This picture is not to scale. The idea is to make a picture like this using the information from the cylinder barrel.	Make one of these. Since I was unable to find the metric graph paper I wanted, I had to use the 10 blocks to an inch variety that was available and scale it to 1 block = 1 mm. It works OK this way, but the overall drawing came out kind of large. Normally that wouldn't be important but when taking photos it was hard to get it all in. I hope I've covered everything needed to get this done. The pictures are a little light, but I think they'll get the point across. Start by drawing a circle on the paper with a radius equal to that of the crankshaft - in this case the radius is 36 mm. Draw a vertical line indicating the center of the bore. Locate TDC by using the connecting rod length (125 mm in this case). Measure it from the top of the circle - crankshaft stroke. Do the same to locate BDC. Measure it from the bottom of the circle. This shows the space swept by the piston. The next thing to do it draw in the ports. It helps to have a protractor to draw it out right - in fact its necessary.
Using the port height measurements, mark off the top of the ports. This can be done by measuring vertically from BDC - if this measurement is taken from TDC the deck height has to be included. The exhaust port is the big square on the left. In the middle is the front transfer port and on the right is the rear transfer port. These lines can be added after they have been discovered by using the degree wheel and piston stop. Either way it's done is OK. Using one method, we can check the accuracy if the other. If you want to do it from a degree wheel see Part 4 for instructions if you need them.	This is very difficult to see but using the connecting rod length I located (on the crankshaft stroke circle) the exhaust and transfer port actual opening point (in degrees ATDC). There's a line at 89° ATDC for the exhaust and 117° ATDC for the transfer ports. If done correctly this can be verified with the degree wheel. The blowdown duration would then be 28°. That's the degrees of crankshaft rotation the engine has to get rid of the exhaust gasses and drop enough in pressure to allow the transfer process to take begin. That's not much time for such an important process. In fact this seems like an area that can be improved upon.
I've added the boost port by extending it from the rear transfer port outline - all the way over to the right - it could have been put inside the front transfer port window. It is equal to the rear transfer + the extra amount shown. It could have been placed beside the other ports but I was out of room on my paper. The dark vertical line is there so I could start from a complete block for the rear port.	The corner radius measurements are then added to the drawing in order to duplicate the ports as closely as possible. There are 4 different radii to the exhaust port. I located the bottom outside radius by using the 18° measurement I found it had on its outer wall. Most software programs don't allow for this many different curves on one port - they're more set up for transfer ports.
The mean port area line has been added - this is the area of the port that is open when the crankshaft has completed 1/2 of its port time open duration for the given port. This horizontal line is added after the port open time (in degrees of crankshaft rotation) has been added. This is half of the crankshaft rotation, not half of the port window opening. This port opens at 89° ATDC - making it open for 91° (until it reaches BDC) The mean of 91° is 45.5°. Locate this point by making a mark at 134.5° ATDC (89 + 45.5) on the crankshaft stroke circle (exactly half way between when the port opened and BDC).	This transfer port opens at 117° ATDC - which means it's open for 63° before it reaches BDC. The mean of 63° is 31.5°. Make a mark at 148.5° ATDC (again the midpoint ; 117 + 31.5) to indicate the average opening point. It's easy to see why removing material at or near the top of exhaust and transfer ports is the most useful. With the mean port area line falling about 70% or more down the port window, it would be almost useless to remove material at or near the bottom of a port . Almost useless but not out of the question or something that's not recommended. Increased transfer port width will usually include the whole port.
The crankshaft stroke circle would then look like this. The dotted lines are the mean port lines - the "mean" lines are in degrees of crankshaft rotation - not half of the port height. There's a big difference between the two.	Heres what it looks like side by side. We're mainly concerned with the area above our mean port opening lines on all ports. Many software programs ask for the total port area or have a way for you to enter this data - we will have that data too.
