[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

Did you titanium nitride the rings or were they just chrome faced?

They were just chrome faced, made in Japan.
We used 0,8 and 1mm rings, 0,8 being the best.

Did you run the ring end gap up the center of the rear C transfer or was it situated to run up the cylinder wall? The reason I ask is because on our rotax engines they came with the end gap running up the cylinder wall between the B port and the C port but when we widen the B port it causes the end of the ring to protrude into the port. To solve this we relocated the ring locating pin to the rear of the piston. We do suffer a bit of trama to the C port where the end gap meets the top of the port but not to a point where it degrades the ring seal.

Our ring end gap was always in the centre of the C-port.
We never had any problems!

Hello Guys,

First: VERY interesting and informative thread. Thank you for this. I love to read in it :)

And I've got a little question that has occured to me: How much HP (or % of power) would an engine like the Aprilia RSA loose, if there was NO C-Port. And in such a case: Would it make sense to use more inclination for the A- and / or the B-Ports?

Regards

I tried a cylinder without C-port.
And I tried to make the B-ports wider.
The engine lost about 2HP.

Jan Thiel a écrit:

I tried a cylinder without C-port.
And I tried to make the B-ports wider.
The engine lost about 2HP.

Hi Jan,
That is interesting and maybe that is why the Japanese do not make the C port very large if it is only worth 2HP. Seems such a waste of port area for only 2HP on a 54HP motor.
Are there other disadvantages of not having a C port such as piston cooling?
Did you perform tests on the axial angle of the C port and did you try the same axial angle as the B ports?

Thank you in advance for answering our question; we are very privileged.

The original Rotax C-port was 19mm wide.
A bit on the limit for the pistonring slot.
I reduced this to 15 and so I could widen the B-ports.
This gave a good improvement.
A C-port 12mm wide was also tried with even wider B-ports but it was worse.
Long ago the C-port flow passed through the piston. (MZ)
This gave good piston cooling but less HP.
Nowadays the best piston cooling is achieved by making all transfer ports as wide as possible.
And even 'short circuiting' from the A-ports into the exhaust helps piston cooling!
This charge is not lost as it is pushed back into the cylinder by the positive pressure wave from the exhaust pipe
It also proved to be VERY important that the lower side of the transfer ports was not lower than
the piston in BDC. Also the C-port!
This is almost just as sensitive as the top of the ports!
In almost all cylinders this is wrong!
Different upward angles of the C-port did not give better results.

In my opinion 2HP is quite a lot!
If you did not have a C-port what else could you do with the cylinder surface?
Nothing!
So it is better to have the C-port!

Hi Jan,
Thanks for the informative answer.
Yes, we would all like to find 2HP easily but what I was wondering, if that area could be used more effectively for additional width on the A and B transfer port instead of having C port. You have answered that question, thank you.

I once cut the reed horizontal cavity off a 1990 Yamaha YZ80 cylinder and replace it vertically with a larger reed and cavity. This also meant that the C port no longer had an upward angle but it’s flow was straight into the cylinder when the situation was correct. The result was the torque range widened and increased slightly. Because I have done two things at once, it was hard to determine which had the most effect but I did want to revisit testing the C port’s direction to flow directly over the piston with the aim of slowing and pushing upward the main transfer column. I suspect that the C port would need to be retarded in it’s opening for this to happen effectively.

Interestingly, the later model Yamaha YZ80 has it’s transfer ports’ bottom edges above the piston by a reasonable amount. I never modified these as I thought that the smaller port fully opened for longer might have a better overall flow effectiveness than a taller port partly opened.

Thank you for your time.

I think having the port lower edges above the piston in BDC might be bad for piston cooling.
The flow will detach from the piston crown.
You could try to modify them, but be very careful with the inside radius of the transfer duct.
It should be as big as possible!
I found every single square mm more port surface improved power!

Jan Thiel a écrit:

It also proved to be VERY important that the lower side of the transfer ports was not lower than
the piston in BDC.

Of course this makes sense... but the piston is not at BDC very long, so the step is almost always there anyway. Silly idea: did you ever try leaving the bottom of the transfers higher than BDC to sacrifice some time-area for better flow?

I guess it would be bad for piston cooling...

Yes, I did.
The power was a bit less.
And you could see the piston was less cooled.
The piston is at BDC for a relatively long time.
Remember it stops and changes direction!
And the more time/area the more flow!

I always thought the c port was there to shape the flow of the air coming out the a and b ports.
I normally aim the c port up at the closes side of the combustion chamber to help flush the chamber out

Yes Wax, you are completely right!

otherwise Coanda effects at the "corners" created>lack of flow[Vous devez être inscrit et connecté pour voir cette image]

Hi Jan and Frits,
I have noticed the position of the EGT sensor in the Aprilia cylinder exhaust flange and I was wondering why it is mount so close to the exhaust port?
Could you tell me what is your temperature target for the EGT at over rpm range, 700 C?

AIM Mychron suggest 150mm from the piston face to the EGT sensor; is this incorrect for an accurate reading?

Thank you,

Measurng after 150mm will give you higher temperatures.
There is nothing wrong with this!
As long as you always do it in the same way!

Thanks Jan,

When are you going to write a book on your 2 stroke experiences -please make it soon

With pictures!

GtG001 a écrit:

Hi Jan and Frits,
I have noticed the position of the EGT sensor in the Aprilia cylinder exhaust flange and I was wondering why it is mount so close to the exhaust port? AIM Mychron suggest 150mm from the piston face to the EGT sensor; is this incorrect for an accurate reading?

Putting the EGT sensor in the exhaust flange has the disadvantage that it will not only be touched by the exhaust gas but also by washed-through fresh mixture, which then gives a lower temperature reading. But mounting the sensor further away, somewhere in the header pipe, has the disadvantage that it wil be subjected to more severe vibrations, shortening its life. That is why the position in the flange was chosen.

GtG001 a écrit:

Thanks Jan,

When are you going to write a book on your 2 stroke experiences -please make it soon

Making a book on 2strokes, I think it makes no sense now they are disappearing.
And maybe I am also too lazy!

Yes, they are not being made in any numbers now but this forum is proof that there are people who would buy such a book ....
I have gain a great deal of knowledge from this forum, so thank you for what you have already shared with us and put me on the list if you change your mind.

On the rsa cylinder you use 3 exhaust ports versus a bridge port.
Did you try it as a bridge port as I was lead to believe they make more power due to there larger exhaust port area

wax a écrit:

" I was lead to believe..."

Not by race results....

Ha ha
Ok frits