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

Hi Jan, Frits and everyone :-)

I have to thank you for your response, Frits. Yet another question occured to me:
As you have made your scravenging for the roof of the ports - is it the same angle on the bottom?

I am asking because on the APC-drawings it is not this way.
(A-Port has got 28,16° on the roof and 16,45° on the floor,
B-Port has 6,82° on the roof and 7,42° on the floor,
C-Port has 37,7° on the roof and 52° on the floor which is never to be concentric with the Piston edge.)

Greatings
Tim

Tim Ey a écrit:

I have to thank you for your response, Frits. Yet another question occured to me:
As you have made your scravenging for the roof of the ports - is it the same angle on the bottom?
I am asking because on the APC-drawings it is not this way. (A-Port has got 28,16° on the roof and 16,45° on the floor, B-Port has 6,82° on the roof and 7,42° on the floor, C-Port has 37,7° on the roof and 52° on the floor which is never to be concentric with the Piston edge.)

Yes, the ends of the inner transfer duct curvatures should end at the same angles as the roofs. The drawings you mentioned are older.

Frits Overmars a écrit:

Yes, the ends of the inner transfer duct curvatures should end at the same angles as the roofs. The drawings you mentioned are older.

Very kind from you! Thanks!

Does the sheer lower angle have an high impact by the way? I can imagine it only having an effect on the Port-Area distribution - as the inner transfer duct curvature and the piston are congruent only a really short time. I assume a high inner wall radius to be the major point.
Am I right?

Greatings
Tim

Frits, would you share your thoughts please.
Sprockets, Small HP engines & O-Ring chains
I wondered if using a larger drive sprocket & the appropriate larger rear sprocket to maintain the previous ratio what may happen, would there be less losses in the drive / chain as each links angle is reduced when rotating about a larger drive sprockets or would there be greater losses ?
Thanks Rick

Tim Ey a écrit:

Does the sheer lower angle have an high impact by the way? I can imagine it only having an effect on the Port-Area distribution - as the inner transfer duct curvature and the piston are congruent only a really short time. I assume a high inner wall radius to be the major point.

It does not have any influence on the angle.areas, but it has quite an influence on the flow coefficient. That, and the direction of flow, are the main points.
You can make the inner wall radius as large as you want; eventually it will become infinite and the wall will become flat, but then you will need a sharp kink at the upper end, where the duct enters the cylinder. And the radius of that kink is decisive for the flow. For good flow you need to keep the minimum radius in a duct as large as possible.

RAW a écrit:

Frits, would you share your thoughts please. Sprockets, Small HP engines & O-Ring chains
I wondered if using a larger drive sprocket & the appropriate larger rear sprocket to maintain the previous ratio what may happen, would there be less losses in the drive / chain as each links angle is reduced when rotating about a larger drive sprockets or would there be greater losses ?

If you enlarge the sprocket sizes, each link of the chain will hinge through a smaller angle, so there will be less friction.
But those large sprockets mean a higher chain velocitity, so each link has to hinge more times per second, which nullifies the benefit.
Large sprockets are good for two other reasons though. First, the pulling force in the chain will become smaller, so you can use a lighter chain. Second, the polygonal effect of the chain drive will be smaller. The irregularity increases as the number of teeth decreases. In theory you should not use sprockets with less than 20 teeth, but that is often impractical. I try not to go below 16 teeth, so the jerking in the chain won't become too much.
O-ring chains: I suppose they create more friction than plain chains, so I would not recommend them for racing.

Thank you Frits. Unfortunately the chain question was not RSA or motorcycle related, it is in my instance upon a 28 HP rotax restricted class superkart, however I felt the topic may have some form of relevance to others also. We use a 16 tooth drive sprocket & a 219 chain. The o-ring chain appears to rob less power once warm / at running temperature compared to the no o-ring of which I notice appears to be binding / awkward to hinge almost like it has no lubricant within the roller / pin hinge joints at the end of each race when checked the rear axle rotation is not free, with the o-ring this is no longer the case but I wonder about the o-ring drag upon each hinge swivel, thus the larger sprocket querrie and the potential less hinge arc & less possible power robbing.
Thanks again Frits,

Running an O-Ring chain doesn't mean DO NOT LUBE.

