2 Stroke Cylinder Mapping - Part 6

Primer - Part 1 - Part 2 - Part 3 - Part 4 - Part 5

A New Angle

Full Disclosure To describe the porting of this barrel in more specific terms it is necessary to find additional data. The horizontal angle of the transfer port roofs need to be discovered and recorded because the angle has affect on the port area too. This can be done with a protractor or an angle finder. Using a combination of these tools will allow you to find the correct data. When it comes time to grind the ports it will be necessary to know what the ports roof angle was before starting in order to make the necessary changes - if such a change is indicated. The upward angle of the port is taken from a position of the barrel being rested in an upright position. It may be necessary and easier to find the angle of the port roof while resting the barrel upside down on a flat bench. The angle must be recorded as though it is upright though. I'll have to find and include the formula for determining the chordal area of horizontal angles - it would seem to make sense to use it on this type of measurement though if enough time and care is given to discovery it can be properly determined using conventional methods. This transfer port has a flat top. This type of roof is typically used for midrange and higher RPM power. This port is aimed 18° up. As a 5th port in combination with the flat top main port it would also make good power. This transfer port is aimed 42° upwards - kind of an unusual number but that's the way I drew it. It could happen. Port Walls In addition to the roof angle it is also necessary to record the (vertical) angle of the port walls - as viewed from above. This information is used to tune an engine to perform within a more specific range - favoring high RPM, or midrange. Where the port walls fall is not an accident. Just like every other parameter of the engine they too must be put into the equation and used to gain advantage. Here's a simple drawing that shows the angle the ports release their charge into the cylinder if looking from above. The centerline of the exhaust port is the zero degree reference point. Find The Sweet Spot  In this drawing the main transfer port has its front wall cut at a 47° angle and its rear wall is cut to 55°. The direction of the charge from the front wall is toward the center of the piston crown - its rear wall falls somewhat further back. The rear transfer port has both of its walls cut at 90°, but the rear wall has a different angle cut into it at the liner - a sort of kicker - to make the charge push forward. The direction of the front wall of this port would be directly in line with the opposing port (if I drew it) - so the released mixture will collide with each other. This helps to keep the fresh mixture in place and helps prevent it from exiting the exhaust port. The boost port is directly opposing the exhaust port, but with its steep upward angle (usually 50° to 60° upward) its charge will be released toward the spark plug. This will help roll out the spent gasses and cool the piston crown. It's loop charged!   These port walls determine where the new mixture goes. In general terms - ports that are more directly opposite each other tend to make more peak power. Range comes from aiming the charge more rearward. The combination of these angles and the upward angles of the port roofs shows a wide range of possibilities within any engine. Where - The Power The 295 cc test engine has the main transfer port roofs cut to a 3 degree upward angle. The rear ports are angled at 5 degrees upward. The boost port has its surface cut to 55 degrees - when calculated, the effective area of this port came down a lot because of that steep angle. This overall port arrangement lends itself well to midrange power. In combination with its port layout it worked well - especially when the front wall of the mains are aimed back a little further. All of its transfer ports were cut to open at the same time - usually this is more favorable to a high RPM engine - but it seemed to work well with that motor. Often the port timing is staggered to favor a wider power band - starting with the mains, ending with the boost - a couple of degrees between each is usually enough to make the power spread a little fatter. The reason an engine displays the running characteristics it does is because the sum total of the parts involved make it have a certain personality. Some of the major components here are the cylinder ports size, shape and angles - both horizontal and vertical. The transfer ports of the engine have as much to do with the way the engine runs and its behavior as the exhaust pipe. It could be said that the transfer ports deliver the spread or range of power while the tuned pipe balances the delivery and availability of that power .

Numbers   One thing I found interesting about different versions of 2 stroke software is the way in which parameters are described or units of measure are calculated or defined - and I'm not just talking about the difference between english and metric measurement. Some software will ask for the dimension of the wrist pin to crown or the wrist pin offset - the offset is normally positive and toward the direction of rotation, others won't.   When dealing with ports, either with or without irregular shapes there are terms like average width per port or low blow - which is a measurement taken on the exhaust port and is defined as being the width of that port when its measured at the height (port opening) of the transfer ports. The top corner radius and bottom corner radius are almost always needed - especially with transfer ports - and they're almost always assumed to be the same on all of these ports and there is no provision for transfer ports that open at different intervals.   One manufacturers instructions require that you guess at the blowdown duration - there is no provision for the input of that little piece of data. Another manufacturer used terminology like port attitude angles, radical attitude angle and the axial attitude angle (I like those phrases) - they also use the term ordinates, a feature used to input the more irregular shape of some ports [an ordinate is defined as being a straight line from any point drawn parallel to one coordinate axis and meeting the other, usually a coordinate measured parallel to the vertical]. The irregular port is broken down into six parts so each area can be determined independently - however there is a "fudge" factor which must be included in the upper and lower section because of the corner radius - more guess work.   I did not find any way to properly enter the shape of the exhaust port of this TRX barrel. There was always a compromise of sorts. I wanted to just enter the actual port area as measured but there is no way to do it. It seems that progress is slow among 2 stroke software developers - and that accurate data taken from a cylinder is not put to its best use.      Cut to the Chase   For the purpose of finishing this project in a reasonable amount of time, I'm just going to show the numbers as calculated from one manufacturer's software - instead of comparing these numbers to the numbers produced by other software. This is because the units of measurement between different software and the terminology used to express values are different enough that a common language needs to be invented to properly compare them - perhaps something along the line of BMEP figures. This would allow everything to be compared accurately, consistently and without compromise. I entered some port timing data into the software to figure out my time area values. I choose a target RPM of 8300 for this big bore kit along with a BMEP of 150. I've already learned that the engine was deficient in the area of blowdown and exhaust port time area - here's how it looks on paper.   RPM Piston Speed Current Values s-sq mm Exhaust port TA value Blow TA s-sq mm Transfer port TA Value 8300 3921 3.9318 13.3195 6.3341 2.9104 9.8595 Exhaust port target values needed Transfer port target acceptable range 4.5873 15.5402 10.4949 2.7351 17.4007 to 8.3342 Modified port size reveals new value 4.5807 15.5178 8.4163   From this information I can see that the exhaust port TA value is too low as well as the Blow TA (blowdown). By making the exhaust port 3 mm wider and raising the port 2 mm the numbers start to look better. Normally this would indicate a radical change - especially when raising the port that much - but in this case it was indicated because the blowdown was far too short. Even with making these changes the Blow TA value is still lagging. That's what happens with big bore sleeve kits - their displacement increases cubed (cm^3) while the port walls only grow by the square (cm^2). Additionally, because of the downward angle of the exhaust port - in relation to the bore - in some cases it is very difficult to "bring back" exhaust port timing that has been severely mellowed out due to cutting the bore so large. The larger the bore, the lower the exhaust port roof becomes. Transfer ports don't present as much of a problem in this area unless they have very steep horizontal roof angles.

Date Last Modified: 11/29/2011
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