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

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Döllinger



Nombre de messages : 32
Localisation : Biblis
Date d'inscription : 14/01/2017

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Sam 14 Avr 2018 - 9:42

Hallo,

theses days i got an ignition from PVL for a Maico 250.
It´s fully adjustable. I found for the start a time/avance sheet of an excisting aftermarket CDI (a update CDI-box which uses the stock Kröber rotor/stator) ) with values that seem weird to me.
Bore 76mm, stroke 54mm, conrod lenght 137mm.
1500-16°
2000-16,5°
2500-17°
3000-15,5°
3500-18,5°
4000-19°
5000-20°
6000-20°
7000-21°
8000-21,5°
9000-21,5°
10.000-21°
11.000-20°
The advance is getting less at 10k rpm, a number the Maico will hardly reach.The original Kröber ignition got a curve with even more advance.
My engine is ported to Frit´s transfer times, got a 38mm Dellorto, and a resonance exhaust, ect, ect.
You think that´s a curve to start with and get on the dyno, or will it end in tears. (piston melting ect.)
I read Frit´s "Zündkurven" and think that the suggestions in this essay don't match with the ignition timing i postet above.(and the other ignition curves i use for other engines)
It´s hard to find an engine with comparable technical data.

Best regards,

Bernd

PS. Bob van der Zijden told me that at these low revs the strenght of the spark is far more important than an advance curve. What is your opinion?
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Frits Overmars

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Nombre de messages : 2000
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Sam 14 Avr 2018 - 13:06

Bernd, with my 'Zündkurven'-essay at your disposal there is an easy way to generate your own ingnition curve.
Al you need to do is multiply the rpm-values in my curve with the rpm-value at which your Maico produces its maximum power, and divide each outcome by the 13000 rpm at which the RSA produces its maximum power.

At low revs the compression pressure and the turbulence in the combustion chamber are not very strong, so I don't think you will need a strong spark in those circumstances. But because the density and purity of the fresh mixture will also be mediocre, chances are that there is not always an igniteable mixture present at the right moment between the spark plug electrodes, so you will need a spark with a long duration, in order to improve the hit-or-miss chances.
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Döllinger



Nombre de messages : 32
Localisation : Biblis
Date d'inscription : 14/01/2017

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Dim 15 Avr 2018 - 3:10

Hi Frits,
thank you for your fast reply. The RSA-curve is close to the one for my Rotax 256.
I calculated the RPM's and supposed to have the max. power at 7500 rpm. I assume the finally max power at 8500, the 2nd curve to switch is calculated for that, but for the first start i want to keep the engine undamaged.
Here´s the comparison between the updated Kröber and the converted RSA curve.
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I assume that the flat characteristic of the Kröber is accounted by it´s wiring with an electronic filter.

Good explanation with the spark duration and an interesting issue. I will have a look at the flashover on the oscilloscope.
Another question: Is it only possible to operate with such a high advance in the power band (Kröber and others) on an engine with low torque at high rpm. A torque that is so poor, that the actually to high advance has no damaging effect?

Best regards,

Bernd
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Frits Overmars

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Nombre de messages : 2000
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Dim 15 Avr 2018 - 9:09

Döllinger a écrit:
Is it only possible to operate with such a high advance in the power band (Kröber and others) on an engine with low torque at high rpm. A torque that is so poor, that the actually to high advance has no damaging effect?
Well, the torque is only so poor outside the power band. And then 30° ignition advance is not too high.
The early ignition results in a high expansion rate after the end of combustion, so the exhaust gases will contain very little energy and the wrongly-timed exhaust pipe pulsations will not upset scavenging too much. It's all explained in the Zündkurven-essay.

A word of advice: do not retard the ignition timing too much after maximum power. An advance of only 5° will yield quite hot exhaust gases and your engine may not be thermally sound enough to endure this. I wouldn't go later than 10° advance.
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Döllinger



Nombre de messages : 32
Localisation : Biblis
Date d'inscription : 14/01/2017

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Dim 15 Avr 2018 - 10:04

Hi Frits,

thank you for your advice. I will change the ignition timing after the power band.

I`m not sure if the aspect i meant came out clear enough. When you see the Kröber ignition timing, it´s got much more advance after, let´s say 7000rpm. (in the power band)
If i would have so much advance in my Rotax, the engine would not survive that. My question is: How can an engine survive an advance as high like on the green curve?
For the first passage you wrote. I understand this. I read your essay many times.
Thanks for your efforts,

Bernd

PS. My CNC operator is coursing the day he agreed to mill the cylinder head. It looks good so far, but he is not ready, yet.
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Frits Overmars

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Nombre de messages : 2000
Age : 69
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Dim 15 Avr 2018 - 13:24

Döllinger a écrit:
When you see the Kröber ignition timing, it´s  got much more advance after, let´s say 7000rpm. (in the power band).
If i would have so much advance in my Rotax, the engine would not survive that.
My question is: How can an engine survive an advance as high like on the green curve?
I'm not sure that it can. It depends on the specific power it's making. In the period when the Maico was built, many racing engines had a fixed 20° ignition timing. You would hardly get away with that on a modern competition engine.
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Döllinger



Nombre de messages : 32
Localisation : Biblis
Date d'inscription : 14/01/2017

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Lun 16 Avr 2018 - 13:51

Hi Frits,

thank you for your efforts. Here are two pics of the Maico cylinder head, nearly ready, only some cosmetics are missing. Combustion chamber exactey like in your drawing.
It´s very interesting to watch and listen how he´s milling that out of a solid block. My CNC wizard needed ca. 25h of work until now, without drawing. (all fins are conical)
Probably i have to weld some thin stripes on the top of each fin to keep them from cracking under vibrations.
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patouille

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Nombre de messages : 24
Age : 28
Localisation : PARIS/CHAUMONT
Date d'inscription : 19/01/2011

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Mer 18 Avr 2018 - 3:15

While on the spark advance curve, do some of you have more information about the coil charging timing curve ?
I'm designing my own ignition module but I'm struggling to find more information on the dwell especially on high revs engines.
I suppose this aspect of the ignition affects the power through the spark intensity.
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http://www.mothersoxer.racing
Döllinger



Nombre de messages : 32
Localisation : Biblis
Date d'inscription : 14/01/2017

MessageSujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)   Ven 20 Avr 2018 - 6:47

Hi,

the cylinder heads are ready. The sphere top is a little kept a few 1/10mm higher. So i can define the volume of the combustion chamber exactely to mill out the missing volume. The surface is approx 3 times the original head.
Thank you Frits for your advice in case of the combustion chamber design! Only little bridges between the single fins are missing aside a few hours of finishing with file and sandpaper. About 55 hours of CNC work.
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