Now to count the actual mean port area of the exhaust port. The full blocks are all one square mm each. Adding up the partial blocks takes some time. The number of full blocks is 495 (not 488 like it says above). I estimated the rest of the partial blocks.	The same thing is done to the front transfer port. There are 303 full blocks and 10 blocks at .8 of a mm width (10 * .8 = 8). There's very few odd sized blocks to count - so carefully estimating them works well.
The rear transfer port has few odd blocks.	The boost port has few odd ones too - though it is angled at 55°.
The drawing is starting to look pretty full.	Here's the final mean transfer port numbers.
From the information gathered I can say... The mean exhaust port area is 539.2 mm^2 * 2 exhaust ports. This comes to 1078.4 mm^2 or 10.78 cm^2. The mean front transfer port area is 318.4 mm^2 * 2 front ports. This comes to a total of 636.8 mm^2 or 6.36 cm^2. The mean rear transfer port area is 241.6 mm^2 * 2 rear ports. This comes to a total of 483.2 mm^2 or 4.86 cm^2. The mean boost port area is 265 mm^2 or 2.65 cm^2. After figuring in its upward angle (55°) its mean area comes down to 210 mm^2 or 2.10 cm^3. I let the software do the work on this port to figure its actual area. The chordal measurement will be fine for use here. The mean area of all the transfer ports is - including the boost port angle - 1330 mm^2 or 13.30 cm^2.. I have not yet done the intake port area. I will do it and include it soon. Since this is a reed valve engine the intake port size is going to measure quite large. I'll also count the total area of each port so we can use that data too. Knowing the mean port areas can be useful for a couple of things. It clearly shows the working area of the ports, and it shows which part of the port needs to be worked for the most improvement. During the crankshaft rotation more time is spent with the piston uncovering the lower 25% to 30% of the port as is spent on the upper 70% to 75% of it. This can cause problems at some RPM range because fresh mixture that would be trapped can sneak out the exhaust port and/or the exhaust pipe can stuff the exhaust port too soon and drive spent gasses down the transfer port tunnels or any one of several different phenomenon. This is as a result of symmetrical port timing - the piston uncovers the port at the same time it covers it back up again. It would be nice if the ports would change their height when we needed them to - in the middle of the stroke! The "mean" area of the port must be used to as much of an advantage as possible. Careful attention to detail (especially when speaking of port shapes and aim) can make a difference when working over this area of the tunnel opening. Normally this mean port time provides a good benchmark for understanding a ports time area.
There's still a lot more to do. Now that we have the actual mean port area numbers, we have good data to work with. We can use this alone or in addition to the actual area of each port without taking into account the mean. Since we know all the port dimensions - including the port corner radius, the information can be feed into 2 stroke modeling software. Exhaust ports are tricky sometimes because of their unique shape(s). Some software doesn't provide for enough different corner radii - so it's best to compute the actual area (or mean area - depending on what the software calls for) and use those numbers for the input figures - it will be much more accurate this way. Notice that once the mean area line was included in the drawings, the bottom corner radius of most ports was below the line; not useful. Drawing them in is only useful to calculate the whole port area. I completed the drawings to show what the information looks like and how it is gathered. Two stroke software will be helpful to find the target values we will be aiming for. Software is good at crunching numbers and that's what is needed next. Check the 2 Stroke Page for places to get some of it for free. Most of it will run on Windoz 95 or earlier - and much of it is DOS - I'm not sure about Windoz 98 compatibility. If you're a Macintosh user like me you can emulate a PC by installing Virtual PC on your Power Mac, then install the 2 stroke software onto its drive partition. I am running Windoz 95 within my G3 Power Mac. It runs the 2 stroke DOS applications well without any problems and is cheaper than buying another computer. You don't need much of a (PC) computer to use most of the software that we're talking about here - I think a 386 would do it justice though I understand a 486 with a math coprocessor will make it much faster. A page was written with a very good explanation of each program