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

Why make it simple when complicated works much better.

Thanks for your reply, Cara. My main concern was about the fact that the return wave will at one moment hit the zone where the 2 pipes become one and then this would generate some flow perturbation or some kind of counter wave in the flow, this is what I call potential loss.

I understand that you certainly found the best shape for this zone through you numerous trials.

While we are at it, maybe you would draw a simple sketch of the shape you adopted for this zone, and also tell us which pipe volume was giving the best results.
For the lengths of the different parts, a simple speed wave calculation will give us the results but I'm curious to know about the diameters: did you consider them as if it was a 350cc single ?

carlovitch1 a écrit:

While we are at it, maybe you would draw a simple sketch of the shape you adopted for this zone, and also tell us which pipe volume was giving the best results.
For the lengths of the different parts, a simple speed wave calculation will give us the results but I'm curious to know about the diameters: did you consider them as if it was a 350cc single ?

While we are at it ?
What should I be part of already?
I only wanted to satisfy your curiosity benevolently, as you put it, somewhat from my fund of memories.
Since your curiosity has developed into a thirst for knowledge, I would like to gently point out to you once again that this took place over forty (>40) years ago, about which you now want sketches and data from me after all.
I have told you in no uncertain terms that -no more documents-are available and that I will pass on everything I still know.
Nevertheless, there will now be a hearing of evidence, which I am supposed to make more precise from my memories with sketches and data on– form factor, —volume and calculations.
I repeat myself!
I have stated several times that it is—unserious—to pass on measurements (data) without detailed documentation. That is why I will not make any sketches!
I am glad that you are curious about the diameters, I believe you promptly already because of the way you formulate your questions in this regard....... It appears that you have a—teaching—profession, just a feeling from me.
To clarify, 2 strokes was not my bread and butter profession, but I spent a lot of time with 2 strokes. During this period, I made well over 100 exhaust systems for various 2-stroke engines and also some for the -360° parallel twin. At that time, believe me, I had a large archive with all the necessary records needed for a 2-stroke exhaust. The emphasis is on--had--!
Therefore, it is difficult for me -today- to provide you with exactly what you want from memory to satisfy your curiosity.
No, I did not use a 350 single cylinder for this project.
Because the differences are considerable.
Two carburettors, -larger intake volume, -two enlarged crankcases, -larger overflow volume, -two-piston areas, -two combustion chambers with a greater combustion speed, -and.... I think you already know all that.
As for the calculations, that won't be a problem for you nowadays with all these electronic possibilities.
And if you now think I'm just attempting to avoid proving my point, well, if that's what you think that's fine with me.
I am at an age where I no longer need to be proven and confirm myself.
Nevertheless, I will gladly answer questions about the best of my knowledge.

Cara, maybe my wording was not the best. In case you felt offended, I'm sorry about it, it was absolutely not intentional (English is not my native language). I have no judgement on anybody here, except admiration for achievements of many people, which you are in. So please make sure my questions are only based on technical curiosity.

I'm not asking at all for the exact quotes of your special pipe. Only a basic handwritten sketch of the important shape tricks to make me understand (as I'm more of a visual guy) what you finally ended as best solution.

I'm only an amateur bike restorator and engine builder, mostly focused on MX/Offroad. The single pipe solution is interesting for me as I eventually plan on converting an old road purpose 125 twin into a kind of scrambler bike.

carlovitch1 a écrit:

Cara, maybe my wording was not the best. In case you felt offended, I'm sorry about it, it was absolutely not intentional (English is not my native language). I have no judgement on anybody here, except admiration for achievements of many people, which you are in. So please make sure my questions are only based on technical curiosity.

I'm not asking at all for the exact quotes of your special pipe. Only a basic handwritten sketch of the important shape tricks to make me understand (as I'm more of a visual guy) what you finally ended as best solution.

I'm only an amateur bike restorator and engine builder, mostly focused on MX/Offroad. The single pipe solution is interesting for me as I eventually plan on converting an old road purpose 125 twin into a kind of scrambler bike.  

