[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

Howard Gifford a écrit:

My thought is to give the port a slight taper starting at 34mm at the carb tapering out to 36mm at the plate and ending at 38mm where it enters the case. My plan is to leave as much material as possiblre in the case and just radius the end where it enters the case. Your thoughts would be appreciated.

If I were you I would use 34mm at the plate, too. And then BIG radii where it enters the case.

But unfortunately I can't tell you if this really works better than your variant.

But what sense would it make to use "only" 38mm and little radii? At this point it is already in the case, so velocity and such things don't seem to be important anymore. Flow seems to be important, like Jan said and like Frits confirmed again.

Just my thoughts, don't take offense if I am wrong

Thank you Frits and Daniel for your suggestions. I decided on 34mm at the carb tapering to 38 mm at the plate then a big radius blending into the case. I wil let you know how it works out. (pictures to follow!)

I have a question for Jan about pistons. We had some forged pistons made to our spec. If we lean down to make good power we suffer loss of heat treatment in the forging material which causes the ring locator pin to either work its way out or work its way further in allowing the ring to rotate , both resulting in catastrophy! The cast pistons we use seem to not suffer from the same problems, however we are forced to relocate the locating pin due to widening of the B port and this is a pain. The stock cast pistons use a wide section of the cylinder wall between the B and C ports to support the ring ends. Widening of the B port causes the ring to snag in the port so we relocate the pin to the center rear of the piston. My question is... Are forged pistons worth the effort or should we stick to the cast ones that are much cheaper and do not go soft on us?
What is your experience?

Nowadays cast pistons are every bit as good as forged ones - and they are cheaper.

Jan or frits ,
By how much was the height of the RAS exhaust port reduced by the power valve when fully lowered , or alternatively
how much was exhaust open duration reduced ?

Many thanks , Mel

Hello everyone. I was thinking in the possibility of building a RAM Air box for my bike. And I have some doubts about this.
This was written in page 2..

koenich a écrit:

so a airbox has several advantages, another project for the winter. it should be also pretty good to get the loudness down quite a bit... but I guess the effect will not be that big on smaller bikes which go max. 100 km/h and only on tight karttracks.

something frontfacing, that is not sucking air from coolers/exhaust and has somekind of stone/trashstop in it should do it. volume as big as possible...thanks graham for the great explenation!!!

The Carburattor is fitted inside the airbox so pressure inside carb vents will be equal to that from inside the airbox, so the carb doesn't go lean at high speed or what?

I am confused because when air pressure arises with lower altitude a carburattor must use bigger jets so the mixture does not get lean!

Is it possible to make a full functional and realiable ram-air box with a simple carburattor, I am using a Mikuni TMX 38 by the way...non power jet

Is there real power to explore using ram air system? Or the biggest improvement comes from the carb air not been heated by radiator and cylinder (so one air box can pull air from the side of the bike, right angle).

Thanks to Frits and Jan Thiel, this thread had help thousands trought the world, and they are working better right now because of this, I have personaly read this topic two or three times.

Regards

Howard Gifford a écrit:

Thank you Frits and Daniel for your suggestions. I decided on 34mm at the carb tapering to 38 mm at the plate then a big radius blending into the case. I wil let you know how it works out. (pictures to follow!)

I have a question for Jan about pistons. We had some forged pistons made to our spec. If we lean down to make good power we suffer loss of heat treatment in the forging material which causes the ring locator pin to either work its way out or work its way further in allowing the ring to rotate , both resulting in catastrophy! The cast pistons we use seem to not suffer from the same problems, however we are forced to relocate the locating pin due to widening of the B port and this is a pain. The stock cast pistons use a wide section of the cylinder wall between the B and C ports to support the ring ends. Widening of the B port causes the ring to snag in the port so we relocate the pin to the center rear of the piston. My question is... Are forged pistons worth the effort or should we stick to the cast ones that are much cheaper and do not go soft on us?
What is your experience?

My experience is that cast pistons gave slightly more power.
To make them even better they should be 'hipped', by the manufacturer.
Forged pistons are not worth the effort.
Is it not possible to get stock pistons without a pin?

melvyn trevor a écrit:

Jan or frits ,
By how much was the height of the RAS exhaust port reduced by the power valve when fully lowered , or alternatively
how much was exhaust open duration reduced ?

Many thanks , Mel

To be honest: I do not remember.
But I think it was 4-5 mm
Too much will cause auto-igtion.

romeuh80 a écrit:

Hello everyone. I was thinking in the possibility of building a RAM Air box for my bike. And I have some doubts about this.
This was written in page 2..

koenich a écrit:

so a airbox has several advantages, another project for the winter. it should be also pretty good to get the loudness down quite a bit... but I guess the effect will not be that big on smaller bikes which go max. 100 km/h and only on tight karttracks.

something frontfacing, that is not sucking air from coolers/exhaust and has somekind of stone/trashstop in it should do it. volume as big as possible...thanks graham for the great explenation!!!

