2 Stroke Cylinder Mapping - Measuring The Ports Correctly

2 Stroke Cylinder Mapping - Part 2

Primer - Part 1 - Part 3 - Part 4 - Part 5 - Part 6

Measuring The Ports Correctly.

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.

Primer - Part 1 - Part 3 - Part 4 - Part 5 - Part 6

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.