It's amazing how you don't let up.
I'm not offended at all, I just gave you my conclusion. You want me to make a sketch for you from the thought of an exhaust design I used -over 40(forty)* years ago!?
You ask for design features, for diameter sizes....
Z. B...
-- which pipe volume gave the best results--.
-- For the lengths of the various parts, a simple velocity wave calculation will give us the results, but I am curious about the diameters--.
-- sketch of the main moulding tricks --.
I had made well over—HUNDRED – exhaust systems for various 2 strokes—and now you think that after so many years*.
Explicitly remember this exhaust system so precisely as to sketch for you a certain design feature. I do not know of a 2 in 1 system for such a small engine unit. Since you will need an engine characteristic for your project that I never aimed for, and therefore my experience with a 350 Twin would not help you much. Furthermore, I don't know your engine concept. I could make some more arguments for and against. But these would not be useful to you.
The one thing I can still offer from my memories.
The joining of the slightly conical pipes should only be done within the—calculated manifold length—and that with a gentle curve to each other. The inside of the manifold tubes at the joint should converge—not razor sharp and pointed—but with a sufficient radius in the middle. The transition to the first short diffuser should also start with a radius and then with a larger opening angle to the next section. If you design motors, you will certainly have a spatial mind and understand these simple indications, without various sketches.

OK, thanks for your reply Cara.

Frits Overmars a écrit:

pojoran a écrit:

Dear Frits.
Would you please advise about 2T cylinder head or FOS cylinder head design.
1) what 's effect to head design in case different racing fuel unlead & lead fuel.  
   (such as Burn perior, Head Volume, Squize area or etc..)
2) If we change fuel from AV gas(Octance100) or unlead racing fuel to VP racing Q16 (Octance116),
   basically i can increase compression ratio correct, if you have any advise please
Thank you for your advice.

Dear Pojoran, if you wish to use leaded fuel, you are on your own. I haven't concerned myself with leaded fuel in decades, and I am not going to.

Apart from this, any combustion head shape, irrespective of its volume, should promote the highest possible burn rate. That way you will only need a small amount of ignition advance, which will limit the undesirable pressure rise acting on the piston before Top Dead Center. Furthermore, shortening the burn duration will limit heat loss to the surrounding metal.

Squish is the most effective way of promoting a fast burn rate. Make the squish area parallel to the piston crown and make the squish gap as narrow as possible. If the crankshaft is of good quality, you can use a squish gap of 1% of the stroke.

Different fuels will permit different compression ratios, but contrary to four-stroke practice, two-strokes do not need a high compression ratio for power, so I do not recommend to blindly use the highest ratio that the fuel will permit.

Good morning everyone,
I'm Potito and I'm passionate about two strokes.
Luckily I came across this forum full of ideas based on the great experience of you all.
Wandering on the internet I found a guy who talked about squish and I found an interesting idea about it, I would like to share it with you and know what you think. In practice, to have an effective parallel squish this maintains, and I agree, that the two areas of the vertical circular rings, external and internal of the squish crown, must be equal in order not to have the flow contraction. I think it makes sense unless you want to increase the speed on the narrowest ring to increase the turbulence. What do you think. Thanks
(Forgive me but I translate with the help of google)

It's friday june 3 in Thailand
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Happy birthday Jan !

Hi Frits,
I wonder if you could offer some advice and input on the subject of exhaust port Mach velocity of exiting gas. I can perhaps imagine that in 15 bar GP engines that Mach flow could be in evidence towards transfer opening. But what of other lesser engines, I`m thinking more of 12 bar engines even 10 bar ones, there are a lot of classes for home tuned road bikes around the world but little authoritative information available. And, are there implications for the exhaust pipe design that come from reduced Mach flows. I have perhaps not explained myself too well but hope you can get the thrust of my enquiry, many thanks in advance, and really appreciate what you give to the world of two-stroke engine enthusiasts.
Cheers, Melvyn

melvyn trevor a écrit:

Hi Frits,
I wonder if you could offer some advice and input on the subject of exhaust port Mach velocity of exiting gas. I can perhaps imagine that in 15 bar GP engines that Mach flow could be in evidence towards transfer opening. But what of other lesser engines, I`m thinking more of 12 bar engines even 10 bar ones, there are a lot of classes for home tuned road bikes around the world but little authoritative information available. And, are there implications for the exhaust pipe design that come from reduced Mach flows. I have perhaps not explained myself too well but hope you can get the thrust of my enquiry, many thanks in advance, and really appreciate what you give to the world of two-stroke engine enthusiasts.
Cheers, Melvyn

Hi Melvyn,
sonic exhaust outflow occurs at the first phase of exhaust blowdown, when the exhaust port starts opening and the ratio from cylinder pressure to exhaust pipe pressure is >2. This is the case in most engines, even those with a Brake Mean Effective Pressure less than 10.