The Carburattor is fitted inside the airbox so pressure inside carb vents will be equal to that from inside the airbox, so the carb doesn't go lean at high speed or what?

I am confused because when air pressure arises with lower altitude a carburattor must use bigger jets so the mixture does not get lean!

Is it possible to make a full functional and realiable ram-air box with a simple carburattor, I am using a Mikuni TMX 38 by the way...non power jet

Is there real power to explore using ram air system? Or the biggest improvement comes from the carb air not been heated by radiator and cylinder (so one air box can pull air from the side of the bike, right angle).

Thanks to Frits and Jan Thiel, this thread had help thousands trought the world, and they are working better right now because of this, I have personaly read this topic two or three times.

Regards

Yes, that is certainly possible!
Do not forget to put the fuel tank vent into the airbox!
The biggest gain comes from preventing hot air entering the carburator.

Jan Thiel a écrit:

melvyn trevor a écrit:

Jan or frits, by how much was the height of the RAS exhaust port reduced by the power valve when fully lowered , or alternatively how much was exhaust open duration reduced ?

To be honest: I do not remember. But I think it was 4-5 mm. Too much will cause auto-igtion.

When both blades were fully closed, the exhaust timing was reduced from 202° to 160° if I remember correctly.

For those who want more precision

Hi , fpayart ,
Teriffic stuff , well done and thank you very much , all explained nicely , Jan and Frits will be most impressed !

Regards , Mel

I don't believe this

About the auto-detonation problems that you have/had, having a secundary power jet injection water, just a fine mist to lower the temperature of the cylinder a little bit, or it would cause lag to achive full power after using the water injection?

It is done in turbo 4 stroke engines.

Jan Thiel a écrit:

romeuh80 a écrit:

Hello everyone. I was thinking in the possibility of building a RAM Air box for my bike. And I have some doubts about this.
This was written in page 2..

koenich a écrit:

so a airbox has several advantages, another project for the winter. it should be also pretty good to get the loudness down quite a bit... but I guess the effect will not be that big on smaller bikes which go max. 100 km/h and only on tight karttracks.

something frontfacing, that is not sucking air from coolers/exhaust and has somekind of stone/trashstop in it should do it. volume as big as possible...thanks graham for the great explenation!!!

The Carburattor is fitted inside the airbox so pressure inside carb vents will be equal to that from inside the airbox, so the carb doesn't go lean at high speed or what?

I am confused because when air pressure arises with lower altitude a carburattor must use bigger jets so the mixture does not get lean!

Is it possible to make a full functional and realiable ram-air box with a simple carburattor, I am using a Mikuni TMX 38 by the way...non power jet

Is there real power to explore using ram air system? Or the biggest improvement comes from the carb air not been heated by radiator and cylinder (so one air box can pull air from the side of the bike, right angle).

Thanks to Frits and Jan Thiel, this thread had help thousands trought the world, and they are working better right now because of this, I have personaly read this topic two or three times.

Regards

Yes, that is certainly possible!
Do not forget to put the fuel tank vent into the airbox!
The biggest gain comes from preventing hot air entering the carburator.

Thank you, I had too ensure that the Carb does not get lean and seize at high speed

senso a écrit:

About the auto-detonation problems that you have/had, having a secundary power jet injection water, just a fine mist to lower the temperature of the cylinder a little bit, or it would cause lag to achive full power after using the water injection?

It is done in turbo 4 stroke engines.

Welcome!

Howard Gifford a écrit:

I have a question for Jan about pistons. Are forged pistons worth the effort or should we stick to the cast ones that are much cheaper and do not go soft on us?
What is your experience?

take a look at the vertex catalog howard - they have some explanations in there why casted is better on 2-strokes and forged is preferable on 4-strokes...

Citation :

Vertex Two-stroke Pistons
Because of a cast piston’s ability to contain higher silicon content compared to forged pistons, and because twostrokes have lower cylinder pressures when compared to four-strokes, cast pistons are ideal for two-stroke engines. Vertex pistons are gravity cast from an aluminum alloy with 18% silicon content. Silicon is important because:

• It decreases thermal expansion, which changes the geometry after the piston reaches operating temperature. With low thermal expansion characteristics, Vertex pistons can be run at tighter tolerances compared with forged pistons. This creates better power and performance, also making them less susceptible to rattling or breaking.

• Silicon gives the pistons better wear characteristics and prolongs piston life by decreasing wear on the piston skirt.

