250cc Air box design

Hello all
I am in England I apologise I do not speak French, but I hope to get some advice.
We race a 250cc twin cylinder 2 stroke road race bike.
I would like to improve the air box by making it larger, at present it is about 5 lts volume.
I can make it larger but I'm wondering is there a good volume to aim for.
Also pressurised or still which is best.
If there are any discussion on air box design could someone point me in the right direction.
Many thanks

Len

Bonjour,

D'abord c'est pour une utilisation "route" ou "circuit" ?

la boîte à air doit faire entre 10 et 20 fois la cylindrée totale soit 250 cc x 10 = 2,5 litres minimum mais mieux 250 cc x 20 = 5 litres

La pressuriser c'est pour le circuit mais les réglages (par exemple diamètre de l'entrée d'air, longueur du manchon, taille des gicleurs, de l'aiguille, du puits d'aiguille, etc) sont difficiles à trouver et souvent cause de serrage.

Voilà ce que je peux dire pour la base de travail. Mais tu ne nous explique pas quelle est ta machine, quel est l'usage que tu veux en faire, quelle est sa préparation (pot d'échappement, squish, etc). Avec ces éléments quelques personnes ici comme Frits Overmars ou Francis Payart pourront être d'un avis éclairé mais à distance sans voir la moto c'est très difficile car il y a beaucoup de paramètres à prendre en compte.

Merlin7 a écrit:

Hello all
I am in England I apologise I do not speak French, but I hope to get some advice.
We race a 250cc twin cylinder 2 stroke road race bike.
I would like to improve the air box by making it larger, at present it is about 5 lts volume.
I can make it larger but I'm wondering is there a good volume to aim for.
Also pressurised or still which is best.
If there are any discussion on air box design could someone point me in the right direction.
Many thanks
Len

Hello Len, maybe the text below is helpful to you.
================================================================
Tuning an airbox is anything but simple. It is not just a basic Helmholtz resonator with one volume and one supply duct because there is a second set of ducts (the carburetors), opening and closing with a varying frequency (the crankshaft rpm).

Speaking of volumes, simultaneously-firing multicylinder engines, for example tandem-twins and the Aprilia V-twin racing engines, have the disadvantage that both cylinders breathe at the same time, which causes large pressure fluctuations in the airbox. This should be counteracted with a large-volume airbox.

An airbox has two advantages: it ensures that the engine does not have to inhale air that has been preheated by the radiator, and at high riding speeds it provides ram air pressure. In the Aprilia RSA250 this extra pressure was 30 mbar at 260 kmh, good for 3% extra power.
But this extra pressure can also cause major carburetion problems. 30 mbar corresponds to a petrol column of about 400 mm height. And what is the height difference between the petrol level in the tank and the petrol level in the float chamber??
Under unfavorable conditions, this 30 mbar ram air pressure pushes petrol back from the carburetor to the tank, resulting in a lean mixture and seizures. So you need to ensure that the pressure in the fuel tank is always at least equal to the pressure in the airbox.

With an old-fashioned tank breather, it was simple: if 1 cc of petrol is consumed from the tank, 1 cc of air must be supplied at the same time to prevent depression in the tank.

But with an airbox you don't want to keep the air pressure in the tank equal to the atmospheric pressure, but you want to keep it equal to the airbox pressure. And if, for example, that pressure rises 3%, then 3% more air must be capable of flowing into the petrol tank within the same time.
With an almost empty tank, that is a considerable volume of air that must be supplied in a short time, requiring an internal air hose diameter of maybe Ø 12 mm.

The aeration nipples located on a carburetor on the left and right sides above the float chamber are normally connected to an overflow bottle. But if you use an airbox, you must ensure that the pressure in those aeration hoses is always equal to the pressure in the airbox.
The simplest way is to simply encase the whole carburetor(s) in the airbox, omit those aeration hoses altogether and connect the lowest point of the airbox to the overflow bottle. That only requires a small-diameter hose.

For the sake of safety, the Aprilia racers used an electric petrol pump. Then you no longer have to worry about the pressure difference between tank and airbox. But then you have to somehow make sure that when the battery runs out, the ignition stops sparking before the petrol pump stops pumping, otherwise things can become expensive ...
Frits Overmars

Frits, ces explications sont uniquement pour les boites à air étanches ou bien pour toutes les boites à air ?

Frits, these explanations are only for airtight air boxes or for all air boxes ? (traduction by google )

crigar a écrit:

Frits, ces explications sont uniquement pour les boites à air étanches ou bien pour toutes les boites à air ?
Frits, these explanations are only for airtight air boxes or for all air boxes ?