By the time the piston is about to open the transfers, the cylinder pressure must have dropped to the level of the pressure in the transfer ducts and the exhaust flow velocity will have dropped to subsonic. So irrespective of an engine's BMEP (not to be confused with the cylinder pressure at exhaust opening, which in most cases is less than half the BMEP value) there is no sonic exhaust flow towards transfer opening.

I see no real influence in exhaust pipe design. Hotter exhaust gases require longer pipes and all that, but I don't consider that a fundamental difference. What I do see is the importance of exhaust port and duct design. A good flow coefficient and a small duct volume can make a lot of difference in the way exhaust gas energy is being put to use.

Crikey that was quick Frits, very many thanks for you response, priceless information as always.

Cheers, Melvyn

Hi Frits, Can I have an answer to my previous question, please?

Architito2002 a écrit:

Hi Frits, Can I have an answer to my previous question, please?

For those who have missed your first question, here it is once again:

Architito2002 a écrit:

[Good morning everyone,  I'm Potito and I'm passionate about two strokes.  Luckily I came across this forum full of ideas based on the great experience of you all.
Wandering on the internet I found a guy who talked about squish and I found an interesting idea about it, I would like to share it with you and know what you think. In practice, to have an effective parallel squish this maintains, and I agree, that the two areas of the vertical circular rings, external and internal of the squish crown, must be equal in order not to have the flow contraction. I think it makes sense unless you want to increase the speed on the narrowest ring to increase the turbulence.
What do you think. Thanks  (Forgive me but I translate with the help of google)

Potito, there were two, or actually three reasons I did not answer your first question.
1: I did not understand your remark about 'vertical circular rings'.
Now that I'm reading it a second time, I suppose that you meant the curtain area between the squish volume and the combustion bowl.
In that case, I disagree because I do want to increase the flow velocity there; the whole point of squish is to generate turbulence.
If you want parallel squish (to which I agree) you cannot have equal external and internal curtain areas anyway because a curtain area is 2π x diameter x height. Parallel squish means the height must be constant, and as the inner and outer diameters of the squish area are unequal, the curtain areas will be unequal too.  
2: I have a pretty good idea of the identity of this 'guy who talked about squish' and I have no intention to get involved in his claims.
3: I have been searching in vain for an indroduction, which would be the common thing to do if you join a discussion.
Could you tell us a bit about yourself?

Frits Overmars a écrit:

Architito2002 a écrit:

Hi Frits, Can I have an answer to my previous question, please?

For those who have missed your first question, here it is once again:

Architito2002 a écrit:

[Good morning everyone,  I'm Potito and I'm passionate about two strokes.  Luckily I came across this forum full of ideas based on the great experience of you all.
Wandering on the internet I found a guy who talked about squish and I found an interesting idea about it, I would like to share it with you and know what you think. In practice, to have an effective parallel squish this maintains, and I agree, that the two areas of the vertical circular rings, external and internal of the squish crown, must be equal in order not to have the flow contraction. I think it makes sense unless you want to increase the speed on the narrowest ring to increase the turbulence.
What do you think. Thanks  (Forgive me but I translate with the help of google)

Potito, there were two, or actually three reasons I did not answer your first question.
1: I did not understand your remark about 'vertical circular rings'.
Now that I'm reading it a second time, I suppose that you meant the curtain area between the squish volume and the combustion bowl.
In that case, I disagree because I do want to increase the flow velocity there; the whole point of squish is to generate turbulence.
If you want parallel squish (to which I agree) you cannot have the external and internal curtain areas equal anyway because a curtain area is π /4 x diameter² x height. Parallel squish means the height must be constant, and as the inner and outer diameters of the squish area are unequal, the curtain areas will be unequal too.  
2: I have a pretty good idea of the identity of this 'guy who talked about squish' and I have no intention to get involved in his claims.
3: I have been searching in vain for an indroduction, which would be the common thing to do if you join a discussion.
Could you tell us a bit about yourself?

I start right from point three:
3.Hi everyone, I'm a 48-year-old boy and I'm fascinated, like you, by the two-stroke engine. They were the first engines on which you started making messes without understanding anything !!! Every now and then I enjoy modifying the engines of the special vespa with discreet results. Over time I have studied mechanics, thermotechnics and fluid dynamics that have allowed me to understand something more about "this 10 combination padlock" (fantastic !!).
In recent years I have also studied something on sound waves, which are also heavily involved in the two-stroke cycle. I find this puzzle great.
Not least were the "revelations" of masters like you, who detonated their heads on us for competitive purposes. Thank you!!!!!!!!!!
I have in mind, like many others, an ecological two-stroke engine, but I have to find the time to experiment and subsequently patent, as long as it works. The architecture would remain quite similar to the current one.

