Janbros' 2-stroke excel for ports, heads and pipes

thought I'd better open a topic to keep track of updates/corrections.

My 2-stroke spreadsheet is ready (for now at least), so I'm throwing it on the net so everybody can use it.
It contains pages for ports, the head and FOS pipe
On the seperate ports pages (exhaust and transfer), it calculates the STA numbers (Specific Time.Area) of your ports and you can matcch them to the target's, determinned by the BMEP
On the Head page, you can design your head and it calculates the MSV (Mean Squish Velocity) 100% accurtly with Blair
On the FOS-pipe page, it calculates a Frits Overmars FOS pipe, and you can ad a Wobbly-duct , and you can add extra header/diffusor/baffle sections to the pipe.

you are free to do with it what you want, as long as it is not for commercial use. If somebody has idea's to improve or add extra features, please share so we can all benefit.
In the manual I've tried to explain how it works, please read it first before you ask questions.

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new link as of 21-07-24 due to update of Frit's FOS concept and a small error in the calculation of "Deck-Aux Open" calculation which had no effects on the STA's:

excel 2.2 : [Vous devez être inscrit et connecté pour voir ce lien]
doc 1.2 : [Vous devez être inscrit et connecté pour voir ce lien]

and a stand alone Exhaust, which has an extra exhaust-comparator sheet
FOS 3.1 [Vous devez être inscrit et connecté pour voir ce lien]

If you put your intellectual property on the net, I strongly recommend that you add a license; a license allows you to avoid the application of your work for purposes you do not endorse (such as, in your case, commercial use). One of the best options is one of the so-called "Creative Commons" licenses, which are specifically designed to license individual creative endeavours. There are several licenses available, check the website:

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You can choose a Creative Commons License, for example, in your case, you could choose "Attribution-Non Commercial-Share Alike International 4.0". The process of adding the license is easy, just type in the original Excel file: "This work is protected under the XXX license". If you go to the website of Creative Commons, they will give you a hyperlink to put into your work.

As another point, I am involved with various "open software" developments and I know from experience that quality control is very important. I encourage you to write a document specifying specifically all of the equations you use in your work. It will make your own development easier, it will help other users to understand your work, and most of all, it will help others to find and solve any mistakes you may have made.

merci JanBros
magnifique travaille

Thanks again Janbros !!

Congratulations Jan Bros,

As my son and I did something similar (only in a raw version and not that extended) we know how much work this must have been for You. Deep respect
We will dive into it ( I hope to start this evening) to look for deviations between our calculations and yours just to verify and learn.

Regards Maurice

good job Jan Bros
thanks !

some more corrections for "IF" errors in situations I hadn't tested yet, and added a page you can print and take into your shed to write down all the needed input data.

Thank you very much for doing this great job and sharing it.

Great work, that's impressive !

Hello, I have some doubts with the JanBros Excel translation. I do not understand any concept to complete your data sheet. I upload a photo with my deduction on how to take the measures to correct me.
Thank you[Vous devez être inscrit et connecté pour voir cette image]

I measure crown height from top of piston to top of piston-pin bore, so you should deduct half of the piston-pin diameter from your crown height.

I already realised piston-pin is not mentionned in the spreadsheet, so I'm reworking it a bit and am seperating all the moving parts from the non-moving parts between old and new, so that when somebody uses a bigger/smaller piston-pin, the difference will be counted for.


If anybody has requests for some changes, now is a good moment ;-)

JanBros a écrit:

I measure crown height from top of piston to top of piston-pin bore, so you should deduct half of the piston-pin diameter from your crown height.

I already realised piston-pin is not mentionned in the spreadsheet, so I'm reworking it a bit and am seperating all the moving parts from the non-moving parts between old and new, so that when somebody uses a bigger/smaller piston-pin, the difference will be counted for.


