2 Stroke Porting and Polishing Tips from TJ
Tips from TJ on Porting and Polishing 2 Strokes
sponsored by www.ccspecialtytool.com/
- Determine the level of porting you wish to achieve before you start cutting or grinding. You need to know what your are aiming for before you start shooting. Determining this goal up front will also help immensely in choosing the proper tools for the job at hand. Is your goal a basic level of porting and polishing? Consider chamfering (bevel) your port openings, radius the corners of port windows and intake bridges (this is discussed in greater detail below), take time to smooth out any casting flaws or rough areas (to a degree), and look closely at the transition from the aluminum of the port to the cylinder liner, often there are mismatches here that disrupt air/gas flow into the combustion chamber. This is the minimum amount of modification I could recommend. As for achieving a full race level of porting, hate to break it to you, but you will need to go a little further than just what you will find on this article. I suggest at a minimum you invest in a nice porting and polishing kit (www.ccspecialtytool.com/catalog/porting-and-polishing-kits) and a few carbide burrs (www.ccspecialtytool.com/catalog/carbide-cutters) also there are several accessories that you may want to look into (www.ccspecialtytool.com/catalog/accessories-spare-parts)
- Take your time and pay attention when chamfering (bevel) port openings (windows). Often I see this step treated like a minor detail, rushed through with little regard to method or consistency. The key reason to chamfer a port window is to allow a smoother transition of the piston, in particular the piston ring across the port opening. This not only extends the life of this particular part, but also reduces wear and tear on the entire combustion chamber while reducing overall mechanical losses. Below are some methods to consider:
1. Your approach should be to give every edge of the port window a consistent recess and angle. Putting a sharper angle on one side of a port and a more shallow angle on the other end, can cause a the piston ring to actually “walk” or move in position (not a desired effect).
2.Use abrasives or other method to further smooth the corner of the chamfered (bevel) port opening. Use of a carbide burr or abrasive stone, by itself, can leave a “burr” to the metal (similar to the burr you would have on the blade of a knife after sharpening) that can actually damage the side of your piston, or in any case it does not help your objective. Remember cylinder liners are generally a much harder metal (Cast iron, Nikasil, and so on) than the surrounding, usually aluminum, cylinder. Use a felt buff and buffing compound to further smooth and deburr the chamfered edge. By taking your time an going through these steps you can insure a perfect and smooth transition of the piston ring into and out of the port windows.
3. Don’t go overboard with chamfering. You're not putting a ramp on the edge of your port window. Dimensions I like to advise are 1mm to 1.5mm, with or parallel to the cylinder wall, and a 0.4mm to 0.5mm depth into the port.
- Radius or round the corners (picture a rectangle, then round the edges) of the port opening (window). A rounded corner offers a much less abrupt angle to the piston and to air flow. Increasing the radius at the corner of a port opening can optimize flow even if an increase in overall port size is not desired, or possible. It is also a great practice to add at least a slight radius to the upper roof and floor of certain ports, this slight curvature will also offer a less abrupt transition for the piston ring.
- Recess and radius (only slightly) the bridge of bridged ports. This usually applies to intake ports, and sometimes exhaust ports, which are often bridged to reduce piston rock. The reason for this is the thinner metal of the bridge will heat faster and expand (due to thermodynamics) at a faster rate. This expansion can cause the bridge to bulge out into the combustion chamber and rub the bottom of the piston skirt. Very little recess is needed, about 0.05mm to 0.08mm at the apex of the radius. The radius should begin and end approximately 1 to 2mm above and below the port.
- Keep in mind port trajectory of the gas into the combustion chamber plays a large role in the characteristics of power production. A air/fuel flow that is directed at the center of the piston area will tend to produce better top end performance, while gas flow aimed at the back wall or upward into the combustion chamber will tend to yield more torque and therefore better bottom end performance.
- There is software out there that will help you immensely with your calculations on timing, port size, trajectory, and so on. (not a bad place to start www.macdizzy.com/19891hopup.htm ) (Also a guy named Tom Turner, on our user's list at www.ccspecialtytool.com/about-cc-specialty-tool , made TSR ) There are free and low cost options out there now(BiMotion and www.porting-programs.com/ comes to mind). Some have limitations but they can start to show you the ideas and principles behind port modification and design. Another option is purchasing templates that have proven design, and making slight tweaks or modifications to them.
- Exhaust Port timing can be critical to your results. The exhaust port timing, and the pipe, will determine a great deal about your cylinders performance. In general the higher the roof, or top edge of the exhaust port window is, the higher the powerband will be in the rpm range. Typically a pressure wave return time is .003 seconds. If the exhaust port begins to open at 80 degrees of rotation, the wave would return exactly when needed at 10,000 rpm because at that point, the time it takes for the piston to move from 80 degrees to 260 degrees is .003 seconds. If the exhaust port is modified to open at 100 degrees of rotation, then the peak of the powerband would lower to around 9,000 rpm, based on the same timing principle. Keep in mind that raising the roof or upper edge of the exhaust port will decrease the trapping volume of the cylinder. The earlier in the cycle that the exhaust port opens during the descent of the piston, also means the later in the rotation cycle that it closes on the up stroke. This also explains why Power Valves are great options for 2 strokes. Power valve setups can allow the exhaust port height, area, and potential volume to be varied. This will yeild a much broader useful rpm range.