2. I understand

1. As bad as I wrote, you understood perfectly what I meant and gave certainty to this other piece of the puzzle. Thank you!!!

Sorry if I was rude !!!

Architito2002 a écrit:

3.Hi everyone, I'm a 48-year-old boy and I'm fascinated, like you, by the two-stroke engine. They were the first engines on which you started making messes without understanding anything !!! Every now and then I enjoy modifying the engines of the special vespa with discreet results. Over time I have studied mechanics, thermotechnics and fluid dynamics that have allowed me to understand something more about "this 10 combination padlock" (fantastic !!).
In recent years I have also studied something on sound waves, which are also heavily involved in the two-stroke cycle. I find this puzzle great.
Not least were the "revelations" of masters like you, who detonated their heads on us for competitive purposes. Thank you!!!!!!!!!!
I have in mind, like many others, an ecological two-stroke engine, but I have to find the time to experiment and subsequently patent, as long as it works. The architecture would remain quite similar to the current one.
As bad as I wrote, you understood perfectly what I meant and gave certainty to this other piece of the puzzle. Thank you!!
Sorry if I was rude !!!

You're welcome Potito. And no worries, I didn't detect any rudeness.

I finally found the time to prepare an English version of my Petrol-Ethanol-Methanol-E5 treatise.
Unfortunately, I don't see a way to post a PDF here, but you can find it at:
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and here:
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l'adresse en lien direct pour le pdf
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Frits Overmars a écrit:

I finally found the time to prepare an English version of my Petrol-Ethanol-Methanol-E5 treatise.
Unfortunately, I don't see a way to post a PDF here, but you can find it at:
[Vous devez être inscrit et connecté pour voir ce lien]

and here:
[Vous devez être inscrit et connecté pour voir ce lien]

Hi Frits

This is a timely and very helpful piece of information for me.

In the Dry Lake Racers Land Australia Speed rules we have two main fuel Classes "Gas" which is for Gasoline/Petrol and we get our fuel tested if we run a Record speed.

Then we have "Fuel" Class for everything else Methanol, Nitromethane etc.

E85 is included in the Fuel Class and I am looking at running that in the future. This has happened as I can't use the type of Race Gasoline/Petrol I have used since 2011 as Leaded Fuels are no longer allowed.

I read in your Petrol-Ethanol-Methanol-E5 treatise you said that low compression is preferable.

What sort of general Static Compression Ratio range are you talking about for a 250 twin as a guide?

Cheers Pete

Cheers

Pete

Bonjour jan , bonjour frits et bonjour tous les autres participants a cette discussion hypnotique qu'il faut relire plusieurs fois pour bien comprendre le potentiel de toutes ces  informations donné par centaine au mettre carré!!
Je m’appelle nono, passionnée par le moteur deux temps (comme vous pouvez vous en douter) depuis que je l'ai découvert avec ma première mobylette, je suis òccitan et j'habite en Pyrénées, donc en France(administrative) pour ceux qui ne connaisse pas!

J'ai pas mal de question, je vais commencer par l'autoallumage, sur l'aprilia rsa le moteur fonctionne en autoallumage en fin de régime( je ne suis plus sur du régime exact a partir de 13700tr/min il me semble), pour quelles raisons il y a autoallumage sur ce moteur, la température de la bougie est-elle trop élevé?
Une bougie plus froide(température) aurait-elle résolu le problème?
Est-ce du au rapport volumétrique trop élevé?
Puisque la combustion ne commence pas avec "l'étincelle" a quelle moment commence t'elle, le savais vous?
Selon vous le moteur serait plus performant en fin de régime si il n'y avait pas d'autoallumage?

Merci d'avance pour vos réponse!!
Cordialement, NONO.

vorlis a écrit:

Bonjour jan , bonjour frits et bonjour tous les autres participants a cette discussion hypnotique qu'il faut relire plusieurs fois pour bien comprendre le potentiel de toutes ces  informations donné par centaine au mettre carré!!
Je m’appelle nono, passionnée par le moteur deux temps (comme vous pouvez vous en douter) depuis que je l'ai découvert avec ma première mobylette, je suis òccitan et j'habite en Pyrénées, donc en France(administrative) pour ceux qui ne connaisse pas!