Moderation in all things ????

Yes I thoroughly agree, however perhaps the assembly grease within the chain has less ability to escape or be forced out past the o-rings, o-ring chains do require lubricant for the o-rings & of course a different lube for the roller / sprocket contact surfaces

Hello Guys,

finally i made it. I was struggeling with the Registration, but now i did it!

First of all a short Introduction of myself: My Name is Martin, i am 33 Years old and i am from Thuringia in Germany. As you can imagine i am interested in 2 Stroke bikes and engines. i own a rg 500 cr 125 and a kx 500. and simsons.... I am mainly interested in simson engines because (or in spite of?) that engines are aircooled and old fashioned. I know that Jan Thiel hates aircooled Engines and maybe Frits Overmars does so too. Jan and Frits, i hope you won`t refuse to answer me the one or other question about aircooled simson engines ;-)


To all the 2 Stroke addicts here: Thank you to be so much interested, thanks for creating that thread and thanks for keeping at least a few of 2 stroke engines alive! And of course, Jan and Frits, thank you for sharing all that knowledge you gained by working hard so many years!!!!

Jan, and Frits, i have a Question regarding the RSA Engine. I hope it wasn`t asked already....

How many square millimeters is the transferport area of the RSA engine and how is its relation to the carburetor area? Was the carburetorsize of 42mm a result of testing or was it calculated based on the transferport area?

Thank you!

Hallo Martin, herzlich willkommen.
You are right, we do not like aircooling; it is unsuitable for modern two-strokes. But I do like Simsons; I like the german SimsonGP class where my friend Jan Schäffer ([Vous devez être inscrit et connecté pour voir ce lien] participates. We do some occasional brainstorming and Jan has been champion in all four classes. Whenever I am in Germany, I try to visit those races.
I posted a red-and-yellow picture of the Aprilia blowdown and transfer areas and angle.areas. I don't remember exactly when it was, so you will have to do some searching of your own.
The carburettor size was the result of testing. 43 mm carbs were also tested but most riders preferred the 42 mm.

Jan & Frits, the 42 & 43mm carburettors, was there a difference in HP & Tq curves with the different carbs & with the riders predominantly choosing the 42mm why was this.

Thank you for the Response, Frits. Yes, you are right aircooling maybe unsuitable. An aircooled engine, especially a racing engine is probably very critical because the temperature of the engine is not as stable as the temp. of a liquid cooled. but in spite of that. i like the aircooled simson engines because of it`s simplicity. no waterpump, no rotary valve, no exhaust valve and so on. that is why i like two stroke engines so much. very few moving parts.

regarding the carburetor size.... i found the rsa`s effective transfer port area: 1302 square mm.
a 42mm carb means 1385 square mm. slightly more than the transfer port area.

so can one say, that the carb area should be roughly the same as the transfer port area? on the other hand, on full throttle the carb area is "always present", the tranfer port area is not because the piston passes the ports from time to time. So the carb. area could be smaller than the transferport area. As the rsa`s transferduct entry area is slightly smaller than the port window area.

Or is it already taken in account when speaking about an EFFECTIVE transfer port area, that the ports aren`t always fully opened?

Yes, people in Sachsen are totally fascinated by the simson mopeds. many tuners, many good ideas i think. but that leads me to a question, as you and also Jan are Dutch. The Dutch seem to LOVE the Kreidler Mopeds. I always wondered why. There was and is also Zündapp, sachs, Hercules.... But the Dutch want the Kreidler

Martin1981 a écrit:

Thank you for the Response, Frits. Yes, you are right aircooling maybe unsuitable. An aircooled engine, especially a racing engine is probably very critical because the temperature of the engine is not as stable as the temp. of a liquid cooled. but in spite of that. i like the aircooled simson engines because of it`s simplicity. no waterpump, no rotary valve, no exhaust valve and so on. that is why i like two stroke engines so much. very few moving parts.

regarding the carburetor size.... i found the rsa`s effective transfer port area: 1302 square mm.
a 42mm carb means 1385 square mm. slightly more than the transfer port area.

so can one say, that the carb area should be roughly the same as the transfer port area? on the other hand, on full throttle the carb area is "always present", the tranfer port area is not because the piston passes the ports from time to time. So the carb. area could be smaller than the transferport area. As the rsa`s transferduct entry area is slightly smaller than the port window area.