If anybody has requests for some changes, now is a good moment ;-)

Thank you very much for your answer, I upload a new corrected image.
Another question, Deckheight = Squish?
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deck = top of the cylinder
deckheight = distance from top of cylinder to pistonedge.

so if the piston sit's lower than the deck, you enter the distance as a positive number. if the piston comes above the deck, you enter the distance as a negative number.

squish = the outside area of the combustionchamber , between points 1 and 2

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squishheight = distance between top of cylinder and squish area of combustion chamber where the distance is lowest (in case of a tangent squish area, when squish is paralel, it should be the same between points 1 and 2).

Thank you very much for your answer, I understand it perfectly. My native language is Spanish, I don't know the technical concepts in English.

Hello Mr. Janbros,

I am trying to understand more about how to tune a 2 stroke by looking at your excel sheet and read the discussion which is managed by Jan Thiel and Frits Overmars. Therefore, I would like to ask some of question about your excel.

1. The whole idea of the excel, apart from the sheet Head and the FOS pipe is we strive for achieving the target Blow down and Transfer STA, right? Considering that these STA numbers for blow down (8.12) and Transfer (66.16) are achieved from RSA 125 which has >1 exhaust port, is it possible to get the same number for a single-exhaust port 2 stroke machine? Or do we have to adjust/tune the port of this single exhaust port cylinder as close as possible to that STA target?

2. I see that in cell AK28: AO45, there is a calculation for STA target. I am interested in the Blair formula for both BD and Transfer. For BD, it is written as (BMEP-1.75)/0.8187/2. In his book, it is (BMEP-1.75)/8187 (pg. 261, basic design 2 stroke, Blair). I assume that 8187 to 0.8187 is conversion from meter to millimeter, but what is the purpose of dividing it by 2? This also works for Transfer where you divided it by 4.

3. The width that we put in the port map data for both exhaust and transfer. What kind of width is that? How should we obtain the width?

4. What is the definition of cylinder height?

Thank you in advance for your help.

rataraditya a écrit:

1. The whole idea of the excel, apart from the sheet Head and the FOS pipe is we strive for achieving the target Blow down and Transfer STA, right? Considering that these STA numbers for blow down (8.12) and Transfer (66.16) are achieved from RSA 125 which has >1 exhaust port, is it possible to get the same number for a single-exhaust port 2 stroke machine? Or do we have to adjust/tune the port of this single exhaust port cylinder as close as possible to that STA target?

2. I see that in cell AK28: AO45, there is a calculation for STA target. I am interested in the Blair formula for both BD and Transfer. For BD, it is written as (BMEP-1.75)/0.8187/2. In his book, it is (BMEP-1.75)/8187 (pg. 261, basic design 2 stroke, Blair). I assume that 8187 to 0.8187 is conversion from meter to millimeter, but what is the purpose of dividing it by 2? This also works for Transfer where you divided it by 4.

3. The width that we put in the port map data for both exhaust and transfer. What kind of width is that? How should we obtain the width?

4. What is the definition of cylinder height?

Hi,

1/ you'll never be able to have enough blowdown-time.area with a single exhaust port, or if you'd try you'd end up with rediculous exhaust timing. compare it with a four-stroke using only 1 exhaust valve vs 2 : it's impossible to exit the same amount of exhaust gas in the same time through a smaller hole.

2/ honestly, I can't remember. probably because it was the only way to make the numbers work with all the data Frits and Jan have given. if you don't devide them , the STA numbers ain't comparable.

3/ that's explained in the doccument youcan download. you enter the circumferential (is that english ?) width , the one you measure on the portmap, not the chordal width.

4/ it's the height drom the top of the cylinder to the flat part at the bottom that sits on the crankcase. but there is a problem with that, as if you give a different height for the new one, it's not specified where the difference is compared to the ports : below or above the ports. I'm reworking it, but it is not on my priority list at the moment.

Dear Mr. Janbros,

Thank you for answering my question . Here's my comment:

1. So basically, maybe what I can do now is taking the recommended exhaust and transfer duration by Frits, 190 deg and 130 respectively, and adjust the ports width up to mechanically accepted value. And compare it to the STA values. What do you think?

