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

I don't know, but first thing anybody else will ask is : "more info"
purpose of engine (racing, MX, dragracing, ...)
max rpm
timing's of cylinder
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.
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Hi,

You asked for a paper regarding the pressure ratio causing choked flow. Have attached such a paper, however not for two stroke but four stroke, but the theory should be the same more or less.
Looking into the formulas in the paper I get that the pressure level for sonic flow is around a pressure ratio of 2 between the cylinder and the exhaust channel. I kindly ask the Professor not to request me to post a proof of the presented formulas. My intention is really not to start up a continued discussion around the topic, just wanted to share it. Maybe it can be to some help.

Regards, Fredrik G.

(http://nopr.niscair.res.in/handle/123456789/29957)

Goodbye.[/quote]

Hello Ken, are you a moderator? If so, then please inform me in detail of any violations on my part of the forum rules, and then take appropriate action, such as an official warning. If you are not a moderator, then you should consider your words carefully. As a forum member you are asking another forum member to go away - because you are "disappointed with my attitude".

I am disappointed with your attitude and manner. I am looking forward to reading more of your posts so that I can react to them with WORDS that __you__ don't like

If you (or anybody else) wants to set some kind of "standard" for what is acceptable and what is not acceptable in this thread or in this forum, then the thread or forum should be closed and all contributions should be reviewed and approved before publication.

I suggest you close your eyes, and go to your Happy Place, where it is Christmas 365 days per year and there are endless renditions of "Imagine" by John Lennon.[/quote]

Fredrikgu a écrit:

Hi,

You asked for a paper regarding the pressure ratio causing choked flow. Have attached such a paper, however not for two stroke but four stroke, but the theory should be the same more or less.
Looking into the formulas in the paper I get that the pressure level for sonic flow is around a pressure ratio of 2 between the cylinder and the exhaust channel. I kindly ask the Professor not to request me to post a proof of the presented formulas. My intention is really not to start up a continued discussion around the topic, just wanted to share it. Maybe it can be to some help.

Regards, Fredrik G.

(http://nopr.niscair.res.in/handle/123456789/29957)

Goodbye.

The Professor is very grateful for this contribution. I will study the paper.

Please note: indeed, in the test of Gordon P. Blair there is a formula to calculate the "critical pressure ratio" in the specific case of flow from a cylinder (large volume), through a "throat" into a pipe. Indeed, the critical pressure ratio is approximately 2. Well, in my calculations, it is between 1 / 0.52 = 1.92 and 1 / 0.602 = 1.66, depending on the properties of the gas.

Please note: I have opened a new thread about gas dynamics in exhaust systems. Interested readers are referred to: [Vous devez être inscrit et connecté pour voir ce lien]

Hello Jan and Frits,

This thread has hundreds of pages and I think that I've read all but I'm not sure if the questions I'm going to ask are already asked. Anyway, there's my doubts:

- First one is about the oil mixure. Have you tested the oil ratio and does it affect to the power? Could you tell us the % of oil used at the RSA? Do you use castor based oil or 100% syntethic? For the mixure process, do you how many liters of gasoline could be mixed with the oil at the same time?

- The second one is about the working temperature of the engine. Have you tested the optimal temperature for that engine?

-The third one is about maintenance schedule; how many hours last the piston / rod / cranshaft bearings etc?

Thank you very much in advance.

Best regards,
Iván.

iizquierdo a écrit:

Hello Jan and Frits, This thread has hundreds of pages and I think that I've read all but I'm not sure if the questions I'm going to ask are already asked. Anyway, there's my doubts:
- First one is about the oil mixure. Have you tested the oil ratio and does it affect to the power? Could you tell us the % of oil used at the RSA? Do you use castor based oil or 100% syntethic? For the mixure process, do you how many liters of gasoline could be mixed with the oil at the same time?
- The second one is about the working temperature of the engine. Have you tested the optimal temperature for that engine?
-The third one is about maintenance schedule; how many hours last the piston / rod / cranshaft bearings etc?
Thank you very much in advance. Best regards,
Iván.

