13 replies to this topic

[Alexandar :D]

Nasao sam jedan jako interesantan link doduse na engleskom ali ima jako lepo sve objasnjeno oko mapinga,time area parametara sa primerima kako i sta raditi pa i formulama.Napominjem da je preterano dobro objasnjeno pa ko hoce da cita nek izvoli

http://www.macdizzy.com/cyl_primer.htm

[perlx*]

Zanimljivo, hvala.
Premestam u dokumentaciju.

[Alexandar :D]

prebaci,doduse ja sam probao da izvucem slike tj text sa sajta ali nisam uspeo :/

[cvele]

Armed with only a piece of paper, a pencil and a piece of tape I'll reveal this engines ports in a matter of minutes.

Start with a piece of fairly stiff paper. It needs to be only a little taller than the liner.

Tape it in place inside the cylinder. It should fit tight against the bore. Make marks to indicate the top and bottom of the liner.

Using the edge of a pencil, rub it against the transfer port walls. The dark port outline will soon be easy to see.

The exhaust port becomes easy to see. You'll find the right amount of tension with the pencil to get an outline like this.

The intake port reveals itself. Usually its necessary to work the pencil from the top and bottom to get good results.

The port walls traced and easy to see. It will look like this when its done.



Take it out and lay it flat.

Tape it together so the marks are on the outside.

There is your basic port map. Even the most advanced porting software needs information like this to make accurate predictions. The next thing to do is to measure the area of the ports. When measuring the ports keep in mind the width of any bevel which might be on the port. Many times a transfer port bevel is .5 mm wide - that's .5 mm all the way around the port, so it is a good idea to subtract this from the measurement before you write it down - or whatever the actual bevel width is. This would be easier if the ports were flat, but they're not so we have to do it the hard way - transfer the real dimensions to graph paper.

To proceed to Part 2 - Measuring The Ports Correctly - Click Here.

[cvele]

When measuring a port window, the idea is to get an accurate width without including the radius of the cylinder wall. Since most ports enter the cylinder at an angle, it has to be accounted for. If you follow the wall when measuring the port your readings will be off - they will show you to have more port area than you actually have.


Measuring the port this way will give much better overall accuracy. The idea is to get the real port window size.


If it is measured like this, the ports will measure much larger than they really are.


I have cut the port window open and left the paper hanging so I can use it as a stop. I put its piston inside to make it rigid.


The exhaust port looks big but when measured properly its size comes down significantly.

Measuring it. With the ports cut open (using the X-acto knife) I can now set the hanging piece of paper so that I can use it to measure against so I will get an accurate port size. I turned the exhaust port inwards since it makes it so easy to measure it this way. Following the "right way" from above I found that the total width of the front transfer port (main transfer port) was actually 25.8 mm. When measured the "wrong way" it showed 27.1 mm. The rear transfer port (5th port) has a larger difference because of the angle it intersects the cylinder. The "right way" shows it to be 20.0 mm and the "wrong way" has it at 24.5 mm. The left exhaust port measures 27.5 mm and 31.2 mm if measured wrong. The boost port measures 22.6 mm across its face.

The idea is to get "real" sizes when measuring the ports so we'll know the actual useful area. Subtract 1 mm from the total width to account for the .5 mm bevel width on the transfer port sides. The final numbers are 24.8 mm (front transfer - main), 19.0 mm (rear transfer - 5th) and 21.6 mm (boost). The exhaust has a wider bevel - it's 1 mm so I end up with 25.5 mm for its total width.

The height of the ports can be measured more directly. Sometimes transfer ports enter the cylinder at steep vertical angles too making things more difficult. Though this cylinder - a 72.25 mm bore ATC 250R barrel - has transfer ports which enter the cylinder nearly perpendicular to the bore. They all measure 17 mm tall - after I subtract the 1 mm (top bevel only) it nets a 16 mm vertical height. The exhaust port is 33.7 mm tall. The port has a 2 mm bevel on its top edge, so the final number is 31.7 mm. Don't count the bottom bevel when it's below BDC.

