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

Hello to all,

I asked few questions few years back and I am happy to see this thread is still alive!

I would like to ask about this port design in the pictures /I hope they will appear /

I assume that those ports under the transfer ports are open when the piston is at TDC and when the piston starts to move tovards BDC those ports help to fill the transfer ducts earlier.  
What gains could be achieved with this design? low or midrange power gains maybe? I would be happy to hear someones more experienced thoughts on this.

Thank you for sharing


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Really useless I would think....

Thank you Jan.

I would like to ask if I can use the axial angles of the FOS scavenging concept for a port arrangement like this? C ports thru ports thru the piston.
Using the FOS angles of A and C ports. Or what axial angles would be suitable?
Its a 55cc engine to be used for MX with 3 speed gearbox.
Bore 40mm stroke 44mm. It has a prretty long rod of 100mm and 100mm diameter crank webs, so pretty large crank case volume.

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The dimensions in the picture arent actual dimensions.

There are no C-ports in your drawing Jenco, only A- and B-ports. The transfer port designation is as follows: the transfer ports closest to the exhaust port are called the A-ports. If there are more sets of transfer ports, they are called the B-ports, then the C-ports, and so on, as you can see in the picture below.
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It is difficult to give a suggestion for the axial angles without even knowing your A-port width. But as a rule of thumb you could take the average of the axial A-port angle and the axial B-port angle from the concept below.
For your stroke/bore ratio of 1,1 that would be the average of 26,8° and 11,2° = 19°.
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Thank you Frits,
I called them C ports because their direction angles /180°/  My bad,
So the direction angles of my B ports are 180°
Their width is 10mm, axial angle 36°
A ports are 16mm wide and their direction angles are leading 60° trailing 70° axial only 2,5°

So maybe I should take the average of B and C of FOS concept.
Thats 34,1° for my B ports and 26,8 for my A. Is that right?

The axial angle calculation in my scavenging concept is valid for a five-port transfer layout; you should not apply it to calculate the axial angle of B-ports that are aimed directly at the exhaust port. 34,1° axial angle would give your B-ports a nice large effective area, but then their flow strength would overpower the flow out of the A-ports and upset the scavenging balance. Besides, 60° leading edge angle for your A-ports is old school. If at all possible, you should aim them more sharply away from the exhaust port.

I have no clue as to the piston speed your engine is going to run at, but judging from the single exhaust port, it probably won't be astronomical, so in order to avoid a scavenging column that moves too fast, you might want to keep the A-ports' axial angle on the low side, like 16°. And 45° to 50° should be a safe axial angle for your B-ports.

Yes it is an oldschool engine, 1975ish.
It revs max 12 000 rpm under load. So thats 17,6 m/s piston speed.
It has a single exhaust port 74% of bore worked out okay.
Transfers are 115° and exhaust 170° I need it to have wide range so it can be quick on the track rather than concentrate on peak power.
I consider to rise the exhaust timing more tho because I found a little carbon in the transfers.

Do you mean 12 000 rpm?

Yes, I fixed it thanks

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Frits this A port coordinates, what engine is it from ?
Amended, Sorry - I’ve discovered it’s your cylinder,
However I’m interested in knowing how this differs from the rsw / rsa duct

Apart from some tiny differences, the above drawing shows the A-port dimensions of both my cylinder and the Aprilia cylinders. They share the principle of applying the largest possible inner radius with the aim of promoting flow attachment and thus reducing turbulence losses.
The same applies to the radius at the cylinder base, at the transfer channel entrances, which is not shown in the drawing above, but can be seen below.
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Incidentally, I have never understood why Honda, below, has always held onto an inner radius that tightens downstream, where the Cross Flow Area decreases. The decreasing CFA may counteract flow separation at the tightening inner radius somewhat, but it increases pressure losses and decreases mass flow.
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I completely agree with your view upon the Honda cylinder.
I am a bit confused with the way the short turn radius & 190mm piston crown radius do not continue this flow attachment [Vous devez être inscrit et connecté pour voir cette image]

RAW a écrit:

I am a bit confused with the way the short turn radius & 190mm piston crown radius do not continue this flow.

It's a compromise, like so often, Rick. That 190 mm radius piston crown has an 8° edge angle. Using this same angle for the A-transfers in order to optimize flow attachment would mess up the flow velocity of the central scavenging column as well as its fore-aft balance. Besides, it would only really work while the piston is in BDC.
There is however a simple way of improving flow attachment to the piston, not only when it is in BDC, but all the way: give the piston timing edge a radius.
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A piston egde radius not only promotes flow; keeping the mixture flow attached to the piston also greatly improves piston cooling.
Note: do not give the top edges of the transfer ports a radius. It would hardly help outflow from the transfers into the cylinder, but it would 'help' inflow from the cylinder contents into the transfer ducts, and we do not want that to happen.