Bienvenido Iván,
Your questions have indeed already been asked and answered here somewhere, sometime. I am not going to try and find the exact places, that could take me hours, so I will just answer what I know from memory.
The fuel had a content of 5% synthetic Agip oil. Less oil gave less power.
I can't say how many liters were prepared at a time; that could vary amongst the various teams. But as a general advice I would say: do not mix more than you need and always keep the gasoline and the mixture in cool, sealed metal containers. Some fuel components have a boiling point of just 50°C and each time you open a warm fuel container, you will here the hissing sound of those componenents escaping.
The optimal engine temperature is: the cooler the better. The limit is only set by the environmental temperature and the size of the radiator.
I do not recall the maintenance schedule; maybe Jan can say more about it. But there will no doubt be differences between track maintenance and the test bench schedule, where, for example, a connecting rod lasted a full season of testing.

iizquierdo a écrit:

Hello Jan and Frits,

This thread has hundreds of pages and I think that I've read all but I'm not sure if the questions I'm going to ask are already asked. Anyway, there's my doubts:

- First one is about the oil mixure. Have you tested the oil ratio and does it affect to the power? Could you tell us the % of oil used at the RSA? Do you use castor based oil or 100% syntethic? For the mixure process, do you how many liters of gasoline could be mixed with the oil at the same time?

- The second one is about the working temperature of the engine. Have you tested the optimal temperature for that engine?

-The third one is about maintenance schedule; how many hours last the piston / rod / cranshaft bearings etc?

Thank you very much in advance.

Best regards,
Iván.

Check this article in Dutch motorcycle magazine Moto73, nr. 13 (2011): [Vous devez être inscrit et connecté pour voir ce lien]

In short:

- crankshaft: "lasts between 2200 and 2500 km"
- drive axle of rotary disc: "approximately 3500 km"
- race weekend: "a new piston for every day" (it is noted: a race weekend is 450 km)
- clutch: new plates every 3 races
- rotary disc: 3 to 4 races
- one season requires 2 to 3 cylinders (it is noted: "You need some luck. That they don't start cracking and things like that.")

About operating costs:
- RSA complete: E 165,000
- Cylinder: E 6,000
- Cylinder head: E 150
- Piston + ring: E 150
- connecting rod set: E 900
- One GP requires 8 to 9 rear tires and 5 to 5 front tires; one set costs E 350
- Fuel: for one weekend, 50 liters, E 5 per liter
- APX2 (electronic control system): E 5,000 (but should not fail under normal circumstances)
- Carburettor: E 10,000
- Carbon fairing kit: E 5,000
- Exhaust: E 1,700

It is noted: "Crashes are most expensive. An original Aprilia-fairing made of carbon fibre costs E 5000 and an exhaust, E1700. Those often break in a crash."

The technical specification lists "premix 1:25" (4%), and a fuel consumption of 1 l per 9.5 - 10 km, "start weight" 76 kg and top speed of 244 km/h.

EDIT. Perhaps I should introduce myself. I'm almost 42 years old, and I'm quite lately (newly) got into this 2-stroke tuning. There is this little community in Finland, where they tune those little Suzuki PV (Suzuki EPO in Japan) mopeds. I had one when I was 13-16 years old, then I got one when I was about 32. Now we have one for my son, who is 15, and he will be driving it...I'm just a mechanic... and wallet  

Wow, what amazing topic this is. I've spent at least four hours today already with parts 1-3, and I guess I will be reading all of these again once I finished. Thank you so much Mr. Overmars and Mr. Thiel. This kind of dedication and "love to sport" is very rare.