It depends on the amount of pressure rise due to riding speed. Obviously, if the airbox is completely open for the air to escape, there will be no ram air pressure rise. But otherwise, even if an airbox is not completely airtight, there may still be some pressure rise, so the safe way would be to treat every airbox as if it were airtight.

Thank you !

Frits Overmars a écrit:

crigar a écrit:

Frits, ces explications sont uniquement pour les boites à air étanches ou bien pour toutes les boites à air ?
Frits, these explanations are only for airtight air boxes or for all air boxes ?

It depends on the amount of pressure rise due to riding speed. Obviously, if the airbox is completely open for the air to escape, there will be no ram air pressure rise. But otherwise, even if an airbox is not completely airtight, there may still be some pressure rise, so the safe way would be to treat every airbox as if it were airtight.

Frith, thank you for the info.
Okay so I'm using an air box on my 250cc twin cylinder, is there a volume to aim for or just as big as I can fit in?

Merlin7 a écrit:

Frits Overmars a écrit:

crigar a écrit:

Frits, ces explications sont uniquement pour les boites à air étanches ou bien pour toutes les boites à air ?
Frits, these explanations are only for airtight air boxes or for all air boxes ?

It depends on the amount of pressure rise due to riding speed. Obviously, if the airbox is completely open for the air to escape, there will be no ram air pressure rise. But otherwise, even if an airbox is not completely airtight, there may still be some pressure rise, so the safe way would be to treat every airbox as if it were airtight.

Frith,  thank you for the info.
Okay so I'm using an air box on my 250cc  twin cylinder, is there a volume to aim for or just as big as I can fit in?

BIG

Hi,

As big as possible.
Can you specify what is your type of engine.
Twin cylinder, OK but which configuration?
Twin parallel ?
Twin tandem ?
180 ° or 360 ° ignition cycle
What is your motorbike?

It is interesting to model your air box and its supply duct then, if possible, to make a flow simulation with CFD software.
Check the different flows and flow rates as well as the overall pressure drop.
Modify internal geometry and start over and over again ...

Read more here:
[Vous devez être inscrit et connecté pour voir ce lien]

Francis.

[quote="Frits Overmars"][quote="Merlin7"]

Frits Overmars a écrit:

crigar a écrit:

Frits, ces explications sont uniquement pour les boites à air étanches ou bien pour toutes les boites à air ?
Frits, these explanations are only for airtight air boxes or for all air boxes ?

It depends on the amount of pressure rise due to riding speed. Obviously, if the airbox is completely open for the air to escape, there will be no ram air pressure rise. But otherwise, even if an airbox is not completely airtight, there may still be some pressure rise, so the safe way would be to treat every airbox as if it were airtight.

Frits my apologies, I didn't check my spelling before posting my reply.
So by BIG I presume the biggest I can fit in.
From you're earlier reply, altho it's not a fully sealed airbox I will try to place carbs fully inside the airbox Inc float bowl breathers. And if I understand correctly I need to run a pipe from airbox to fuel tank to maintain equal pressure in both tank and airbox.
Many thanks.
Len.

fpayart a écrit:

Hi,

As big as possible.
Can you specify what is your type of engine.
Twin cylinder, OK but which configuration?
Twin parallel ?
Twin tandem ?
180 ° or 360 ° ignition cycle
What is your motorbike?

It is interesting to model your air box and its supply duct then, if possible, to make a flow simulation with CFD software.
Check the different flows and flow rates as well as the overall pressure drop.
Modify internal geometry and start over and over again ...

Read more here:
[Vous devez être inscrit et connecté pour voir ce lien]

Francis.

Yamaha tzr250 2ma,
parallel twin
180
Crank case reed valve

Dear Frits, I am interested to try you're exhaust design programme.
Is there a step by step explanation of this, as some of the terms I do not understand.
I appologise for asking, but I'd rather get instruction from the man who designed it rather than of some other website.
Thank you.

Merlin7 a écrit:

Dear Frits,  I am interested to try you're exhaust design programme.
Is there a step by step explanation of this, as some of the terms I do not understand.
I appologise for asking, but I'd rather get instruction from the man who designed it rather than of some other website.
Thank you.

Len, I assume you are referring to my FOS Exhaust Concept. I tried to make it so simple that anybody could use it without help.
The drawing below should be self-explaining; all you need to do is to enter the values marked in yellow.
Which are the terms that you do not understand? Maybe with your help I can make the concept even more user-friendly.[Vous devez être inscrit et connecté pour voir cette image]

Thank you very much for the reply Frits, I shall have a go this weekend and report back to you if that's okay.

here Merlin : [Vous devez être inscrit et connecté pour voir ce lien]

Frits' concept he just posted in an excel, in which you can include a "Wobble exhaust duct"