) divided by the remaining volume in the crankcase with the piston crown edge located at the exact height of the highest transfer port's roof (just before the "break open" point).  It is very time consuming to measure and changes with any crankcase modifications, transfer porting, base gasket machining or spacing, reed spacing or rotary valve "tunnel porting".


R.E.D. Estimated Crankcase Compression     If you are unable or unwilling to measure your true Crankcase Compression Ratio, I have a vast archive of various models and can normally estimate it accurately enough for quality simulation output data.  The good news here is that although it is one of the most difficult items to ascertain in a two stroke engine, it is one of the lesser sensitive areas in modification planning (although not one to be ignored).


Start RPM     This is the lowest beginning RPM of your powerband that you are interested in analyzing through simulation.  A word of caution...  the lower the RPM simulated, the slower the simulator runs so be realistic in your powerband needs and don't request simulation RPM's that are not useful or real world for your application.  It just takes unnecessary time and costs more money!


Finish RPM     This is the highest realistic RPM you wish to be simulated.


RPM step size     This is the RPM increment of change from lowest to highest requested RPM's simulated.  For most applications, 250 RPM increments are suitable to usefully map powerband characteristics and are generally recommended.


Carburetor ID     This is the inside diameter measured at the smallest cross section in the carb venturi.


Total Intake Tract Length


Total Inlet Track Length     This is
the centerline length from the end
of the carb bellmouth, velocity
stack or air filtration system
conduit to the end of the reed
block, rotary disc or piston skirt
(piston port model shown
in the diagram at right).



Piston Port Intake Angle

Port Down Sweep Angle     This is the average
port angle of tilt downward when referenced from a
horizontal plane (as shown in the diagram at right).




Inlet Skirt Length      This is the length of the intake skirt of the piston from the crown edge (or top edge of ring if using an "L" or Dyke's type uppermost ring) to the skirt's bottom center.   If the skirt's bottom edge has an inverted "U" or other shaped cutaway, measure to the highest cutaway point up the skirt's center.


Port floor height      This is the vertical distance from the top of the cylinder to the lowest point along the floor of the intake port window.


Port roof height      This is the vertical distance from the top of the cylinder to the highest point at the roof of the intake port window.


Effective port width      This is the average width of the window just inside of the chamfered sides of the intake port 90 to the direction of air flow.


Radius @ top corners      This is the average radius that creates the upper corner shapes of the intake port window.


Radius @ bottom corners      This is the average radius that creates the lower corner shapes of the intake port window.


Opens @      This is the exact point, in crankshaft degrees of rotation before Top Dead Center, that the intake port window is first cracked open by the rotary disc valve.


Closes @      This is the exact point, in crankshaft degrees of rotation after Top Dead Center, that the intake port window is finally fully closed by the rotary disc valve.


Port ID      This is the average port body inside diameter at the window covered and uncovered by the disc valve.  When the port is not round, I prefer to calculate and/or measure its' window area and then select a round port ID that has the same area for simulation input.


RV Port to Crank Center Radius



Radius from crankshaft center to port center      This is the distance from the crankshaft centerline to the disc valve's port window center, as shown in the diagram at right.







Reed thickness      The thickness of a reed petal (Item "A" as shown in the diagram below right) ideally determined with a micrometer or vernier caliper.


Typical Reed Block Example


Reed width      The actual measured width of the reed petal only (Item "B" in the diagram at right).


Reed length      The measured reed petal length from the reed petal end to its' retaining screw's shank (Item "C" in the diagram at right).



Reed block port width      The actual width of the reed block opening beneath the reed petal (Item "D" in the diagram above right).


Reed block port length      The actual length of the reed block opening beneath the reed petal (Item "E" in the diagram above right).


Reed block angle      This is the total included angle formed between the reed block window openings (Item "F" in the diagram above right).


Length from clamp      This is the closest distance measured inside the reed block from the beginning of the reed window opening to the shank of the reed petal retaining screw (Item "G" in the diagram above right).


Reed Stop Measurement Diagram


Stop plate radius      (Item "H" in the diagram above right)   To measure and calculate, proceed as follows using diagram at right.   Remove and lay the reed stop plate on a flat surface upside down.   Calculate the rise height "R" by measuring from the flat surface to the highest point of the stop plate's curve on what is now its' top side and subtracting from it the vertically measured stop thickness "VT", as shown.   Now measure the chordal width "W" across the curve from edge to edge.   Calculate the stop radius using the following formula:

     (W + (4 x R)) (8 x R)


Number of ducts      This is the total number of transfer passages in the cylinder and would include main, auxiliary and "boost" type ports.


Transfer duct length measurement




Avrg. duct length      Measure the centerline length of each transfer passage from entry point adjacent to crank flywheel in crankcase to exit window in upper cylinder (as in diagram at right).   Add all lengths together and divide by the number of transfer passages to get the average duct length.






Effective transfer duct width measurement



Effective duct width      This is the true chordal width when measured 90 to the exiting air movement as is illustrated "W" in the diagram at right.






Transfer roof angle measurement

Upsweep angle      This is the measured angle upward from
an imaginary line drawn horizontally across the cylinder bore
as shown in the diagram at right.   If the port roof is "flat"
with no upward tilt, enter "0" here.


Corner radius      This is the average radius that forms the corner portion of a port window at the cylinder wall surface.


Duct roof height      This is the vertical distance from the top of the cylinder to the highest point of the roof of a port.


1st pair      This refers to the "Main" transfer passages that immediately flank each side of the exhaust port(s).   Some vintage model two stroke engines only employed a single pair of "Main" transfer ports.


2nd pair      This refers to the "Auxiliary" transfer passages that are adjacent to the "Main" transfers and second away from the exhaust port(s).   Sometimes they are referred to by some tuners as the "Secondary" transfers.   Many two stroke engines have only the two sets of "Main" and "Auxiliary" transfer passages.


3rd pair      When a two stroke cylinder employs a pair of transfer ports in the rear of the cylinder, opposite the exhaust port and adjacent to the "Auxiliary" transfer passages, it is sometimes described as having "Twin Boost" ports.   Some Suzuki models and a handful of others utilize this six transfer passage arrangement.


Boost duct      This refers to a fairly common transfer passage layout wherein there is a single "Boost" type transfer port located exactly opposite the exhaust passage(s) creating a five transfer port system.   In the case of the reed valve type intake design, this "Boost" port is often cut right into the top of the intake window.


Variable Height Exh. Guillotine      This would include any type of mechanism designed to alter effective exhaust roof height as engine RPM changes.  "Full down" position is the lowest exhaust roof height that this mechanism would provide as measured from the top of the cylinder deck.


Exhaust Down Sweep Angle      This is the average angle (in degrees) downward from horizontal that the exhaust passage points as it exits the cylinder bore.


Exhaust Centerline Length      This is the average centerline length from the exhaust window at the cylinder bore to the gasket surface plane where the exhaust manifold attaches.


Exhaust Port Outlet ID      This is the average inside diameter at the exhaust outlet where the manifold and its' gasket attach.  If the outlet is not round, describe it as accurately as possible to allow area to be deduced.


Auxiliary Exhaust Ports      These are small "sub" exhaust ports that some manufacturers incorporate flanking left and right of the main exhaust window and above the main transfer windows.  They join with the main exhaust passage before the exhaust manifold and assist with upper cylinder scavenging of spent exhaust product.












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