at the site - click here to view these detailed instructions (saved me a bunch of time)! There is also a RZ350 port map (read - Banshee with power valves). I noticed the full open exhaust port height of the RZ was quite a bit higher than the normal modified Banshee. I like the - Sin( (Arc_Width x 180 ) / (pi x Bore) ) x Bore = Chordal_Width - to come up with the width of the ports. Though the formula is neat - I always like math which can speed things along. In this case the formula will tell you the port window opening (width) along the radius of the bore if you've measured them from your cylinder map while it was opened up and laying flat. In other words it converts flat to curved - it does not figure out the actual port sizes as we have so carefully done as outlined in Part 2 - Measuring The Ports Correctly. Consider the "right way" the right way - and forget about using the formula for anything other than "eyeballing" data, however when using 2 stroke software, the chordal port width is sometimes the number needed to plug into the formula - so it is still necessary to have though I find these inaccuracies disturbing. In order to plan an engine purpose it's necessary know what it is needed for. Is it a full blown race engine? What kind of racing? Will it be used to trail ride and occasionally blast up some dunes? Is race gas a requirement? These and many other considerations are necessary to achieve success. In the example above - a big bore 300 kit - it is (was) necessary to plan its ports for midrange power. Race gas was used since the motor's uncorrected compression ratio is 14:1 (see TRX Compression / Compression Ratio Chart) and Engine Building Formulas). We'll also use this information to calculate each ports value in sec-cm^2/cm^3. To get these numbers we'll divide cylinder volume (cm^3) into the mean areas we found (in cm^2). Then we'll multiply that number by the total time in seconds the port is open - calculated from the RPM the motor is spinning and the duration of the ports timing. See the Engine Building Formulas page and specifically the Port Open Time formula. This is the old way - before software - to do the work. The software does make it faster though.
UPDATE 4/8/99 - I (finally) counted the total number of blocks of each port so I would know the actual area of each port. The total port area numbers are as follows: The total exhaust port area is 1230 mm^2 or 12.3 cm^2. The total front transfer port area is 750 mm^2 or 7.5 cm^2. The total rear transfer port area is 578 mm^2 or 7.78 cm^2 The total boost port area is 329 mm^2 or 3.29 cm^2. The total amount of transfer port area is 1657 mm^2 or 16.57 cm^2. The total amount of intake port area is 1936 mm^2 or 19.36 cm^2. Of that, 3.29 cm^2 makes the boost port.
Briefly Speaking Quite some time ago (1973) Gordon Jennings wrote a book called The Two Stroke Tuners Handbook . For it's time it was a very good book - I was lucky enough to come across a copy of it many years ago. It had a lot of information about 2 stroke engines, porting, how to understand time areas (the time a port is open at the RPM the engine is turning) and a great way to design a high output exhaust. If you could do some math, you could understand most of the book's content. Much of the information in that book has not changed as much as you would think. There have been many other books written since then about 2 strokes that are very, very good too. Many of them are publications from the SAE (The Society of Automotive Engineers), though there are others. I took a quick look at amazon.com for books on two stroke engines. Below are a few of what is listed when using "two stroke" as the search string: Two-Stroke Performance Tuning in Theory and Practice, by A. Graham Bell - 1983 Motorcycle Tuning : Two Stroke by John Robinson - 1994 Design and Application of Two-Stroke Engines (SAE), Sp 1254 - 1997 Design and Simulation of Two-Stroke Engines (R161) - 1996 The Two-Stroke Cycle Engine; Its Development, Operation, and Design, John B. Heywood, Eran Sher - 1999 Today we're still dealing with porting, time areas, angle areas (sometimes) and exhaust systems tuned for high output. Reed valves are now common and modern combustion chamber designs with flat top pistons are letting these engines make tremendous power. Another thing that has changed over the years are the materials from which the parts are made from - they are much better. In addition to this, the quality control has improved a great deal too. Also, we can use computers to help us design every aspect of an engine. Today's desktop computer is a great addition to the garage/shop. To proceed to Part 4 - Finding Top Dead Center - Click Here.