Citation :

Vertex Two-stroke Pistons
Because of a cast piston’s ability to contain higher silicon content compared to forged pistons, and because two-strokes have lower cylinder pressures when compared to four-strokes, cast pistons are ideal for two-stroke engines. Vertex pistons are gravity cast from an aluminum alloy with 18% silicon content.

That is not correct. The pistons of our MB40 aero engines are fabricated (by electro-erosion) out of forged RSP material with 30% silicon (http://www.rsp-technology.com). Such a high silicon content is impossible to cast.
Nevertheless, in general I would advise cast pistons as opposed to forged ones.

Mitaka tries to give an explanation on the subject too:

Citation :

On the subject of forged versus cast pistons, it should be understood that there are pros and cons which are well understood by the Motorcycle manufacturers. Cost is not an issue since the difference at high volumes is negligible. No manufacturer would compromise the performance of a racing bike for a small cost reduction. A forged piston is stronger but cannot not be made from the optimum silicon content alloy and this will require a larger clearance which will always compromise its performance. Of the four Japanese manufacturers, only Kawasaki has used forged pistons in its two strokes and they abandoned the practice 20 years ago. The highest specific power output achieved by a modern mainstream two stroke is the Honda RS125 G.P. Racer. 50 + h.p. - Yes 400 h.p. per litre (more than an F1 car or a Moto GP bike) and this is using a cast piston (the KTM85 is also close to this performance with a cast piston). Why - because this is the only way to achieve the clearances needed to maximise the power output. A forged piston may perform better when working outside the service limits, but this should not be an attraction for a racer.

The reason they use is the same, but come to a different conclusion : the clearance.

edit: It's not different, actually it's the same.

Vagelis a écrit:

Mitaka tries to give an explanation on the subject too:

Citation :

On the subject of forged versus cast pistons, it should be understood that there are pros and cons which are well understood by the Motorcycle manufacturers. Cost is not an issue since the difference at high volumes is negligible. No manufacturer would compromise the performance of a racing bike for a small cost reduction. A forged piston is stronger but cannot not be made from the optimum silicon content alloy and this will require a larger clearance which will always compromise its performance. Of the four Japanese manufacturers, only Kawasaki has used forged pistons in its two strokes and they abandoned the practice 20 years ago. The highest specific power output achieved by a modern mainstream two stroke is the Honda RS125 G.P. Racer. 50 + h.p. - Yes 400 h.p. per litre (more than an F1 car or a Moto GP bike) and this is using a cast piston (the KTM85 is also close to this performance with a cast piston). Why - because this is the only way to achieve the clearances needed to maximise the power output. A forged piston may perform better when working outside the service limits, but this should not be an attraction for a racer.

The reason they use is the same, but come to a different conclusion : the clearance.

They Are also very cheap, quality seems OK!

I have another question Regarding 125GP.

I hav read somewhere, maybe here, that Duck tape is used on the cylinder head to determine that the engine temperature is ideal to go..

Is there some strong indicator that the actual cooling system is insuficient? Without a termo sensor mounted on the cylinder head...

And if that could tell us the problem before piston seize inside the cylinder

What was the total volume of cooling liquid used on the RSA/RSW?

I see that less liquid and bigger radiators was the way the things develop...

I've got a little question regarding the A- and B-Ports, because I can't really figure it out on the drawings.

One flows with 6,82° on the upper side and 7,42° on the under side, the other one flows with 28,16° on the upper side and 16,45° on the under side.

Which is which?

I guess the one with low angles is B and the one with sharp angles is A, but I'm not sure.

Regards

Daniel A. a écrit:

I've got a little question regarding the A- and B-Ports, because I can't really figure it out on the drawings.

One flows with 6,82° on the upper side and 7,42° on the under side, the other one flows with 28,16° on the upper side and 16,45° on the under side.

Which is which?

I guess the one with low angles is B and the one with sharp angles is A, but I'm not sure.

Regards

That is right.

Frits and Jan

Lastest years motocross 125cc bikes have come with ceramic or anodized? Dome pistons.
[Vous devez être inscrit et connecté pour voir cette image]

why haven't the 125 gp adopted them even before the motocross?

My guess is, this piston will not absorb soo mutch heat/energy from the cumbustion, so with more heat the pressure will be higher and more energy will go throut the exhaust making better power, is this true?

Besides, if the piston doesn't heat soo muth it will need lower clearances and can suitable higher power and RPM?

PVO a écrit:

Daniel A. a écrit:

I've got a little question regarding the A- and B-Ports, because I can't really figure it out on the drawings.

One flows with 6,82° on the upper side and 7,42° on the under side, the other one flows with 28,16° on the upper side and 16,45° on the under side.

Which is which?

I guess the one with low angles is B and the one with sharp angles is A, but I'm not sure.

Regards

That is right.

The ones with the low angles are the B-ports.