The intake port is 47.2 mm wide and only has a .3 mm bevel width. Subtract .6 to end up with 47.6 mm for its final number. The total port height is 62.2 mm - so I'll lower it to 61.6 to account for its bevel.

Before drawing the ports on paper it is necessary to measure the corner radius of all the ports so their shape may be duplicated. I went around and checked the port corners against my circle template so I could determine the port corner radius. The transfer ports and boost port have a corner radius of 3.5 mm. The exhaust port has a radius of 6.7 mm on the top inside, 4.7 on the top outside, 12.5 mm on the bottom outside and 7.1 mm on the bottom inside. The exhaust port also has an angle to its outside wall which connect the outer sides. This angle is 18 degrees.

When all the ports have been measured it is time to reconstruct the ports on graph paper to find the mean port area and/or actual port area. This is a little tricky - especially with unusual exhaust port shapes. All of the above information will help recreate these very accurately. The engine I'm working on has a stroke of 72 mm and a connecting rod length of 125.3 - I'm going to use 125 mm for my drawing.

I'll recap the final measurements of each port.

Exhaust port - Width : 25.5 mm. Height : 31.7 mm. Corner radius : Inside top - 6.7 mm. Outside top - 4.7 mm. Outside bottom - 12.5 mm. Inside bottom - 7.1 mm. The angle of the outside wall when viewed from the front is 18 degrees.

Front transfer port (main port) - Width : 24.8 mm. Height : 16 mm. Corner radius : 3.5 mm.

Rear transfer port (5th port) - Width : 19.0 mm. Height : 16 mm. Corner radius : 3.5 mm.

Boost transfer port (bust port) - Width : 21.6 mm. Height : 16 mm. Corner radius : 3.5 mm top sides only. The steep upward angle of this port (55°) still has to be accounted for.

Connecting rod lengths to use. TRX250R 1986, ATC250R 1985-1986 - 125.3 mm

TRX250R 1987-1989 - 130.3

YFS Blaster 200 1987-1998 - 110

YFZ Banshee 350 1987-1998 - 110

Piston Strokes for the same engines. TRX250R 1986, ATC250R 1985-1986 - 72 mm

TRX250R 1987-1989 - 72 mm

YFS Blaster 200 1987-1998 - 57 mm

YFZ Banshee 350 1987-1998 - 54 mm

What's Next? With this information we can now transfer it to graph paper. To proceed to Part 3 - Putting It On Paper - Click Here.

[cvele]

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.

[cvele]

With the help of a few tools, the engine porting can be discovered. There are software programs which aid in engine development but cylinder mapping still has to be done first. Without accurate input, port changes will not be correct.


Some basic tools - rulers, depth gauge, inside caliper, micrometer, X-acto knife, pen, pencil, compass, feeler gauge, black felt tip marker and a small hole template.


This is an indicator set which locates TDC by using a plunger to touch the piston crown. It threads into the spark plug hole. A piston stop is slightly more accurate for it though.


The basic piston stop. Using this tool you will be able to locate true Top Dead Center (TDC) - even with a flat top piston. It is used with a degree wheel. I had to draw this because I can't find mine - I know it's here somewhere.


I looked all over my garage for this thing - it was on my Yamaha GP 760. I started mapping it recently in order to calculate the port sizes before cutting the ports. This engine has been dialed in already. The bent cotter pin is indicating TDC.

Finding TDC Using A Piston Stop Locating TDC is necessary to properly degree in an engine. With the piston installed in the cylinder - and the head off you can do it easily. The piston stop is a very important tool and it is very easy to make. Measure the distance between centers of two opposing cylinder head studs and make a note of this distance. Start with a piece of 3/8" thick aluminum about 1 inch wide and long enough to cross your cylinder. Mark the aluminum for drilling the holes (your measurement). Use a center punch to help locate the holes while drilling. Drill the holes large enough to fit over your cylinder studs. If it doesn't fit you may want to dress it with a small round file to open up the holes to make it fit.

If your motor has a domed piston and its dome goes above the deck at TDC - you're all done. If you have a flat top piston you'll need to install a bolt in the center of the piece, directly over the center of the piston - as close as you can get it. Drill a hole and tap it for a 1" long x 1/4" thickness fine thread bolt. Put a nut on the bolt then screw it in the aluminum.