For more background on scavenging I tried to attach a file called Pisa.pdf, but the forum software won't let me.
But you can find Pisa.pdf in the folder FOS Tips & Concepts , which you'll find here:
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Thank you Frits, I will keep reading & learning,

grazie ing. Overmars
sei il nostro maestro dei motori a 2 tempi
spero che tu capisca l'Italiano purtroppo conosco solo il francese scolastico

motori49 a écrit:

grazie ing. Overmars. sei il nostro maestro dei motori a 2 tempi. spero che tu capisca l'Italiano purtroppo conosco solo il francese scolastico

Ciao Motori49, grazie mille per le tue parole di elogio. Ho vissuto e lavorato in Italia; amo le persone, la lingua e il cibo.
Ma poiché dobbiamo tenere conto degli altri membri del forum, non possiamo scrivere troppo in italiano qui.
A proposito, il vero maestro dei motori a due tempi è sempre il mio vecchio amico Jan Thiel.

ciao Frits
grazie per la risposta in Italiano
Frits e Jan per me sono entrambi maestri del 2 tempi
je m'excuse pour les amis du forum si j'ai écrit en italienne
per l'inglese uso il traduttore

Aucun problème

Hello Jan, Frits, and the rest of you 2 stroke tuners.  My name is Alan Stuck from Michigan, USA. I am now a retired CNC machinist with his own work shop at home. I have been modifying 2 stroke engines sense the 80's and have built mostly drag race snowmobile engines but also dirt bike engines for enduros, and MX. I currently use TRS software which is out dated by todays standards and I have started to look into engine mod2 software.

As I have read through each section I have been taking notes and looking to see if any of my questions have been answered. I may have missed a few and if the question has already been asked and a answer given please forgive me. I would like to also thank everyone for some very usefully information that has been posted through all 5 sections of the thread. Wow this translation is not working very well.

So my first question is for Jan or Frits. With triple ports. Should the port's tunnel/passage be of the same size as the window at the cylinder liner. In chordal area?? Jan had mentioned earlier that the Aprilia RSA triple port size was 12mm X 13mm = 156 sq.mm. So should the window at the liner be of the same sq.mm or should it be bigger?? If bigger is there a ratio to the size it should be??

Welcome Alan.
As you'll see below (colour pics by GighenZzMarco) the triple ports are no rectangles but more akin to triangles.
Their cross flow area is indeed about 156 mm². It is advisable to maintain this cross flow area all the way until the triple tunnels merge into the main exhaust duct, the reason being that the total duct volume should be kept as small as possible.

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Thank you Frits, Those pics and blue print gave me a great understanding on how they should look. A few pages back you were talking to polcat88 and he had questions about the exhaust port angles. Being I also modify race snowmobile engines I thought of this while reading those post. You told him (The roof angle of the exhaust should be about the same as the roof angle of the A-transfers: around 25 °). Now for drag race snowmobile engine for maximum power with a narrow power band would it be better to have a shallower A transfer port angle like 14*?? And, If you have a shallower angle on the A transfer port would you need a shallower angle on the exhaust port as well?? I did read and understood the tower of Pisa. Which made perfect sense why you wanted a higher angled A transfer port. My thinking is for maximum power only.

Zrt1200 a écrit:

In the center of the blue print you have in red writing. Maximum over lap of 57*. Would this be at the back of the triple port window or over the entire triple port??

If I'm not mistaken, it's the point in the stroke at which the short-circuiting between auxiliary and A-port through the piston-pin is biggest, and the reason aprilia used closed-welded pison pins.

Thank You JanBros. I had just figured out what Frits had meant and edited my post accordingly. Well I think I figured it out. Now I am not sure.

Zrt1200 a écrit:

for drag race snowmobile engine for maximum power with a narrow power band would it be better to have a shallower A transfer port angle like 14*?? And, If you have a shallower angle on the A transfer port would you need a shallower angle on the exhaust port as well?My thinking is for maximum power only.

The Aprilia RSA cylinder and exhaust system have been developed for maximum power only; no compromises.
Any changes on these parts would be unlikely to improve that maximum. The RSA's powerband was optimized via the ignition, powervalve and powerjet mappings.

Thank you Frits for the explanation as I understand exactly what you mean. We use a MSD Digtal 7 on our Drag race snowmobiles and we change the timing curve depending on starting line traction.

With the triple ports when used in a bigger CC engine that the bore and stroke are square. Is there a general rule for maximum triple port size. If the Aprilia 125cc uses a 12mm X 13MM, Could a 250cc use twice the size of triple ports in reality??