[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Hi Frits,

Thank you very much for your responses. They are most helpful!

I think i understand what you are referring to regarding the coanda effect. In the current situation the flow would rather adhere to the cylinder head than make the turn and follow the piston crown at the point where it becomes flat? By matching the piston profile and cylinder head profile this situation would be improved. BUT a domed piston and parallel squish would be better still in terms of piston cooling.

The 1mm gauge wire is an ingenious solution. Square root 2 at its best!

Thanks again,

Dan

maccas a écrit:

I think i understand what you are referring to regarding the coanda effect. In the current situation the flow would rather adhere to the cylinder head than make the turn and follow the piston crown at the point where it becomes flat?

Right!

hi
i need an advice. On my 94ccm rotary valve Scooter engine the Cylinder has a t port exhaust, but not as wide as optimal. at the moment its about 90%.
my problem are the holes of the stud bolts as you can see in the picture.

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i can grind the port to 100% (red) but i can not make the port shape as it should by because i must let the material for the stud. The folw has to go like the green arrow.

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Frits or Jan has it a sense or is there no gain?
has anyone done this before?

Thanks Manuel

the Exh flow is radial, so the shape you're drawing isn't optimal

the problem with T port is that you cant achieve the 100% of bore that you can obtain with a 3 ports

Bonjour ,
une question sur la valve rotative ,pourquoi l'ouverture se fait elle si tard ,+-150 deg av pmh ?
que se passe-t-il si l'ouverture se fait beaucoup plus tot ,+- 170 deg av pmh ? ,sans changer la fermeture
merci

why do you ask about rotativ disk mico ? 125 NF4 have not this !!    

Hello Gilles ,your nf4 have reed valves but...  

One solution might be to bore the front stud holes to accept 12mm, put a screw in each side to bolt the cylinder down and then put some studs in the top to bolt the head, its sort of common practice in limited engines, and least you could gain a lot of duct space to work, maybe some machined alu plugs so the stud holes dont create strange ressonances.

Why is your plating in the exhaust divider a different color?

Manuel Rainer a écrit:

hi
i need an advice. On my 94ccm rotary valve Scooter engine the Cylinder has a t port exhaust, but not as wide as optimal. at the moment its about 90%.
my problem are the holes of the stud bolts as you can see in the picture.
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One big oval exhaust would be far the best in this case. That does not mean that you can remove the bridge.

At least in intakes its also common to remove the aluminiun to gain all the possible area and leave the stud visibel, but without orings in the cylinder around the studs it might leak.

senso a écrit:

One solution might be to bore the front stud holes to accept 12mm, put a screw in each side to bolt the cylinder down and then put some studs in the top to bolt the head, its sort of common practice in limited engines, and least you could gain a lot of duct space to work, maybe some machined alu plugs so the stud holes dont create strange ressonances.

Why is your plating in the exhaust divider a different color?

hi

senso how does you mean. close the original stud hole and make some oders?

the color difference is because the divisor is grinded back. Original

An oval is no possibility.

the only thing i can do is to grind the shape like i signed in red.
my only doubt is if it has any sense

micowoy a écrit:

une question sur la valve rotative ,pourquoi l'ouverture se fait elle si tard ,+-150 deg av pmh ?
que se passe-t-il si l'ouverture se fait beaucoup plus tot ,+- 170 deg av pmh ? ,sans changer la fermeture.

You can start to open the inlet disk as soon as the crankcase pressure has dropped below the pressure in the inlet tract. That may well be around 170° before Top Dead Center.
The advantage will be that the crankcase pressure will not drop much further, so it will not slow down the transfer flow so much.
The disadvange is that the inlet already opens when there is very little suction; the suction signal to the carburetter will be very weak and it will be difficult to get clean carburation.

Manuel Rainer a écrit:

the only thing i can do is to grind the shape like i signed in red.

Don't do it.

the front head bolts are very long and go all the way into the exh port and thats whats creating the step ? ive never seen anything like that before. yes maybe use studs on the two in front. is the exh tunell walls plenty thick ? sometimes you can punch a hole to the water jacket if you dont pay attention

Frits Overmars a écrit:

micowoy a écrit:

une question sur la valve rotative ,pourquoi l'ouverture se fait elle si tard ,+-150 deg av pmh ?
que se passe-t-il si l'ouverture se fait beaucoup plus tot ,+- 170 deg av pmh ? ,sans changer la fermeture.

You can start to open the inlet disk as soon as the crankcase pressure has dropped below the pressure in the inlet tract. That may well be around 170° before Top Dead Center.
The advantage will be that the crankcase pressure will not drop much further, so it will not slow down the transfer flow so much.
The disadvange is that the inlet already opens when there is very little suction; the suction signal to the carburetter will be very weak and it will be difficult to get clean carburation.

Thank you for your reply

micowoy a écrit:

Frits Overmars a écrit:

micowoy a écrit:

une question sur la valve rotative ,pourquoi l'ouverture se fait elle si tard ,+-150 deg av pmh ?
que se passe-t-il si l'ouverture se fait beaucoup plus tot ,+- 170 deg av pmh ? ,sans changer la fermeture.