In order to locate TDC you'll now need a degree wheel installed on the end of your crankshaft. Many degree wheels come with washer sets so they can be used on a variety of engines. It is not necessary but it is easier if you remove the flywheel during this procedure. The flywheel magnets will want to be pulling - so it's best to remove it. Once the degree wheel is installed leave it a little bit loose so you can move it when you need to. You'll also need to provide a pointer to indicate your wheel. The pointer is easy to make from a nail or similar piece of metal. Twist it so it can be tightened under an engine side cover bolt. Make sure it is fairly rigid. It doesn't matter where it points - yet.

Turn the engine over by hand until the piston is at TDC - eyeball close is good enough. Rotate the degree wheel and set it to TDC. Now put the piston about half way down the cylinder bore. Use a couple of cylinder head nuts to tighten the piston stop to the engine - it has to be snug. Turn the crankshaft clockwise until it touches the piston stop - do not force it - this is a light touch procedure. Make a note of the reading on the degree wheel. Now rotate the crankshaft the other direction until it touches the piston stop again. Note the reading on the degree wheel. True TDC is directly in the middle of the two readings. Adjust the degree wheel so that when you rotate the crankshaft in either direction it will stop at the same reading though one will be BTDC and the other will be ATDC.

If you set the degree wheel so the piston stops at (for instance) 20 degrees BTDC when you turn it clockwise and 20 degrees ATDC when you turn it counter clockwise you have located true TDC. It may take a couple of tries to pin point it. When it's where it should be tighten the degree wheel so it won't move. Do the procedure again. You can make small changes by bending the pointer.

Finding TDC Using A Dial Indicator Similar to above except that after the indicator is screwed into the spark plug hole the piston is brought up until it touches the plunger. Stop it when the indicator needle has traveled 1 mm. Make a note of the reading on the degree wheel. Turn the crankshaft the other direction. Stop when it reads exactly the same as it did when turned the other direction (1 mm travel). Make a note of the reading on the degree wheel. Again, true TDC is directly between the two readings. Adjust the degree wheel accordingly. It is also possible to just let the indicator travel until it indicates the piston will not travel any further. However, there is a small margin or error in this method.

That Degree Wheel Since the degree wheel is now set up, why not make a note of the port timing. Rotate the engine over until the edge of the piston is just at the bevel to the exhaust port. You know you're very close to where it actually opens. Within a degree or two you'll see the actual port open. This is the mark to write down. Do the same for the transfer ports and boost ports. All this information is useful when trying to determine the state of tune of an engine.


This piston is at TDC. Note that it's not flush with the deck - the top of the cylinder liner. We need to know its distance below the deck. We can use a feeler gauge to find out what it is.



Carefully match the thickness of the feeler gauge to the space that's there. Feel it with your finger. If there's a lip, change it until it's smooth. In this case the piston is .024" (.61 mm) below the deck.

What The Deck This motor has a -.61 mm deck. That is to say, the piston when at TDC is not flush with the edge of the cylinder liner - it is .61 mm below it. If there is a deck height difference to consider, it should be accounted for when transferring the images to paper. When making a drawing by taking the measurements from the top of the ports and proceeding from there - the drawing must be adjusted to compensate for the difference in the deck difference. Also, engines with base plate spacers will need to have the spacer included when calculating the port timing this way. It is necessary to add the thickness of the extra base gasket as well.

It is necessary to know the deck height of a cylinder that is having its compression changed or when it is getting the squish band set properly. If I wanted to set the squish at 1 mm, I would have .039 mm to work with. I could use a copper head gasket of that thickness (about .015") to achieve it.

See Part 5 What To Do With The Data.

[cvele]

What is known about this test engine. This engine has a bore and stroke of 72.25 mm x 72 mm. This calculates to an engine displacement of 295 cm^3. It's connecting rod is 125.3 mm - center to center. The exhaust port opens at 89° ATDC for an open duration of 182°. The transfer ports open at 117° ATDC for a duration of 126°, leaving a 28° blowdown period. The cylinder head volume has been cut to 25 cc's, the head gasket is 0.5 mm thick and it is 72 mm in diameter. The deck height is -.61 mm. This makes the engine have a trapped volume of 162.76 cm^3 (figured from the top of the exhaust port) for a compression ratio of 8.17:1 or an uncorrected compression ratio (UCCR) of 14:1. This engine produces 180 PSI static compression. The minimum octane gasoline run through it should be about 100.