Or is it already taken in account when speaking about an EFFECTIVE transfer port area, that the ports aren`t always fully opened?

Yes, people in Sachsen are totally fascinated by the simson mopeds. many tuners, many good ideas i think. but that leads me to a question, as you and also Jan are Dutch. The Dutch seem to LOVE the Kreidler Mopeds. I always wondered why. There was and is also Zündapp, sachs, Hercules.... But the Dutch want the Kreidler

I certainly NEVER loved Kreidler!

Interesting to know the tranfer port aerea was 1302 mm2, I really never cared about this, simply made them as big as possible, until I lost power! This happened when I made the A-port too near the exhaust!
Another thing that may be interesting: the underside of the transfer ports had as much influence on power
than the topside. If they were lower than the piston in BDC, like most cylinders are, that would cost a LOT of power!

Martin1981 a écrit:

... is it already taken in account when speaking about an EFFECTIVE transfer port area, that the ports aren`t always fully opened?

That the ports are not always fully opened, is covered in the angle.area concept.
It is the sum of all the little steps of (open area x the number of crank degrees that each area is open).

Hello, okay Jan, then you are the only Dutch that does not like the Kreidlers :-) . joke, apart, it was just a feeling of me that the kreidlers are very popular in the Nederlands. Of course, not everybody like them. 

Okay Frits, so there is no general formula to calculate the carb size? But one of my calculations seemed to be right at least. there was a thread of a guy that wanted to build a road racer with kx 500 engine. my first thought was about the carb size he would have to use. i calculated that the port area that is available is about 2,5 times more than the RSA`s. so the carb area also must be 2,5 times more then the RSA`s. I ended up with a 67mm Carb. Then i happilie noticed that Jan recommended a 65mm Carb. Which is not available unfortunately.


Jan, you mentioned the floor of the transfer ports and i took that in account when rebuilding my KX 500 Engine. I rised the cylinder by a spacer and now the floor of the ports is at the same height as the edge of the piston in BDC. except the C Port not. Because it does not have a floor. Almost every cylinder i saw shows a not very well matching c port floor or no floor at all. I will not make the transfer ports wider at the moment. i am not very experienced and i don`t want to mess up the scavenging by changing angles or make any other mistakes. 

anyway, i may had a little luck when buying that cylinder. It is a little bit different than a usual KX 500 cylinder and i haven`t seen this before. The Main exhaust port has a totally different shape in comparison to a normal kx 500 cylinder. It makes the powervalve unusable. But i don`t care because i don`t like the KIPS. On the one Hand the KIPS does a lot. it changes the port timing, it opens and closes resonance chambers in the cylinder and head and it "switches" the auxiliary ducts on and of. But it is much to complicated. And in my opinion a 500ccm cylinder does not need a powervalve. And the 2 stroke engine in therms of gas dynamics, cylinder scavenging and so on is already complicated enough. so there is no need for an additional mechanical "complexication".

The other thing, that is different: normally the 2 aux. ports are devided, i don`t know why. and usually there are "notches" in the top of the aux. ports witch i have closed by welding before the cylinder was replated.

And finally, i guess you already noticed that the exhaust floor is higher than the transfer port floor. It is 5mm higher.

Jan, just for interest, what do you think: are the a ports too faar away from the exhaust port?[Vous devez être inscrit et connecté pour voir cette image] 
[Vous devez être inscrit et connecté pour voir cette image]

Need more carburetion ?

You can always run an S&S Super D with 2 1/4 " bore ( venturi is slightly smaller ), or read on about the old Maico 501's & mods.

