[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Hello everybody ! I have a question about the shape of the exhaust port. Is this a good shape for a racing engine with a width 70 % of the bore ? Are the radiuses to big or to small ? What would you change ? Thank you in advance.

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ice t a écrit:

Hello everybody ! I have a question about the shape of the exhaust port. Is this a good shape for a racing engine with a width 70 % of the bore ? Are the radiuses to big or to small ? What would you change ?

A port width of 70% of the bore is fine, ice t. But the straight upper and lower port edges are not.
An elliptical shape for the upper and lower edges offers the optimum guidance for the piston ring.
Such shapes can be produced with CNC machinery, but for those who have no access to CNC I wrote a program to convert the elliptical shapes into multi-radius shapes that are easier to produce with conventional tools.
The drawing below shows the safe limits for an engine with a good quality piston ring.

A word of explanation concerning the optimum port width of 70%: if you make the port any wider, the necessary safe corner radii will need to be so much larger that you lose valuable port area in the top corners which will reduce the available blowdown angle.area.
You can make the bottom corner radii larger in the interest of reliability if you wish; the areas at the bottom do not play a role in blowdown.
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The vertical half-axes in the above drawing can be calculated with: Half-axis = 0,7 * (port width / bore)^4,57 * bore
Converting the elliptical shape into a multi-radius shape is more complicated and I see no possibility to upload the program I wrote for it (Marc, how can I attach a zipfile here?), but programs like SolidWorks and Autocad may offer a solution.

hi frits. seems like theres been several rsa125 photos but i dont recall any mention of the specific roof and top corner radiuses. would your drawing represent the roof and top corner radiuses that were used on rsa125 ?

Than you for the answer Frits but i am a little confused right now. My cylinder is a 47,6 bore . For the half-axis calculation i took 70% bore width for the port width that gives 33,3 mm , and got the result 6,5. Those that mean the vertical half axis should be 6,5 % of the bore ? I also made a portmap of my cylinder which shows the original shape of the exhaust port (the portmap width of the exhaust port is 36 mm).

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brokedown a écrit:

hi frits. seems like theres been several rsa125 photos but i dont recall any mention of the specific roof and top corner radiuses. would your drawing represent the roof and top corner radiuses that were used on rsa125 ?

Pretty much. But the lower part of the RSA exhaust port was different; it was narrowed and raised above BDC. You might also want to take a look at this RSW-drawing.
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ice t a écrit:

Than you for the answer Frits but i am a little confused right now. My cylinder is a 47,6 bore . For the half-axis calculation i took 70% bore width for the port width that gives 33,3 mm , and got the result 6,5. Those that mean the vertical half axis should be 6,5 % of the bore ? I also made a portmap of my cylinder which shows the original shape of the exhaust port (the portmap width of the exhaust port is 36 mm).

0,7 * (33,3 mm / 47,6 mm ) ^4,57 * 47,6 mm = 6,5 mm

Frits Overmars a écrit:

Pretty much. But the lower part of the RSA exhaust port was different; it was narrowed and raised above BDC. You might also want to take a look at this RSW-drawing.

thnx frits. im pretty familiar with the rsw drawings.  i knew the rsa exh window lower portion was different from the rsw from looking at photos.  just was never sure what the rsa roof  radius and top corners were. but it sounds like they were similar to your drawing

i have a new project and the main window roof/ topcorner radiuses will be like your blue and red drawing, but the lower portion will narrow inward like the rsa. also im raising the window floor above BDC. the duct floor will be raised with weld also. from my calculations that will put the blowdown area about the same as the duct exit area

brokedown a écrit:

i have a new project and the main window roof/ topcorner radiuses will be like your blue and red drawing, but the lower portion will narrow inward like the rsa. also im raising the window floor above BDC. the duct floor will be raised with weld also. from my calculations that will put the blowdown area about the same as the duct exit area

