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

Hi Frits,

i've got a question about the power valve of the RSW.

My engine got originally pneumatic activated power valve (RAVE)
Due to the disadvantages of the pneumatic i build my own, electronic activation. (2-step)
[Vous devez être inscrit et connecté pour voir cette image]
The servo is Honda NX5.

Here you wrote about the guillotine shaped exhaust valve of the RSW.
[Vous devez être inscrit et connecté pour voir ce lien]
Can you tell me about the opening rpm in the Aprilia RSW?
Best regards,
Bernd

Döllinger a écrit:

Here you wrote about the guillotine shaped exhaust valve of the RSW.
[Vous devez être inscrit et connecté pour voir ce lien]
Can you tell me about the opening rpm in the Aprilia RSW?

It started opening at 10.000 rpm and was fully open at 12.000 rpm.

Hi Frits,

thanks for your information. When you see my slides; you think that is comparable to the RSW system?
I never saw an Aprilia slide in real life.
Best regards,
Bernd

Döllinger a écrit:

Hi Frits, thanks for your information. When you see my slides; you think that is comparable to the RSW system? I never saw an Aprilia slide in real life.

Here you go Bernd: the twin-blade Aprilia power valve.
[Vous devez être inscrit et connecté pour voir cette image]
Rendering by GighenZz Marco

Hi Frits,

the slides don´t look that much different.
Of course the Aprilia activation is much smaller, but not applicable on my cylinders.
Thank you very much for the efforts!

The gear housings were welded yesterday.
[Vous devez être inscrit et connecté pour voir cette image]

Best regards,
Bernd

Nice work !

Good evening everyone !

I have 2 questions for Frits: from your explanation based on the "Pisa tower" concept, it appears that we need to give some axial angle to the transfer channels, and that the A need to have way more than the B.
I have an old 125 TZR that I use some times on the racetracks.
To my surprise, the transfer channels of the stock cylinder show exactly the reverse: the A have a small angle and the B are angled much more upwards. Do you think it is really catastrophic or may I do something to improve the situation and still use this cylinder?
I also have a Suzuki 125 TSR, and it has a flat top piston, which I understand is not ideal (unfortunately there's no possibility to get a domed one). The transfer ducts have no upwards angle. I understand that I may need to change this to get some angle. What do you think would be a correct upwards angle to both avoid slowing down the transfer streams too much and keep some fresh gas close to the top of the flat top piston?

Carlo, correcting the axial scavenging angles on that TZR 125 cylinder would certainly improve it, but I don't know if there is sufficient material in the right places to carry out that improvement.
The flat-top piston in your Suzuki will not make much difference for the optimal axial scavenging angles. All I could say about these angles, I have already said in the 'Tower of Pisa'-story; I would only repeat myself here. But how hard can it be to find a domed piston with the correct bore and pin diameters for that engine?

Thanks for your replies, Frits !
The point is that I will certainly not have enough material to completely correct the TZR cylinder.
I certainly can give more angle to the A ports, but probably not correct the B ones in sufficient proportion. Do you think having both A and B with about the same angle will make things worse or improve?

About the Suzuki, I checked all the pistons catalogs I could find, at the moment I did not find something satisfactory (bore is 56mm, with a 54 it would certainly have been much easier), I have trouble to find one with an acceptable crown height but will go on.

carlovitch1 a écrit:

Thanks for your replies, Frits !
The point is that I will certainly not have enough material to completely correct the TZR cylinder.
I certainly can give more angle to the A ports, but probably not correct the B ones in sufficient proportion. Do you think having both A and B with about the same angle will make things worse or improve?

About the Suzuki, I checked all the pistons catalogs I could find, at the moment I did not find something satisfactory (bore is 56mm, with a 54 it would certainly have been much easier), I have trouble to find one with an acceptable crown height but will go on.

You don't make it easy for me Carlo. "the A have a small angle and the B are angled much more upwards" is not exactly numerical information, so you ask me to gamble with a cylinder that I do not know. But I suppose you could give the A-transfers a 15° axial angle and make the axial angle for the B-transfers as low as material will permit.

You wil find more options for your Suzuki piston if you remove the compression height from your wish list. You can correct this dimension with cylinder base gaskets, or by removing base material from the cylinder, although I would not do this as a first step.