Yeah, two different sized carbs were mounted.

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

Hell, I thought " everybody " knew about this one.

Dear Frits and Jan,

this is the first time I write in this topic, I am very thankful to have you two grandfathers of 2-stroke
racing to share your knowledge here!

I have some questions regarding the RSA engine. You mentioned the problems with part-load
detonations. How did the part-load detonations (or residuals) improve with the reed-valve engine? Was it a big difference?

The next thing I wonder is about transfer duct shape.
Could you say what was your priority for max power given that you have reached the max mean
area of the cylinder?
By that I mean the distribution of massflow over the port open period.

I guess that the simple answer is of course max power, but could you develop it more?

Did you do flow testing at a given port opening to see the effect of a certain change to the duct
or did you do flow testing at fully open ports only?
Or did you do only do engine testing?

At the stage of development did you have any incylinder measurements or simulations or just
rear wheel output?

thanks a lot for your time

best regards
Truls Herland
Sweden

Ambike, thank you. That is an interesting story. i also hat the notion using 2 44mm Mikunis. but that is against my relation to a two stroke engine. a second carb means a major complication to me. and i don`t want to complicate the engine. i can imagine that carb. setting could get very funny with 2 carbs. but maybe lectron has a solution. not 65mm perhaps. but 55mm at least. that is not enough for the 130-140 HP Jan expects a good 500ccm single with 86mm bore and stroke could produce if one makes everything right. but anyway, i do not expect me to do everything right. so a 55mm carb should be enough. better than the stock 39mm carb at least. imagine..... 39mm carbs are used on 125ccm engines usually. and the manufactures bolted them onto 250ccm and even 500ccm also... so this is surely an answer to the question, why a stock 500ccm single mx bike makes "only" 50-60 HP. But on the other hand, who wants to ride a 500ccm mx bike with 130-140 in the woods?   before i rode my kx 500 the first time, i laughed about all the stories about people that where climbing of that bikes with shaking knees and hands after riding it the first time. but my knees and hands were also shaking when i stopped.

By the way, i have another kx 500 cylinder. i once took a look at it and wondered, why the pre owner used black colour for the transfer and intake ports..... taking a closer look a noticed that it wasn`t black colour but carbon deposit, a thin and even layer.... i wondered about that but in this thread i found the reason for that.

Another question regarding the 65 mm carb. is: Is the inlet duct and the reed able to handle a 65mm carbs area....[img][Vous devez être inscrit et connecté pour voir cette image][/img]

So much tiny dividers, are not those prone to cracking being so thin and the engine vibrating quite a bit, plus all the force that the piston skirt exerts on them?

I think you need at least the same area in the reeds(probably more due to losses) than your carburettor area, so having reeds with the needed area might be a problem, maybe with a huge spacer and custom made ultra long reed block?

Senso, there are 2 dividers and they are not as tiny as they look on the picture. but you are right concerning about cracking. the kx500 cylinders are very well known for cracking in this area. should i remove the deviders? then they can not crack anymore.

There is also thin walls in the boyessen ports and between the cylinder liner and the direct crankcase intake holes, removing the dividers might make them even more prone to cracking.
But I dont know those engines, so all I can say is good luck with your project, and keep us updated.

from the standpoint of piston support, i dont think i would remove those dividers. i also have a 500cc engine with a thin inlet divider. it has never cracked.

Martin1981 a écrit:

Senso, there are 2 dividers and they are not as tiny as they look on the picture. but you are right concerning about cracking. the kx500 cylinders are very well known for cracking in this area. should i remove the deviders? then they can not crack anymore.

Hi Martin,

Don't do it!
Auf keinen Fall wegmachen!

Keine Sorge, ich werde die Stege natürlich nicht entfernen.

Don`t worry guys, i won`t remove the bridges of course. 

I think one can not expect that engine make 120-130 hp although it might be possible i certainly won`t do everything right making the engine asking for a 65mm carb. so i think there is no need to make the inlet larger. but even if it will "only" make 70-80 hp it will scare the **** out of me in the woods.