[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

MINGRET01 a écrit:: Hello Frits, Do you have articles in good writing for translation to MOTO 73 where did you spoke to exhaust pipe. They had five articles if I'm remenber . Thank you

Mingret, I uploaded that series of articles, called 'Moto 73 expansie-uitlaten.PDF', to my 'FOS tips & concepts' for you.
You can find it at
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But keep in mind that I wrote those articles 45 years ago!

Nombre de messages : 27 Localisation : lyon Date d'inscription : 19/11/2011

Thank you for your concern. Even 45 after the basics must be identical. Do you know when Aprilia was developing their exhaust pipe, which racks did it start from?

Nombre de messages : 361 Localisation : Belgique Date d'inscription : 05/12/2011

most important : don't use the formula's from Moto73, use these latest instead :

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I'have find two different model.
Wich is the newest?
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Nombre de messages : 361 Localisation : Belgique Date d'inscription : 05/12/2011

the one I posted

Nombre de messages : 27 Localisation : lyon Date d'inscription : 19/11/2011

J'ai déjà ce document, mais comment fait on des cônes multiples avec celui-ci?

Nombre de messages : 361 Localisation : Belgique Date d'inscription : 05/12/2011

the FOS concept is a starting point. you can do with it what you want. only by doing and trying (and a lot of reading here and also in the ESE topic on Kiwi-biker) you will learn more and know what to do with multiple cones.
but first you have to find out the Speed of Sound for your engine. so start with building simple pipes until the calculated rpm of maximum power corresponds to the rpm of max power your engine makes with the pipe you built. Than you have the correct Speed Of Sound for your engine and only than you can realy start experimenting. And when things improve, you have to change the SoS again ....
it never ends, tuning an engine. the stream of "what if..." thoughts in my head never ends lol!

you can easily calculate FOS pipes with one of my excell's. here for free : [Vous devez être inscrit et connecté pour voir ce lien]

JanBros a écrit:: most important : don't use the formula's from Moto73, use these latest instead

Using the latest version is a good advice Jan, but the one you posted is not the latest. This one is:
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When in doubt, always use the FOS exhaust concept in
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JanBros a écrit:: you can easily calculate FOS pipes with one of my excell's. here for free :
[Vous devez être inscrit et connecté pour voir ce lien]

You're welcome Jan. But please do not forget to update your Excel file.

Nombre de messages : 361 Localisation : Belgique Date d'inscription : 05/12/2011

Tnx Frits.

maybe you can add a date again in your concept, like you did in the one above from 2021. than it would be easier to determine which one is the latest.

JanBros a écrit:: Tnx Frits. maybe you can add a date again in your concept, like you did in the one above from 2021. than it would be easier to determine which one is the latest.

I'll think about it Jan. My main problem is that my concepts are frequently used in third-party excel sheets.Then, if I update a concept, those existing excel sheets become obsolete but they keep floating around on the internet.
As I keep saying, I cannot take responsibility for excel sheets that were based on my concepts and I want to stress that I personally have never posted an excel sheet, so any excel sheet that carries the name FOS, may be based on my work but it is not made by me.

Nombre de messages : 361 Localisation : Belgique Date d'inscription : 05/12/2011

I'm not talking about excels Frits, but about the image you posted with your concept.
the one on the right posted by Teamlisa has "2021" in it, the next ones no longer have a date, so that's where the confusion starts .
If you could just put a date in them, it would be clear to all which is the most recent one.
I fully understand your concern's about excel's and that is why I always include your's in my excel's.

and thanks again for all your info shared with us [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 20 241515

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 20 241515

JanBros a écrit:: I'm not talking about excels Frits, but about the image you posted with your concept.
the one on the right posted by Teamlisa has "2021" in it, the next ones no longer have a date, so that's where the confusion starts .
If you could just put a date in them, it would be clear to all which is the most recent one.
I fully understand your concern's about excel's and that is why I always include your's in my excel's.

and thanks again for all your info shared with us

On the right is writen "2011" not "2021", sorry for the bad resolution.

Nombre de messages : 104 Localisation : Biblis Date d'inscription : 14/01/2017

Hello,

one question for Frits. Thanks for Frits´s special help and calculations for this engine a few years ago our Maico MD 250 ist very fast.
I´m about building a second engine and was thinking about a bigger rotary valve diameter in connection with a bigger carburetor.
Now we use a 112mm rotary valve with a 38mm Dellorto. The inlet area doesn´t allow a bigger carburetor. Usable powerband is from 7000-10.500rpm.
I want to modify the crankcase to a 125mm rotary valve to host a 42mm carburator. VM or DEA.
Frits: would you think this is a usefull method to increase midrange power? What are your thoughts?
Maico MD 250: bore 76mm, stroke 54mm rod length 137mm

Best regards,
Bernd

Döllinger a écrit:: Hello, one question for Frits. Thanks for Frits´s special help and calculations for this engine a few years ago our Maico MD 250 ist very fast.
I´m about building a second engine and was thinking about a bigger rotary valve diameter in connection with a bigger carburetor.
Now we use a 112mm rotary valve with a 38mm Dellorto. The inlet area doesn´t allow a bigger carburetor. Usable powerband is from 7000-10.500rpm.
I want to modify the crankcase to a 125mm rotary valve to host a 42mm carburator. VM or DEA.
Frits: would you think this is a usefull method to increase midrange power? What are your thoughts?
Maico MD 250: bore 76mm, stroke 54mm rod length 137mm
Best regards,
Bernd

Hi Bernd, with 76 mm bore and 54 mm stroke, the Maico MD250 is an ultra-short stroke engine and for a two-stroke this is unfavourable because of the limited blowdown and transfer angle areas. 10,500 rpm is a very high rpm for this engine.
A 42 mm Ø carburettor on a 245cc engine is fine by itself and if you can achieve that, you may be able to advance the closing timing of the intake disc 5° to 10° from the value you currently have. That may well improve the mid-range.

Nombre de messages : 104 Localisation : Biblis Date d'inscription : 14/01/2017

Hi Frits,

thanks for your fast reply! So far we had no problems with the revolution limit. The crankshaft is quite steady and got big bearings 25x62x17mm.
Never had any issues with broken needle cages or crank webs driftring apart.
For the bigger rotary diameter i have to cut out a piece of the crankcase and bolt in a titanium part with integrated (solid) cylinder-studs because there will be no space for threads going inside the crankcase. This piece will form the upper part of the rotary inlet aswell.
The primary sprocket will have more teeth to save the super-mini Maico clutch for overload.
Photo of the DIY-primary cover with DIC-clutch action in titanium, beryllium-copper and hardened steel gears.
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Best regards,
Bernd

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