Sorry for my imprecision, Frits.
The TZR cylinder A transfer port channel is almost flat, I would say somewhat 5° upwards, as the B is 20° upwards. In the meantime, I could measure deeper and there's enough material to make the A at 15° and the B lowered to 10-15°. Would this work better?

About the piston, I also found in the Mitaka catalog that the Kawasaki KR1S 250 model may fit on the Suzuki with a small plate under the cylinder and of course some head machining. My concern is more about ringpeg position: all the pistons I see which may work with some adaptation have a way wider one (like 14mm for my original 10mm for the KR1S). I know some guys who have managed to re-locate the ringpeg, but I feel it quite risky with my actual skills and equipment. I still need to think about it to decide whether I try or not.

KR1S piston has holes in it's skirt to lube the exhaust bridges, they will also have to match or you'd need to fill them.

Thanks for info JanBros. This worries me less because it is also the case on the 125 TSR piston.

quelqu'un reconnait ces échapement ?

[Vous devez être inscrit et connecté pour voir cette image]

ils avaient un étiquette "KR1" mais le support au midi de l'échapement est faut  et la spipe-sortie est trop bas pour un KR1

[Vous devez être inscrit et connecté pour voir cette image]

on peut pas le monter sur un KR1 (ça manque pas grand chose) mais aussi le support est complètement faux



ils sont "after market" et tres légère

If given the chance to build another 125cc 2-stroke engine, would you choose the 54 mm x 54 mm bore and stroke of the RSA 125, go to 56x50, or even more oversquare than that?

Charles Kaneb a écrit:

If given the chance to build another 125cc 2-stroke engine, would you choose the 54 mm x 54 mm bore and stroke of the RSA 125, go to 56x50, or even more oversquare than that?

Neither the one nor the other, Charles. With 54 mm x 54 mm you would only have 123,67 cc, giving 1,06 % of your allowed cylinder capacity and your engine power away to your competitors. Generosity may be commendable, but not if you want to win races. The Aprilia RSA and just about all other modern 125 cc engines have 54 mm bore x 54,5 mm stroke.

Oversquare is good for four-strokes, where the diameter of the valves that fit into the head is determined by the cylinder bore.
For two-strokes it's the other way around. I'll give you an example, with a simple rectangular exhaust port, with a port height of 50% of the stroke and a port width of 70% of the bore. For an engine with 100 mm bore and 100 mm stroke (that's 785,4 cc) this would mean a port height of 50 mm, a port width of 70 mm and a port area of 50 x 70 = 3500 mm².

Let us see what happens when we make that engine really oversquare. We double its bore: 200 mm instead of 100 mm.
But in order to maintain the original cylinder capacity, the stroke must shrink from 100 mm to 25 mm.

Now let's look at that exhaust port again. Its height becomes 50% of 25 mm = 12,5 mm, its width becomes 70% of 200 mm = 140 mm, and its port area becomes 12,5 x 140 = 1750 mm². We've lost 50% of our original port area!
This means that our oversquare engine can rev only half as high as our original engine before running out of breath.
It can produce only half the power of the original engine. Oversquare is not good for two-strokes.

Frits Overmars a écrit:

Charles Kaneb a écrit:

If given the chance to build another 125cc 2-stroke engine, would you choose the 54 mm x 54 mm bore and stroke of the RSA 125, go to 56x50, or even more oversquare than that?

Neither the one nor the other, Charles. With 54 mm x 54 mm you would only have 123,67 cc, giving 1,06 % of your allowed cylinder capacity and your engine power away to your competitors. Generosity may be commendable, but not if you want to win races. The Aprilia RSA and just about all other modern 125 cc engines have 54 mm bore x 54,5 mm stroke.

Oversquare is good for four-strokes, where the diameter of the valves that fit into the head is determined by the cylinder bore.
For two-strokes it's the other way around. I'll give you an example, with a simple rectangular exhaust port, with a port height of 50% of the stroke and a port width of 70% of the bore. For an engine with 100 mm bore and 100 mm stroke (that's 785,4 cc) that would mean a port height of 50 mm, a port width of 70 mm and a port area of 50 x 70 = 3500 mm².

