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

In my above answer I mentioned the threedimensional engine control mapping of the RSA, but strictly speaking that was not correct. If an Engine Control Unit with rpm input and ignition timing output is regarded as twodimensional, then what should we call the RSA ECU with four input parameters and three output parameters?
Never mind, take a look at the blue curve in the graph below. That engine had no throttle position sensor, no deto sensor, no EGT sensor, no programmable ignition, no power valve and no power jet, but it had a torque curve like a tractor, making it unbeatable for six years in a row. Why? It was developed by a certain Jan Thiel, about 25 years before the Aprilia RSA.
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For a twin 125cc achieving such torque and power curves is indeed remarkable.

Simply incredible,
Mr. Jan Thiel, if possible, write a little about the main characteristics and about the engine development process that Mr. Frits mentioned.

And I'm sorry if this information has already been written here, I don't remember seeing it

It was never written, I'll do now, as far as I can remember after 33 years.
The engine was designed by Jorg Moller in 1977. It did not win the championship in 1978.
But in 1979 it did, with Angel Nieto as the rider.
Then Moller declared that the engine was at the end of it's development, and that he needed to make a new one. Mr. Minarelli did not agree, and after an  argument Moller left.
At the same time Bultaco, where Martin and me worked at the time had to close.
Minarelli called us so we had a new job almost immediately-
We started work there near the end of January, a bit too late...
There were 4 engines, one with 41,5 HP, 2 with 43 HP and one with 44.
Clearly the cylinders had been 'modified' to give less power..Moller used very rough castings, that needed about 1 week of work to finish.
It was decided to make a very precise new model, following one of the 44 HP cylinders but with a lower exhaust port, as I saw at once that it had been made higher...
It took about 9 weeks to make the model.
But after that you could make a cylinder in 1 day.
My idea was to do all testing work  on a single cylinder engine. so a special crankshaft had to be made. Once this had been made we could finally start serious testing.
We started with the exhaust pipe, making one similar to the Bultaco's
It went very well, after that we made an even better one!
A bit more HP and much more at low rpm
Next problem was to fit it under the bike, a curve in the belly cost a lot of power....
It took us a  week of hard work to get the same power again with the curved belly but we succeeded!
Now the first race was 1 week away, in Austria.
And we still didn't have the new cylinders! Fortunately it snowed, no race!!!
For the next race we were nearly ready, we made 4th and 5th
Then to Jarama, were we could have won easily, but for the rear wheel tube blocking the rear  wheel.
Nieto thought the engine seized and stopped....
The next race at Paul Ricard was won easily with a brand new engine, never tried before.
So the year went on, with various ups and downs. we came 3rd in the championship.
To be continued...

Thanks for this nice story, Jan !

What was the most important factor in your opinion, to get this tractor-like power curve without any of the modern tricks (power valve and electronics)?

Thank you Mr. Jan, waiting for another part of this story...

The secret was the very short timings, exhaust less than 190" with very good flow.

do you still have an idea of transfer timings ?

I was about to ask the same question, wondering whether the "130° rule" applies also to this case.

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Frits would you explain where this head drawing fits within the scale please

Transfer timings were: A 128°, B and C 130°
Inlet 135/75

Thanks a lot, Jan ! Waiting now for the continuation of the story

I don't think I will continue as I think it is of little interest....

+1 to continue! Those storys are always great!

Jan Thiel a écrit:

I don't think I will continue as I think it is of little interest....

for once you are wrong Jan ;-)
stories from first hand are always the most interesting !

JanBros a écrit:

Jan Thiel a écrit:

I don't think I will continue as I think it is of little interest....

for once you are wrong Jan ;-)
stories from first hand are always the most interesting !

I agree strongly!

I also agree strongly.

RAW a écrit:

Frits would you explain where this head drawing fits within the scale please.

That would be AC37.

Thank you

Hey jan!

Be sure! There are thousends quiet listerners who are waiting for your exiting stories...explaniments...tecnical knowledge :)

Tank you very much!

Wolfgang

wos a écrit:

Hey jan!

Be sure!  There are thousends quiet listerners who are waiting for your exiting stories...explaniments...tecnical knowledge :)

Tank you very much!

Wolfgang

I had a look at on the fisrt day this text appears on PLB, and for sure, and the tongue on my knees.... so good; so good !
I hope to read a little more ... as soon as possible !

And the Garelli, so beautiful, one of the most original !
I think i'm right now fully and clearly addicted !


Rudg

Hi Jan, I'd be very interested to hear the rest of the story. I don't think there's anyone here that wouldn't be actually.

Adam

Hi Frits. As you have probably figured out I love to think outside the box. The main two purposes of the two stroke exhaust is to use the energy contained in the wave to 1. aid in scavenge and 2. supercharge the cylinder.  This all has to be done in approximatel .003 seconds for an engine spinning at 10000 rpm. A certain amount of the energy is lost to the atmosphere as noise when the exhaust gases are expelled out the tail pipe. My question is has anyone ever tried to capture any of this lost energy to assist the process of supercharging. My thought was to connect a tune length of tubing from one pipe to the next that transfers some of the energy to the adjacent pipe of a 180 degree firing twin cylinder. The timed pulse would add energy to the reflected wave and should in theory improve the supercharging. Obviously the diameter of the outlet would have to be increased to compensate for the area of the inserted pipe.
As always I value your thoughts or criticism.

Howard, if I understand you correctly, you propose to couple the two tailpipes of an 180°-firing twin, sending a wave
that exits from tailpipe A, upstream into tailpipe B. But with the flow velocity in a tailpipe being sonic for a part of the cycle, a wave cannot move upstream. It would be like a bird flying against a storm: it gets tired but it won't move forward.

My thought is that if you time it right it would add energy to the reflected wave and thus aid in supercharging. At orst it would momentarily raise the static pressure which would have an effect. My question to you is have you or anyone else tried this.