When raising the exhaust duct floor, make sure not to lose any blowdown area. The blowdown should be able to continue unimpeded until the transfer ports are effectively opening.
After filling the exhaust duct floor with weld, it might be a good idea to remove some of the original material below the weld so that coolant can get close to the new duct floor.
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thnx for the help frits but theres one thing that isnt real clear to me. alot of cylinders have the A port angled forward toward the exh, like in your diagram. my cylinder happens to be just a sqaured rectangle A port. i could easily angle it forward with my porting tool but whats the pros and cons of having it angled forward vs leaving it a sqaured rectangle ?

brokedown a écrit:

thnx for the help frits but theres one thing that isnt real clear to me. alot of cylinders have the A port angled forward toward the exh, like in your diagram. my cylinder happens to be just a sqaured rectangle A port. i could easily angle it forward with my porting tool but whats the pros and cons of having it angled forward vs leaving it a sqaured rectangle ?

It's not angled forward; it's beveled off at the top. If you just angle it forward where it enters the bore, you will get a massive short-circuiting of fresh charge into the exhaust port.
What you see is the intersection with the cylinder bore of a transfer duct that is wide over its entire length.
The transfer window is just beveled off to make room for the auxiliary exhaust window directly above it (in the case of the Aprilia racers) and to create some distance between the scavenging flow and the main exhaust port.

frits i dont understand what your saying.  ok i think i understand what your saying. i wasnt refering to inside the duct. just the outside front edge of the window. you see how it slants forward toward the exh ?

what i meant is this: is it better to have the front edge of the window slanting forward  or keep it sqaure ?

you can see this A port is slanted forward

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here you can see A port is sqaure shape with all  corners being 90*

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Hi Guys,

This configuration works very well for me, what do you think about?

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By by

Something about what you said about porttiming is not really clear to me Frits.
You said that for all racing engines, it would be ideal to have 190 degrees exh port duration and 130 degrees transfer duration. (sometimes you would obviously need to change that for bigger area...)
I understand that that's right for a GP engine running maybe 25m/s of mean piston speed.
But is that also right for an enduro bike running maybe 19m/s of mean piston speed? Because this approach is completely new for me!
So what do you mean with 'all racing engines' ? I hope that you find time to make this clear for me!
De groeten van een mede nederlandstalige!  

I meant all two-stroke engines that depend on exhaust resonance for power, and have a transmission that allows them to be kept in the resulting power band. That includes enduro bikes.

Frits a few questions;

How many liters per minute flowed through the RSA coolant system for optimum performance? We're any coolant enhancers added to coolant?

Also in earlier posts you mentioned a domed piston was always better then a flat top. Did you test various domes radius? And what was your results? Thanks in advance!

Thanks a lot Frits!

Frits Overmars a écrit:

brokedown a écrit:

thnx for the help frits but theres one thing that isnt real clear to me. alot of cylinders have the A port angled forward toward the exh, like in your diagram. my cylinder happens to be just a sqaured rectangle A port. i could easily angle it forward with my porting tool but whats the pros and cons of having it angled forward vs leaving it a sqaured rectangle ?

It's not angled forward; it's beveled off at the top. If you just angle it forward where it enters the bore, you will get a massive short-circuiting of fresh charge into the exhaust port.
What you see is the intersection with the cylinder bore of a transfer duct that is wide over its entire length.
The transfer window is just beveled off to make room for the auxiliary exhaust window directly above it (in the case of the Aprilia racers) and to create some distance between the scavenging flow and the main exhaust port.

Hi Frits. I'm working on an cylinder and I did angle the part of the primary near the exhaust to a lower angle (aprox to the angle of the piston dome), not only the top of the port, like on many cylinders I saw, but also the floor. Then the angle rises to the usual 30°. The objective is to lessen the short circuiting in the exhaust. Does this make two-stroke sense?

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Thanks.

Your idea is good, but the piston doesn't stay at the bottom end for a while.
It is not so simple, the flow will be modified by the %age of port opened and the residual pressure in the pre-compression chamber.

jasonsmithrealtor a écrit:

Frits a few questions;
How many liters per minute flowed through the RSA coolant system for optimum performance? We're any coolant enhancers added to coolant?

The coolant flow through the RSA125 was 60 liters per minute. You can't talk about optimum performance though, because, who knows, even more coolant flow might have yielded more power, but that would have meant fitting an even larger pump, but none was available.
Adding anything to the coolant is not allowed in the rulebook (spilled additives might make the racetrack unusable for much longer than water would).