You can start to open the inlet disk as soon as the crankcase pressure has dropped below the pressure in the inlet tract. That may well be around 170° before Top Dead Center.
The advantage will be that the crankcase pressure will not drop much further, so it will not slow down the transfer flow so much.
The disadvange is that the inlet already opens when there is very little suction; the suction signal to the carburetter will be very weak and it will be difficult to get clean carburation.

Thank you for your reply

What model cycle / scooter is that thing ?

The exhaust port is a strange one.

Is there a performance, or " kit " cylinder available ?

Frits Overmars a écrit:

Manuel Rainer a écrit:

the only thing i can do is to grind the shape like i signed in red.

Don't do it.

thanks frits

does this hinders the exhaust flow, or what is the reason?


brokedown

this engine has 4 studs all the way from the carter to the head and so the cylinder has the holes all the way through. i can not close the holes and there is not that much material around the duct.


ambike

the pics are from my polini 94 ccm cylinder. it is from a piaggio 50 ccm scooter.
the cylinder is bored on the max. and so the exhaust has also to be big. that is the reason because there are the studs in the way in the exhaust duct.

Manuel Rainer a écrit:

Frits Overmars a écrit:

Manuel Rainer a écrit:

the only thing i can do is to grind the shape like i signed in red.

Don't do it.

thanks frits

does this hinders the exhaust flow, or what is the reason?


brokedown

this engine has 4 studs all the way from the carter to the head and so the cylinder has the holes all the way through. i can not close the holes and there is not that much material around the duct.


ambike

the pics are from my polini 94 ccm cylinder. it is from a piaggio 50 ccm scooter.
the cylinder is bored on the max. and so the exhaust has also to be big. that is the reason because there are the studs in the way in the exhaust duct.

OK, thanks for the reply.

I'm in Cow Town, Texas. We're long past the scooter stage, but I understand and appreciate what you guys are doing.

And, I do like to keep up.

I also KNOW that most young guys are limited with what they can afford to buy ! It's always been that way & I don't see the situation changing. Might be that it's even worse today.

In my early days companies such as Powroll had it " going on " so that guys could punch out & improve the smaller Hondas.

What's odd is that during those same years, big clunkers like Harleys could be purchased for a few hundred dollars and that's what I did many, many times. The big iron was the price of most small new Hondas ( & Yamahas ), and much cheaper than 350 and 450 models. Yeah, I started BEFORE the CB750 four-cylinders came out.

But, at the same time, the two cycle revolution was on. I've owned, raced, and modified tons of them. Most everything from A to Y, but no Zundapps unless a Sachs is close enough to a Z.

The quality is much better today and likewise, so is the power / reliability.

So, all things considered, keep on learning and experimenting with any / all ...

It's great fun especially when the extra performance is achieved, but never get too greedy !

There's always a logical limit both physically & economically, and sometimes moving onto a bigger machine, or a genuine race bike is the way to go.

I do a lot of collecting, building, and restoration. I've snagged bikes that I could never afford when I was young. One recent example is an Aermacchi RR-250.

How do we post photos here ?

What am I missing ?

Maybe, it's because I can't read " manual " ?

Sleeve valve cylinder, Crecy style

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uniflow a écrit:

Sleeve valve cylinder, Crecy style

It looks great, Neil. But could you please attach your pictures directly (see the picture below) ? I try to avoid Photobucket.

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Frits Overmars a écrit:

uniflow a écrit:

Sleeve valve cylinder, Crecy style

It looks great, Neil. But could you please attach your pictures directly (see the picture below) ? I try to avoid Photobucket.
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I've spent all evening just trying to just attach these, never did take French at school. Next time I'll try to attach directly. If you can see on photo bucket I'm moving along with the crank case pattern, although these pictures are old. Case reed, might be able to fit an FOS cylinder on this crank case ( when the sleeve valve seizes ). This one will just run a carburetor. Crank for the EFI 360 rotary valve engine is finished also. I hope you don't mind me posting these here.

uniflow a écrit:

Frits Overmars a écrit:

...could you please attach your pictures directly (see the picture below) ? I try to avoid Photobucket.

I've spent all evening just trying to just attach these, never did take French at school.

I did take French, and it took me about a week to figure out how to attach pictures here.

Citation :

If you can see on photo bucket....

I can't see anything on photobucket because they compel me to register, which I refuse.

Citation :

..might be able to fit a FOS cylinder on this crankcase (when the sleeve valve seizes).

Now should I hope it works or should I hope it seizes?   

Citation :

I hope you don't mind me posting these here.

Are you kidding me? I'm delighted!

uniflow a écrit:

Sleeve valve cylinder, Crecy style

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This looks familiar

to post photobucket pics directly to the forum all you do is log on to your photobucket and at the top it will say library and you click that. then all you photos will be shown and you put your mouse over what photo you want and a cog wheel will appear in the top right corner of the pic and you put the mouse over the cog wheel and it will have a list of choices. click on 'get links' and then it will have a drop down screen of codes and then just click on the IMG code and it will automatically copy it and then paste it in your post. pretty easy once you do it a couple times

hi frits i might as well ask a question since im here. of all the aspects of a 2t engine, what has the most influence on rpm limit ? pipe, cylinder ports, reed cage, ign timing ?  im gonna try to advance and retard the ign and see if it has any affect on max rpm. it seems my engine is hitting a wall and just cant rev any more.  the ign curve im using is the purple line. i suspect some where between 7-8K (maybe closer to 7K but im not sure) is where the wall is

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