2. Okay, I will follow that formula then.

3. Yes, after I check your excel cell again, I see that there is a formula to convert the width that we enter in the port map cell into chordal width. Moreover, it is written in the manual. Sorry for not reading that

4. I see. I try to change it and it doesn't affect the STA, Power or duration. It only change the values under the Port Data in Data Sheet. So the best way to put the correct number of this cylinder height is by entering any number of cylinder height so that the value under the Port Data has the same value with real distances of the ports ( example: Deck-Main Bottom in Port Data = Deck-Main Bottom in real measurement). What do you think?

Thanks for your help.

rataraditya a écrit:

Dear Mr. Janbros,

Thank you for answering my question . Here's my comment:

1. So basically, maybe what I can do now is taking the recommended exhaust and transfer duration by Frits, 190 deg and 130 respectively, and adjust the ports width up to mechanically accepted value. And compare it to the STA values. What do you think?

2. Okay, I will follow that formula then.

3. Yes, after I check your excel cell again, I see that there is a formula to convert the width that we enter in the port map cell into chordal width. Moreover, it is written in the manual. Sorry for not reading that

4. I see. I try to change it and it doesn't affect the STA, Power or duration. It only change the values under the Port Data in Data Sheet. So the best way to put the correct number of this cylinder height is by entering any number of cylinder height so that the value under the Port Data has the same value with real distances of the ports ( example: Deck-Main Bottom in Port Data = Deck-Main Bottom in real measurement). What do you think?

Thanks for your help.

1/ you can not expect your engine to get the same BMEP as an RSA. enter the BMEP you think you'l be able to distract from it, and than try to match the numbers. if you can't lower BMEP.

4/ as it is now, the height difference is always at the top, so if you enter a higher height for the new cylinder, it only changes the distance deck-ports. that change has no effect on the ports themselfs.

Hi Janbros,
first of all: Great Job
There is a thing I would like to know about this "out of breath".
Is it the rpm where the torque (or power) curve is not raising any more and drops?
Or is it the rpm where the engine doesn´t rev higher?
I have read the RSA topic but couldn´t find an answer.

Thanks in advance
Frank S.

Frits explained it acouple of times, but it is the rpm after which torque lowers again, because the engine can't get more mixture in the cylinder to raise torque further.
for example : if your goal was to get max torque at 10000 and "out of breath" only says 9000, you either aimed too high or you have to enlarge the corresponding angle.area.

Merci beaucoup,
I thought so, but I was confused because my Inputs gave the exakt RPM, where my engine stalls. Strange coincidence 🤪
Now it is clear and I habe readjusted my inputs.
Greets Frank

JanBros a écrit:

thought I'd better open a topic to keep track of updates/corrections.

My 2-stroke spreadsheet is ready (for now at least), so I'm throwing it on the net so everybody can use it.
It contains pages for ports, the head and FOS pipe
On the seperate ports pages (exhaust and transfer), it calculates the STA numbers (Specific Time.Area) of your ports and you can matcch them to the target's, determinned by the BMEP
On the Head page, you can design your head and it calculates the MSV (Mean Squish Velocity) 100% accurtly with Blair
On the FOS-pipe page, it calculates a Frits Overmars FOS pipe, and you can ad a Wobbly-duct , and you can add extra header/diffusor/baffle sections to the pipe.

Hi JanBros,
On and off all my life trying to write my excel file (in the 90s were using Lotus 123), just for the expansion chamber, most of the time run out of branches, thoughts, ideas and energy.
I think you did a fantastic job! - even thou it might be not flawless.
I will compare the outcome on paper, I like to see what differences we ending up from you and me.
It will take me some time (which I never have), but hope to see the outcome soon.
Bless
AKos

another smal error reported, unnoticed as I always use the wobbly-duct

The error is in cell P19. It refers to the percentage of the 2 stage diffusor in cell M18
but it should refer to cell R19, which is the percentage of the 3 stage diffusor.

change it yourself or download a new version, use same link in the first post.

Hi JanBros,
Is the link still working. I can’t download it.
Thanks
Dan