Citation :

Also in earlier posts you mentioned a domed piston was always better then a flat top. Did you test various domes radius? And what was your results?

Jan Thiel tested every possible shape. The result was that domed was best  .
Now a question of my own: where is Watertown?

roost a écrit:

Hi Frits. I'm working on an cylinder and I did angle the part of the primary near the exhaust to a lower angle (aprox to the angle of the piston dome), not only the top of the port, like on many cylinders I saw, but also the floor. Then the angle rises to the usual 30°. The objective is to lessen the short circuiting in the exhaust. Does this make two-stroke sense?

You would have to try it. My hunch is that the sloping port roof would concentrate the bulk of the flow at the exhaust side of the port, which is what you want to avoid.

Frits Overmars a écrit:

jasonsmithrealtor a écrit:

Frits a few questions;
How many liters per minute flowed through the RSA coolant system for optimum performance? We're any coolant enhancers added to coolant?

The coolant flow through the RSA125 was 60 liters per minute. You can't talk about optimum performance though, because, who knows, even more coolant flow might have yielded more power, but that would have meant fitting an even larger pump, but none was available.
Adding anything to the coolant is not allowed in the rulebook (spilled additives might make the racetrack unusable for much longer than water would).

Citation :

Also in earlier posts you mentioned a domed piston was always better then a flat top. Did you test various domes radius? And what was your results?

Jan Thiel tested every possible shape. The result was that domed was best  .
Now a question of my own: where is Watertown?

Thank you Frits !

Watertown is a small city in NY state 20 minutes south of the Canadian border. The area is often called the 1000 Islands region. it's a gorgeous area to visit!

bentou a écrit:

Your idea is good, but the piston doesn't stay at the bottom end for a while.
It is not so simple, the flow will be modified by the %age of port opened and the residual pressure in the pre-compression chamber.

I don't fully understand your explanation. My idea is not far from the conventional designs. On many other cylinders, I saw that the roof of the transfer near the exhaust port is "cut down", so it's vertical angle is much lower than the rest of the transfer roof. While the bottom of the transfer is all equally aimed about 30° up.
In my design the flow near the exhaust (under the boost port), is aimed so it follows the angle of the piston dome, roughly 8°. On the roof and on the bottom of the transfer. Further away from the exhaust, the duct aims the usual 30°. The flow attaches a bit more on the piston crown (cooling it), and stays more out of the way of the exhaust gasses. Hopefully :)

Frits Overmars a écrit:

roost a écrit:

Hi Frits. I'm working on an cylinder and I did angle the part of the primary near the exhaust to a lower angle (aprox to the angle of the piston dome), not only the top of the port, like on many cylinders I saw, but also the floor. Then the angle rises to the usual 30°. The objective is to lessen the short circuiting in the exhaust. Does this make two-stroke sense?

You would have to try it. My hunch is that the sloping port roof would concentrate the bulk of the flow at the exhaust side of the port, which is what you want to avoid.

I think I'll try it. Is there any simple way that can tell us when we have a lot of short-circuiting?

roost a écrit:

Is there any simple way that can tell us when we have a lot of short-circuiting?

Yep. Look at the exhaust gas temperature.
The official method is to measure the power and the fuel consumption, and calculate the specific consumption.

To frits and everyone used to real racing engines, how detrimental/bad is to not have the exhaust duct cooled all the way around, in some engines that I mess with the exhaust duct only has cooling in the top and part of the sides as there is no room to cool it in the underside.

senso a écrit:

To frits and everyone used to real racing engines, how detrimental/bad is to not have the exhaust duct cooled all the way around, in some engines that I mess with the exhaust duct only has cooling in the top and part of the sides as there is no room to cool it in the underside.

Without cooling of the exhaust duct and the cylinder area below the exhaust you cannot have a thermally sound and powerful engine. I can't give you exact figures because I have worked with well-cooled engines since 1978 and apart from some methanol-burning glow plug engines I never performed serious work on anything else since then.