Let us see what happens when we make that engine really oversquare. We double its bore: 200 mm instead of 100 mm. But in order to maintain the original cylinder capacity, the stroke must shrink from 100 mm to 25 mm.

Now let's look at that exhaust port again. Its height becomes 50% of 25 mm = 12,5 mm, its width becomes 70% of 200 mm = 140 mm, and its a port area becomes 12,5 x 140 = 1750 mm². We've lost 50% of our original port area!
This means that our oversquare engine can rev only half as hight as our original engine before running out of breath. It will produce only half the power of the original engine. Oversquare is not good for two-strokes.

Frits, wouldn't that mean that undersquare is then preferable? Displacement goes with bore squared and straight with stroke, while port area goes with bore straight and stroke straight, so maximizing port area would require the bore to be 2/3 that of stroke.

Charles Kaneb a écrit:

Frits, wouldn't that mean that undersquare is then preferable? Displacement goes with bore squared and straight with stroke, while port area goes with bore straight and stroke straight, so maximizing port area would require the bore to be 2/3 that of stroke.

Yep. But the long stroke would give a high mean piston speed. Normally this is not a problem for a two-stroke; it runs out of breath before that piston speed starts to threaten mechanical reliability. But we shouldn't overdo it.
Look at today's 250 cc MX two-strokes: all manufacturers seem to agree that 66,4 mm bore x 72 mm stroke is the optimum.

Hi everyone, hope you all are doing well.

Frits, I fully agree with your explanation, historically do you however know why Yamaha for example stuck for such a long time with 56 x 50,6mm in their race engines ? Was there a specific technical argument behind this?

carlovitch1 a écrit:

Frits, I fully agree with your explanation, historically do you however know why Yamaha for example stuck for such a long time with 56 x 50,6mm in their race engines ? Was there a specific technical argument behind this?

Not that I know of, Carlo. By the way, the classic Yamaha dimensions were 56 x 50,7 mm but we won't argue about one tenth of a millimeter more or less. All Japanese manufacturers have used these dimensions at one time or another and if I remember correctly, Suzuki was the first to switch to a 54 mm bore, followed by Honda, and finally Yamaha and Kawasaki.
[Vous devez être inscrit et connecté pour voir cette image]

Thanks for your reply, Frits. The table is even longer than I was thinking about.

Frits Overmars a écrit:

carlovitch1 a écrit:

Frits, I fully agree with your explanation, historically do you however know why Yamaha for example stuck for such a long time with 56 x 50,6mm in their race engines ? Was there a specific technical argument behind this?

Not that I know of, Carlo. By the way, the classic Yamaha dimensions were 56 x 50,7 mm but we won't argue about one tenth of a millimeter more or less. All Japanese manufacturers have used these dimensions at one time or another and if I remember correctly, Suzuki was the first to switch to a 54 mm bore, followed by Honda, and finally Yamaha and Kawasaki.
[Vous devez être inscrit et connecté pour voir cette image]

please !y tise french ! MERCI FRITS

Jette un oeil ici : [Vous devez être inscrit et connecté pour voir ce lien]

Toop a écrit:

OWJ1 (1996) 180cv - Adoption d'un piston à alésage de 54 mm . Pole en République Tchèque de Jean-Michel Bayle

Toop a écrit:

Lacconi 1998 OWK1 (1998-99) +180cv – Arrivée du sans plomb

alain combemale a écrit:

please !y tise french !

?
Google Translator could not help me, so I will have to guess what you mean Alain, but I think I know:
[Vous devez être inscrit et connecté pour voir ce lien]

Hello everyone, hope you are doing well.

I have a question for Frits and Jan about the transfer ducts orientation.
If my understanding of the "tower of Pisa" transfer flow concept is correct, the A and B ducts are angled upwards at different angles (A somewhat 18-20°, B somewhat 8-10°) so the flow doesn't lose too much energy by a collision at right angle and is directed upwards.
What if we put a slightly different upward angle to the A ducts (left 16° and right 20° for example) and the same for the B ones, so the collision is even less energy consuming but the general flow direction is kept ?
Was this tested once (at least maybe because of cylinders having a foundry not that symmetrical)?