Time Area It should be clear to anyone working on the ports of a 2 stroke cylinder that as the speed of the crankshaft increases, the time a port is open becomes shorter. The fast moving piston opens and closes the ports by sliding by them at incredible speed. That causes problems to a certain extent. Big ports will pass a lot of mixture at high RPM, but it may make the power delivery unacceptable at moderate RPM's - where the engine spends most of its time. Raising ports to make them open longer is one way to make them larger but doing that usually has trade off's as well - it may raise the speed at which good power delivery is available so high that the engine becomes unusable, too peaky or unpleasant to drive. Fortunately ports can be made wider, which will not increase their duration but still give increased flow due to increased area. Doing this is called increasing the time area because the port is able to pass more mixture (or exhaust) in the same amount of time due to an increase in the port width. The port tuner finds a combination of time and area to match to needs of the engine and its intended purpose.

The volume of gasses that can flow through a port tunnel is limited by the tunnel size and/or the port opening. Therefore if the port is opened up in the cylinder it may not pass any additional gasses unless its tunnel is increased in size as well. If you push air down a hallway which has to exit through a door at its end which is the same size as the hall, simply installing a larger door will not let more air pass through the hall - even if the end of the hall gets bell mouthed to accept its additional size. Even if the door gets attached at an angle so that it has an increased area - the restriction is still the diameter of the hall. It is necessary to widen the hall and install a wider door to get more air through.

Sec-cm^2/cm^3 Sec-cm^2/cm^3 is the way to say that we divided the cylinder volume (in cm^3) into the mean area (in cm^2) of the port we're questioning. We then multiply that number by the time in seconds the port is open. We can figure out how much time that is by looking at the Port Open Time formula which converts the engines port timing in degrees of crankshaft rotation into real time.

T = ( 60 / N ) x ( Z / 360 ) or T = Z / ( N x 6) T is time, in secondsN is crankshaft speed, in RPMZ is port open duration, in degrees Using this simple formula we can find out how long a port is open at any given RPM and from that we can determine its time area. Using an 8000 RPM limit as an example I can tell that the exhaust port of this test engine is open for 0.00379 seconds (60 [seconds in a minute] divided by 8000 RPM [crankshaft speed] multiplied by 182 [duration in degrees of crankshaft rotation the port is open] divided by 360 degrees [a full circle] - or 60 / 8000 * 182 / 360). Since the displacement of this cylinder is 295 cm^3 and the mean exhaust port area is 10.78 cm^2 we divide the displacement into the mean port area and arrive at 0.0365. Multiply that by the time the port is open - I get 0.000138 sec-cm^2/cm^3. Additional units of measure One of the other popular units of measure you'll see is s-sq mm (sec-square mm). This figure represents the port duration in degrees of crankshaft rotation divided by the time the port is open in seconds, multiplied by the mean port area (in square mm). Another one is s-mm^2/cc*10^3 - also referred to as TA value or time area per unit of displacement. This formula takes the mean port area in (cm^2) and divides it by the displacement of the engine (in cm^3) then multiply that by the time the port is open in seconds - it looks an awful lot like sec-cm^2/cm^3 - time area. Angle Area The angle area is a number which can be helpful to determine proper port timing as well. It might be thought of as a sort of short cut to quickly determine port areas. Because angle area does not consider the speed of the crankshaft it is necessary to use both the time/area and angle/area calculations to help determine the final area. Using the example above - start with the mean area of the exhaust port already divided into the displacement of the cylinder (10.78 cm^2 / 295 cm^3 = 0.0365) and multiply that number by the 182 degrees of the ports open duration (0.0365 * 182 = 6.643). The angle area for this port would then be 6.643 deg-cm^2/cm^3. This number is useful to compare to a chart which has some established values outlined for particular types and sizes of engines. The example below is very basic. For an engine turning at 8000 RPM the "most useful" range for the exhaust port angle/area is from about 5.8 to 9.2 - looking at the chart I can see that the 6.643 deg-cm^2/cm^3 falls on the low side of the range.
Update - 5/4/99 The area needed to obtain the target RPM varies within the range of engine parameters - there is overlap. This chart provides no exact answer - it is only a guide. Look at the exhaust and transfer port angle / area numbers independently. The red and blue lines represent extremes. As an engine spins faster, more time area / angle area is needed for the port to do its job. In 1973 Jennings decided on transfer port time area boundaries of .00008 to .00010 sec-cm^2/cm^3. The exhaust port time area boundaries were .00014 to .00015 sec-cm^2/cm^3. For a piston port engine the intake time area was .00014 to .00016 sec-cm^2/cm^3 and a rotary intake valve time area is.00018 to .00019 sec-cm^2/cm^3. I made this chart to reflect a more modern engine design. Its peak values are spread quite a bit farther apart than they were 25 years ago. The range is quite different today because we are dealing with engine designs that take advantage of as many design improvements as possible. Today we have exhaust valves, boost bottles, super efficient exhaust systems, huge crankcase volumes, reed valves, high output digital ignitions, liquid cooling and other wonderful advances in technology like Nikasil and other super slippery bores. All of these things stretch the limits that were established back when 2 stroke engines were being uncovered and discovered - they all added power, reliability, fuel economy or drivability to the machines that carry them.

