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

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Frits please, could you be so kind and answer for this one. I have tried to measure and calculate those angle areas, so my question is, that how would you estimate max.rpm (and HP) from these figures, so that I can make matching pipe? Is it bad to have blowdown much higher than transfers or vice versa? It doesn't have to be bulletproof analysis, just you quick estimation will do.

Case 1.
50cc (bore 41, stroke 37,8) with transfers reaching 66.16°mm²/cc/1000rpm @ 11136rpm and blowdown 8.72 @ 17847. Transfer timing 130, exhaust 196. Carburetor 22mm.

Case 2.
80cc (bore 52, stroke 37,8) with transfers 66.16 @ 9997, blowdown 8.72 @ 6429, timings 130 and 200. Carburetor 28mm (or bigger?)

Both would get same kind of head that in RSA. Compression ratio 11:1

(Latter one is existing cylinder, not looking too good)

I try to avoid giving quick estimations based on insufficient input. You provided 7 relevant values per engine; I would need about another 100 values, plus at least two full days, if I were to give some meaningful answers. Almost all the questions that you asked during the past week have already been answered by Jan Thiel and myself over the past eight years in the threads I proposed:
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Please read those.

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Ok. I guess I just need to start testing then.

Btw. I already read all these 5 parts...but I still don't really know where to start to solve this puzzle. I had thought, that I calculate these angle areas, and get some kind of estimation about rev range, which could perhaps give me some kind of estimation about HP. Using these values I could then build a pipe, which wouldn't be too far off.

I don't have much money to spend for bad experiments aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 2878

aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 2878

But I will build my own inertia dyno right away, because I don't have possibility to test at road very often.

Nombre de messages : 3 Localisation : Indonesia Date d'inscription : 08/09/2018

Hi all,
i'm an unknown tuner and i really appreciate this amazing forum and all the huge amount of knowledge and experiences gave from Mr. Frits and Mr. Jan, i can just say WOW thank you to share this "two strokes wise". aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 241515

aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 241515

thankspipe a écrit:: Hi all,
i'm an unknown tuner and i really appreciate this amazing forum and all the huge amount of knowledge and experiences gave from Mr. Frits and Mr. Jan, i can just say WOW thank you to share this "two strokes wise".

Thank you so much Thankspipe Very Happy

.

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Frits or Jan, what was the piston to cylinder clearance on RSA, and did you try, how much more or less of that would have affected to power?

And what was the breaking in procedure (did you try any other on that)?

It's commonly said to put little more oil on 2-stroke (like adding 1% to your gas) when you are breaking in. I remember that I read somewhere, that using more oil in 2-strokes would even benefit power, do you have any information of that? How much oil is the optimum for power?

Frits said: 5% of synthetic oil, whith less the power decrease ( page 18).
My question is: does the power increase whith a lot of oil only by good lubrication or also by a lean air fuel mix? ( more oil ,less fuel).

Frits a précisé: 5% d'huile de synthèse, avec mois d'huile la puissance décroit ( v. page 18).
Ma question est: est-ce que la puissance augmente avec la quantité d'huile seulement par une meilleure lubrification et /ou aussi par le fait d'un mélange plus pauvre? ( plus d'huile donne moins d'essence dans le mélange).
Dan

op76 a écrit:: Frits or Jan, what was the piston to cylinder clearance on RSA, and did you try, how much more or less of that would have affected to power?

And what was the breaking in procedure (did you try any other on that)?

It's commonly said to put little more oil on 2-stroke (like adding 1% to your gas) when you are breaking in. I remember that I read somewhere, that using more oil in 2-strokes would even benefit power, do you have any information of that? How much oil is the optimum for power?

0,04mm was recommended, 0,05 gave more power, and 0,06 gave the same power as 0,04
5% oil gave more power than 4%, 6% was not tried.
More oil gave better sealing I think.

Dernière édition par op76 le Ven 14 Sep 2018 - 17:53, édité 1 fois

Ok, thanks. What is the right way to measure the clearance?

I believe it was Gordon Jennings, who's findings were that 1:15 oil/fuel ratio gave about 15% more power than 1:30 ratio (on quite modest powered machine). That is huge difference. Difference between 1:15 and 1:20 was not huge, but still remarkable I recall.

There was also some study, that 0.05mm clearance gave about 10% more power than 0.10mm. Do you guys agree with these numbers, or are they less significant for garage tuners?

And if someone could please explain to me this RSA exhaust duct, as I cannot seem to find it anywhere again. I believe Frits didn't implement same kind of "step" to his FOS cylinder concept, why is that?

If I tune my own cylinder, should I have suppression in exhaust duct and step where pipe begins? If my exhaust main area is 650mm² (corresponding 28.8mm diameter circle) then I suppress it to 585mm² (27.3mm). And then again have larger exhaust pipe by some per cent? Or how should I do it?

Dernière édition par Frits Overmars le Ven 14 Sep 2018 - 16:04, édité 1 fois

Measuring piston to cylinder clearance is generally indicated in your owner's manual and can differ from engine to engine (the spot where the piston size shall be measured). It usually requires micrometers.
However if you don't have such equipment (an inside micrometer can be quite expensive), you can check with clearance gauges when putting your piston inside the cylinder and trying to find the clearance between the bottom of the sleeve and the piston skirt bottom. It is less precise of course.

