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

I came for copper and found gold! Thank you so much!

superbe

Magnifique, j'en bave.....
La culasse est fixée au cadre par des équerres en alu , montage rigide sans silenbloc ?

2strokeman a écrit:

La culasse est fixée au cadre par des équerres en alu , montage rigide sans silenbloc ?

Contrarotating twins like the Aprilia RSW250 and RSA250, and, previously, the Rotax tandemtwins, can be pretty free of external vibrations.

Thanks for your clarifications Frits.

Hi Frits and jan, I have a questioni for you.
I have an TM 125 k7 cranckshaft unbalanced forward (cranck fall down in direction of rotation). What about this type of modificati in?
Thanks so much
Eric[Vous devez être inscrit et connecté pour voir cette image]

TM K7, c’est du kart ?

Adco a écrit:

TM K7, c’est du kart ?

Yes it's kart cranckshaft

Au cas où ! Ce qu’en disent classiquement les préparateurs de karts….
[Vous devez être inscrit et connecté pour voir cette image]

Fantastico, thanks so much adco

Dear Frits.
Would you please advise about 2T cylinder head or FOS cylinder head design.
1) what 's effect to head design in case different racing fuel unlead & lead fuel.
(such as Burn perior, Head Volume, Squize area or etc..)
2) If we change fuel from AV gas(Octance100) or unlead racing fuel to VP racing Q16 (Octance116),
basically i can increase compression ratio correct, if you have any advise please
Thank you for your advice.

pojoran a écrit:

Dear Frits.
Would you please advise about 2T cylinder head or FOS cylinder head design.
1) what 's effect to head design in case different racing fuel unlead & lead fuel.  
   (such as Burn perior, Head Volume, Squize area or etc..)
2) If we change fuel from AV gas(Octance100) or unlead racing fuel to VP racing Q16 (Octance116),
   basically i can increase compression ratio correct, if you have any advise please
Thank you for your advice.

Dear Pojoran, if you wish to use leaded fuel, you are on your own. I haven't concerned myself with leaded fuel in decades, and I am not going to.

Apart from this, any combustion head shape, irrespective of its volume, should promote the highest possible burn rate. That way you will only need a small amount of ignition advance, which will limit the undesirable pressure rise acting on the piston before Top Dead Center. Furthermore, shortening the burn duration will limit heat loss to the surrounding metal.

Squish is the most effective way of promoting a fast burn rate. Make the squish area parallel to the piston crown and make the squish gap as narrow as possible. If the crankshaft is of good quality, you can use a squish gap of 1% of the stroke.

Different fuels will permit different compression ratios, but contrary to four-stroke practice, two-strokes do not need a high compression ratio for power, so I do not recommend to blindly use the highest ratio that the fuel will permit.

I have a question to ask Frits. We all know the benefits of having a functional "C" port in both a reed valved engine and a rotary valve egine like the RSA but has anyone tried a "C" port in a standard piston port engine. My thought is that a C port in the roof of the intake port that opens after the A and B ports would take advantage of the suction phase of the pipe and allow more fuel charge to enter the cylinder. Has anyone tried this?

Howard Gifford a écrit:

I have a question to ask Frits. We all know the benefits of having a functional "C" port in both a reed valved engine and a rotary valve egine like the RSA but has anyone tried a "C" port in a standard piston port engine. My thought is that a C port in the roof of the intake port that opens after the A and B ports would take advantage of the suction phase of the pipe and allow more fuel charge to enter the cylinder. Has anyone tried this?

There have been plenty engines with a C-port above the inlet, fed through a window in the piston just below the piston ring. But I assume that you mean a C-port in open contact with the inlet port. My friend Ferry Brouwer modified a cylinder in this way and he claimed a power increase. I do not remember the timing of this C-port (it's more than 40 years ago) but it was clearly lower than the A- and B-ports.

I think making the C lower than the others makes sense. I'm afraid that if you close the C-port after the B and A, it may blow the fresh charge direct into the exhaust.

Hallo, if I may tell you about my experiments regarding the -C- additional channel on slot-controlled cylinders.
I have done this on Kawa, TZ 250/350 and some other cylinders. I had to be careful because of the cylinder wall above the intake port. I have milled many different channel shapes in the cylinder wall, with very different results, from -0- to noticeable improvement.  Of course, the drivers' statements as well as my experiences were very varied, as this small change also depended on the rest of the engine's configuration.
I made the -C- slot height dependent on the other overflow windows by milling only 70% to about 95% of their height in the different cylinders. I also changed the piston window in all directions and stuck with a transverse oval shape.
The higher the engine revved, the higher I made the -C-slot.
Unfortunately, this also made push-starting a bit more difficult (at the time).
I also noticed that the peak power did not increase, but the torque improved from the middle, to about 75-80% of the maximum RPM.
I spent a lot of time and cylinders on these experiments (back then) until it worked reasonably well.
Greetings Cara

Thanks for sharing your experience.
When you say from 70 to 95% of the other transfer, do you mean the bottom line of the C-port was positioned at the same level as the other ports or it could have closed after the others as Howard suggested? In other words did you try what Howard says?

My understanding of -Howard- was ; if someone had routed (milled) a direct duct from the upper intake duct wall (roof), straight up an upright steep C duct into the cylinder on a -slot- controlled engine.
Yes, I did this (tried it) many years ago. So, when the piston is at -BDC-, the extra -C window was only opened 70 to 95% from the height of the other overflow windows. So the -C duct window was only 70 to 95% -high- compared to the other overflow window.

OK. Not sure but I think this was done also on the Aermacchi/HD 250 and 350 at their time of glory (74-75).

Quand on regarde les docs APRILA sur les dotations de gicleurs principaux il y a une sacré différence entre ceux pour RSA et ceux pour RSW.
Les gicleurs principaux sont bien plus petit pour la RSA quelle explication peut il y avoir à ça?
La position du disque rotatif?

Hello quod and probation:
At what RPM was your low rpm trial and what RPM was your high rpm trial? And about the channel shapes, what exactly was the outcome and what did you learn from the outcomes?
Did the piston window have a axial roof also and did the axial roof angle of the c-port match closely with what Frits suggests?
Thank you, Jeff

MINGRET01 a écrit:

Quand on regarde les docs APRILA sur les dotations de gicleurs principaux il y a une sacré différence entre ceux pour RSA et ceux pour RSW.
Les gicleurs principaux sont bien plus petit pour la RSA quelle explication peut il y avoir à ça?
La position du disque rotatif?

There should be no difference. But maybe the RSW-data came from an older model, without power jet. That could explain the difference.