-Carbide burrs are an essential part of porting (www.ccspecialtytool.com/catalog/carbide-cutters), but you need to select the right ones for what you are doing. Long shank cutters may be able to reach farther, but the shortest shank burr you can get away with will offer a greater level of control and precision. The differing patterns and cut options(diamond cut, spiral cut, and an aggressive Alumni-cut) will yeild different results, and for that matter, varying the rpm of the tool rotation will have optimal results with different cut patterns. We have several customers that find it necessary to invest in custom carbide burrs (www.ccspecialtytool.com/custom-order-carbide-cutters-and-burrs ) for a particular application.
- Dont buy into the "CNC Myth" that you gotta take your stuff to a shop with a million dollar CNC machine to get your cylinder ported. CC Specialty has several CNC machines and related equipment, and we still use the tools we sell for any porting and polishing, and even the detail work we do on tools and equipment around the shop. We have clients all over the world that use their multi-million dollar CNC's to do some of the major cutting, but 100% of our customers still use our tools to do the detail work (hint: thats why they are our customers). We have several customers that do porting work, by hand, that has ouperformed cylinders modified by the latest and greatest CNC and CAD design machines and software.
- There are some other performance enhancers you may wish to look into for certain build projects. Among these are Boost Bottles / Boost tubes, these help maintain and regulate or utilize pressure pulses in the carbs. The addition of boost ports may be another consideration. Turbo and even supercharger setups of various configurations can be very useful, however, they are often more complicated with 2 stroke tuning than with 4 stroke applications (on a tangent, I did witness a 50cc turbo 2 stroke bike that would do over 150mph, but I digress).
- Consider the altitude and average weather and humidity of the track or race you are building for, these factors can have a considerable influence on performance due to changes in Air Density. When evaluating altitude, remember there is approximately a 3% decrease in air density for every 1000 ft of elevation increase above sea level, in relation to standard atmospheric (you can find much more info about this on the web, more than you could ever hope to use). Weather and humidity also play a big role because they can change air density as well. Temperature will change air density by causing an increase when the air is colder and a decrease when it warms up. As a general rule you should enrich the air/fuel mixture as air density increases, and lean it out when there is a decrease in density. (Fun little side note, our race group at CC Specialty got reminded of this the hard way in jetski racing. We had a monster of a 550cc ported and race ready out in Texas, where this little red and yellow bullet would scream and fly faster than just about any parallel twin known, but when we brought it to Alabama for a sponsored race, it stalled every time, went back out to Texas for repairs, and BAM! it was back to being a monster, all because of the air density change!)
- MacDizzy is a long time customer and a great source of information on 2 strokes. There are several terms you will need to become familiar with when you start modifying 2 stroke cylinders so please go over this link and the related pages: www.macdizzy.com/cyl_primer.htm . MacDizzy is also kind enough to provide you with some very useful engine building formulas www.macdizzy.com/formulas.htm .
- As far as information and good advice on porting and polishing, forums can be a double edged sword. I have read some great advice and insight on certain forums. That being said, I have read a good deal of pure BS and poorly educated speculation as well. One of my favorite is that porting makes little difference, and to be honest, if I was worried about beating you in a race, I would tell you the same thing : ).
- This is a great free resource: edj.net/2stroke/jennings/
- Some advice on making a port map www.macdizzy.com/cylinder_map.htm . I recommend knowing what you got before you start changing it too much.
- Typically I don't endorse too many products, but if you are starting out in porting these little Sport Port templates can take a whole lot of the guess work out of the equation www.racelogic.com/sportport/index.shtml . If you are unable to find a template for your project at this site, there are ways to cut out templates on CNC machines. A template can help tremendously, since what may appear to be a simple enlargement of the port, might actually involve several complex radius changes. At any rate be sure to measure twice and cut once, the aluminum is easy to take out of the cylinder, but tricky to put back in.
-When you are doing major porting for a race level of tuning, remember to take an "overall" approach to the project. Increasing horsepower by increasing the amount of fuel and air that your motor can move in and out is only one piece of the puzzle (a very big piece, but still just part of the goal). Modifications to reed valves / reed blocks, or sometime disc valves are often required. Take into consideration the pulses that occur (in particular on the intake side) during normal engine operation, often these pulses will change based on rpm. Expansion chambers on the exhaust are a big consideration! Lubrication and cooling need to be taken into account, some of our customers use our tools too also increase the openings for the oil flow and modify the cooling systems (water jackets on liquid cooled) in various ways. Even the gear ratios for the transmission will need to be considered, since the power production characteristics will be altered at this level of tune.
- A few videos that can tell you a little more about using the correct porting tools, what you can do with the right equipment, and you get to see some of the RA porting tools in action youtu.be/T_BXeXoGzD8 , youtu.be/U_2JkElSAis , youtu.be/agXWUoCkGVU , youtu.be/MNJy59ZIdGY , youtu.be/s4iLffvAjbQ , youtu.be/UJbtzxtPpSs , youtu.be/rvX-BFqt2lw , youtu.be/4RLLbL4pAQ4 , youtu.be/kMvvdc8R9oU , youtu.be/GwtquXIAOrk , youtu.be/WDKiv-nQtwA , youtu.be/UK1kzEjw1Sg , youtu.be/xY1ThQIr-J8, youtu.be/T_BXeXoGzD8
-- Here is a handy link with some very good information on porting and polishing. I am not saying it is perfect but it is very handy for the DIY guy out there looking to do some 2 stroke porting www.singaporebikes.com/forums/showthread.php/68266-Porting-the-2-stroke
Do it yourself DIY porting and polishing for the begginer and intermediate tuner.
Here is a quick video about some of the basic tools I will use for porting and polishing youtu.be/N7xmKQ0_H7A