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

@ op76 : what the exhaust duct should look like is explained in the excell you have from me. It is the "Wobbly exhaust duct", and is all explained in the ESE-topic at Kiwi-biker.

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Haha, I had skipped that text-part completely, and I thought Wobbly meant some shaky spring fastening system aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 809516

aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 809516

Thanks JB

aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 980796

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

I know this is not Aprilia question, but what is the optimum shape on inlet port on piston port controlled intake? Let's say I have 30mm carburetor and inlet. Should cross-section area remain the same at the whole distance to the crankcase, or should it expand or even contract by some percent?

Nombre de messages : 86 Localisation : Australia Date d'inscription : 05/05/2012

Here is a Rotax 129 water pump impeller & a Rotax 258 impeller,
Frits if I recall correctly you mentioned that the RSA used a 250cc engines water pump,
This made me wonder, would you know if the RSW was tried with a 250cc pump ? And in your opinion is it conceivable to fit a 258 pump impeller to a 129 engine ?
The area surrounding the impellers outside diameter, clearance requirements & shape, should it increasingly open up throughout the circumference towards the passage opening ?
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RAW a écrit:: Here is a Rotax 129 water pump impeller & a Rotax 258 impeller. Frits if I recall correctly you mentioned that the RSA used a 250cc engines water pump. This made me wonder, would you know if the RSW was tried with a 250cc pump ?
And in your opinion is it conceivable to fit a 258 pump impeller to a 129 engine ? The area surrounding the impellers outside diameter, clearance requirements & shape, should it increasingly open up throughout the circumference towards the passage opening ?

Of course the RSW was tried with a 250 cc pump. At least, the RSW250 aprilia - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 19 809516

.
But I get your drift: the RSW125 was never tried with a 250-impeller. It wouldn't fit.
It's been over 10 years now, and I haven't got any drawings at hand here, but I don't think you can fit a 258-impeller in a Rotax 129 engine.
The area surrounding the impeller's outside diameter should increasingly open up throughout the circumference towards the passage opening, but the axial clearance between impeller and pump cover should be minimal.

In 2005 Aprilia and Derbi worked together for a short time on the RSA engine. We at Derbi used Aprilia cylinders, and Aprilia did some cooling tests for us. An electric central heating pump was used for this, with a variable output. This was done using a RSW engine. The electric pump used on the Derbi proved to be insufficient in a big way.... The RSW pump was good, but more water circulation gave more power.
So it was decided to give the RSA more water circulation, fitting a pump from a 250 Aprilia.
This may have caused a power loss through too much resistance.....quite possible.
No further tests were done I think.

Thanks Jan and Frits for all these valuable info.

Could you please give us an idea of the flow rate obtained by the water pump on the RSA?

carlovitch1 a écrit:: Thanks Jan and Frits for all these valuable info.
Could you please give us an idea of the flow rate obtained by the water pump on the RSA?

You're welcome Carlo. The RSA125 had a coolant circulation rate of 60 litres per minute. The RSA250 obviously had more than that, but I don't recall how much more.
As a rule of thumb you might say that the optimum coolant flow requirement is one litre per crankshaft-HP per minute.
Like Jan wrote, a smaller pump on the RSA125 would have absorbed less engine power, but the accompanying rise to a higher coolant temperature might have nullified that initial gain. The RSA produced best power at 40° C and keeping this temperature low over the total duration of a race is very important for a two-stroke.

Thanks for your reply, Frits.
I'm impressed by how low the temperature needs to be. If I compare to what I have on my 2 strokes MX bikes (250 KTM SX and Honda 500 CR) I'm pretty sure the temperature is at least twice higher.

Nombre de messages : 3 Localisation : Indonesia Date d'inscription : 08/09/2018

Good afternoon to all, maybe this is an answer to direct to Mr Jan Thiel, a friend is attempting to rebuild an old 125cc cross with rotary valve, which angles would you suggest for a good powerband (max power at 10800rpm) and ease of use?
Sorry if it's not directly related to the RSA , but i think there is a good pool of experts in rotary valve here ;-)

Hello, I am Kristjan from Slovenia and I race with classic 50cc Tomos, usually at Grobnik circuit in Croatia. In my free time I tune mopeds and fabricate exhaust pipes with classic and hydroformed method.

I've been reading this thread since 2013 and learned a lot, so thank you Frits, Jan and others, for sharing your knowledge with us.

I have a question about exhaust valve. Jan wrote many posts back, that he would make exhaust slider out of 1,5mm thick stainless steel plate. Would not that kind of slider cause galling with aluminium? And about design, how one choose slider angle? You can see on picture what I mean. Which is better and why?

Thank you.

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Dobrodošli Kristjan, in theory galling between the steel blade and the aluminium might indeed occur. But in practice this situation is comparable to a piston ring in its groove, where the temperature is even higher. The top and bottom faces of piston rings are often coated and this same coating could be applied to a steel power valve.

The slider angle in drawing A would give the best exhaust timing function, but with such a steep angle it will be difficult to cool the part of the cylinder between the bore and the slider slot, so it may be necessary to find a compromise.

Nombre de messages : 1832 Localisation : GE-CH Date d'inscription : 04/01/2016

Frits Overmars a écrit:: so it may be necessary to find a compromise.

As usual and the better is not alwas the best!
So it has to be tried to know.

» [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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