The angle / areas that Jennings listed looked something like this:

Angle Area - deg-cm^2/cm^3 RPM Exhaust Transfer Piston Port Intake Rotary Valve Intake 4000

3.5 to 3.7 1.8 to 2.5 3.5 to 3.9 4.5 to 4.8 5000

4.2 to 4.5 2.4 to 3.0 4.2 to 4.8 5.5 to 5.8 6000

5.1 to 5.4 2.8 to 3.6 5.1 to 5.7 6.5 to 6.9 7000

5.9 to 6.2 3.4 to 4.2 5.9 to 6.7 7.6 to 8.0 8000

6.7 to 7.2 3.9 to 4.9 6.7 to7.7 8.7 to 9.1 9000

7.6 to 8.1 4.4 to 5.4 7.6 to 8.6 9.8 to 10.3 10000

8.4 to 9.0 4.8 to 6.0 8.4 to 9.6 10.8 to 11.4 11000

9.3 to 9.8 5.4 to 6.7 9.3 to 10.6 11.9 to 12.6 12000

10.1 to 10.7 5.7 to 7.2 10.1 to 11.5 12.9 to 13.7 13000

10.8 to 11.6 6.2 to 7.8 10.8 to 12.4 14.0 to 14.9 I noticed the time area of the exhaust port on the test engine was less than the minimum and its angle area shows up here as having too low of a value as well - this would back up what I suspected early on when transferring the port dimensions to paper. The blow down duration is too short for good power at high RPM. With an efficient pipe working on the engine it is very possible that the reflected positive (stuffing) wave that returns to the exhaust port outlet will arrive too soon and perhaps stuff some exhaust back into the transfer ports - not just prevent the escape of fresh mixture out the exhaust.

To proceed to Part 6 - A New Angle - Click here.

[cvele]

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.

[cvele]

E sad jos biaxe u sake i -udri ......

[Boza]

Kolko sam uspeo da provalim sa slika(ne mogu da citam) on je analizirao cilindar bez programa?

[Mario]

i Jennings je dao obrasce i metode u svojoj knjizi kako se to radi pre 40 godina, a nije bilo programa

[Boza]

Ne isplati se citati onda... Program kosta 15e, a da bi mi neko ovo strucno preveo da bih mogao da razumem sve, puko bih bar 10x vise..
Ko odlicno zna engleski i poznaje strucne termmine lepo je imati ovo...

[Alexandar :D]

Pa evo ja mogu reci da nema toliko strucnih termina koliko mislis da ima,a one sto ne razumes nadjes na google.Jedino sto treba znati ipak malo vise od prosecnog engleskog.