J'ai pas mal de question, je vais commencer par l'autoallumage, sur l'aprilia rsa le moteur fonctionne en autoallumage en fin de régime( je ne suis plus sur du régime exact a partir de 13700tr/min il me semble), pour quelles raisons il y a autoallumage sur ce moteur, la température de la bougie est-elle trop élevé?
Une bougie plus froide(température) aurait-elle résolu le problème?
Est-ce du au rapport volumétrique trop élevé?
Puisque la combustion ne commence pas avec "l'étincelle" a quelle moment commence t'elle, le savais vous?
Selon vous le moteur serait plus performant en fin de régime si il n'y avait pas d'autoallumage?
Merci d'avance pour vos réponse!!
Cordialement, NONO.

Welcome Nono,
In certain circumstances the Aprilia RSA seemed to run in Homogeneous Charge, Compression Ignition (HCCI) mode. This made itself noticed when the engine no longer reacted to ignition interruptions or delays during gear shifting. In order to correct this problem it was found necessary to make the fuel/air mixture richer than the optimum ratio for maximum power.
Apart from this, HCCI is highly desirable because it gives a much faster, more complete combustion, even with mixtures that are so lean that they can no longer be ignited by means of a spark. As a result, the engine power, the specific consumption and the heat losses are favorably influenced.

HCCI was not entirely new to us. Jan and I first encountered this phenomenon in 1987 when in ignition experiments at Garelli the engine on the test bench, once running at full power, did not react to changes in ignition timing any more. Only years later did we realize that this must have been HCCI.

A high fuel/air mixture temperature, a high spark plug temperature and a high compression ratio will affect the occurrence of HCCI, but I think the presence of hot residual combustion gases in the cylinder is the most important factor.

These days a lot of research is being done on exhaust gas recycling (EGR) in four-stroke engines. Two-stroke engines do not need the extra plumbing; they have a naturally built-in EGR. I call it exhaust gas retention.

Do I think the engine would be more efficient at the end of the regime if there was no self-ignition?
Quite the contrary; I think HCCI is the way to go. But controlling it at full throttle is still not easy.....

Hi Frits

This HCCI mode you talk about, wouldn't it be similar to the AR-Combustion (Activated Radicals) that Honda applied to the CRM 250 trail-bike, the Pantheon scooter, and the EXP-2 Dakar bike ? I mean, trying to keep the compression ratio at the limit of auto-ignition in any circumstance ?

Merci pour vos réponses FRITS, je les ai bien comprises!

connaissez-vous le taux de remplissage de l'aprilia rsa au régime de couple maximal?

Avez-vous estimé la quantité de gaz frais qui rentre dans le conduit d'échappement avant de retourner dans le cylindre au régime de couple maximal?

Pensez-vous que ce volume de gaz qui rentre dans le conduit d'échappement(toujours au régime de couple maximal) retourne entièrement dans le cylindre ou en reste t'il une part dans le conduit?

Cette quantité de gaz correspond t'elle au volume du conduit d'échappement, ou du moins au volume du conduit sur ça partie refroidi?

dga a écrit:

Hi Frits, This HCCI mode you talk about, wouldn't it be similar to the AR-Combustion (Activated Radicals) that Honda applied to the CRM 250 trail-bike, the Pantheon scooter, and the EXP-2 Dakar bike ? I mean, trying to keep the compression ratio at the limit of auto-ignition in any circumstance ?

Yes, these are all names for one and the same phenomenon.
Honda tried to control the exhaust gas retention rate with an exhaust valve that could almost entirely close the exhaust port.
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vorlis a écrit:

Merci pour vos réponses FRITS, je les ai bien comprises! connaissez-vous le taux de remplissage de l'aprilia rsa au régime de couple maximal?

It must have been approximately 140%. I once calculated it at 143% but I hesitate to be too precice; it's a process in which many factors play a role.

Citation :

Avez-vous estimé la quantité de gaz frais qui rentre dans le conduit d'échappement avant de retourner dans le cylindre au régime de couple maximal?
Pensez-vous que ce volume de gaz qui rentre dans le conduit d'échappement(toujours au régime de couple maximal) retourne entièrement dans le cylindre ou en reste t'il une part dans le conduit?
Cette quantité de gaz correspond t'elle au volume du conduit d'échappement, ou du moins au volume du conduit sur ça partie refroidi?

The answers to these three questions are interconnected.
I can calculate the amount of mixture that has to be present in the cylinder at the moment of exhaust closure, but more mixture may be entering the exhaust duct than the amount that gets shoved back.
The cooled part of the exhaust duct has a volume of about 75 cc, so it should be able to contain the total amount of mixture that enters the duct.