My questions still aren't about Aprillia RSA 125, but old piston controlled inlet port 2-strokes, without any reed or rotary valves:

1. What port timings and angle-areas would you recommend for 50cc, where bore is 41mm and stroke 37,8mm? Conrod length is only 80mm, but I'm stuck with these, as it is forbidden to change parts, and this cylinder has only one piece exhaust port (but you might be able to drill auxillary ports if needed). I was planning to use 20mm carb, and build my own pipe using this: [Vous devez être inscrit et connecté pour voir cette image]

2. And how about, when bore increases to 52mm, as it is our other race class. I guess 28mm carb is needed there.

3. And lastly, I have a dream to build my own cylinder to it with 58mm bore (99.9cc), and implement Mr. Overmars's idea to have exhaust ports completely above transfers. My question is, how would you design two exhaust pipes for this kind of one cylinder two exhaust system? In my case I would probably need two separate pipes.

I'm sorry if some of these were already asked in this monstrous thread. I promise to read the whole thing at least once =) Btw. Have you guys wrote a book? You definitely should.

Why using a 20 mm carb on a 50cc ?

You think it's too big? I guess I came up with that, because there is this motocross version of the same Suzuki engine (RM50), which has 20mm carb.

is it for dirt bike or road race bike ?

It's top speed really...but as these mopeds are what they are, far from perfect top speed machines, you should be very happy to tune it like dirt bike as well. They have very poor aerodynamics and far from optimal gearing, and I guess you should stay below 13000rpm as well, if you don't wish to rebuild after every "race".

I used a 29.5mm carb ( pwk 28 over bored) on my derbi 50cc in the past

Ok, I was wondering if you considered it too large, as many of "our community" uses 18mm, even 16mm...but maybe I go with even larger then. It's just, that you need to use eccentric bolts to install really big carb, and I think that there is not enough material on that oval flange either for 24mm.

EDIT. Is this message translated to some other language (French maybe), and then back to english when I read this. I guess no one understands it anymore

EDIT2. Now it is fine again

op76 a écrit:

Wow, what amazing topic this is. I've spent at least four hours today already with parts 1-3, and I guess I will be reading all of these again once I finished. Thank you so much Mr. Overmars and Mr. Thiel. This kind of dedication and "love to sport" is very rare.
What port timings and angle-areas would you recommend for 50cc, where bore is 41mm and stroke 37,8mm? Conrod length is only 80mm, but I'm stuck with these, as it is forbidden to change parts, and this cylinder has only one piece exhaust port (but you might be able to drill auxillary ports if needed). I was planning to use 20mm carb, and build my own pipe.

Thank you for your kind words, Op76.
I would need a lot more information in order to give profound answers, but then again I would not be able to dedicate sufficient time to those answers, so let us stick to what we have here.
A short-stroke engine is not ideal for a two-stroke; the specific port areas tend to be too small, especially the blowdown area, so you will either need a huge exhaust timing, maybe more than 200°, or you will need to make auxiliary exhaust ports. I recommend the latter.
The limited blowdown will also limit the transfer timing, which must probably be less than 130°.
The best inlet timing depends on factors like the number of gears, how close they are, the type of use for the engine, and the experience of the rider, all of those factors being unknown to me.
170° is a safe value; anything higher should be tried in little steps. A 20 mm caburettor is not too big; even a 26 mm carb would be OK, depending on the inlet timing: the lower the timing, the bigger the carb.

Citation :

when bore increases to 52mm, as it is our other race class. I guess 28mm carb is needed there.

You guessed right; you might even try 34 mm in combination with a modest inlet timing.

Citation :

I have a dream to build my own cylinder to it with 58mm bore (99.9cc), and implement Mr. Overmars' idea to have exhaust ports completely above transfers. My question is, how would you design two exhaust pipes for this kind of one cylinder two exhaust system? In my case I would probably need two separate pipes.

Yes, you'd need two pipes, but it's quite simple: just design a pipe for an engine with half the total cylinder capacity and half the total horsepower.
By the way, here is an update for the FOS exhaust concept:[Vous devez être inscrit et connecté pour voir cette image]

Citation :

Have you guys wrote a book? You definitely should.

Jan Thiel and I considered writing a book about Jans achievements, but I would have been too much of a hassle considering the revenues. But we both write on a number of forums. I can recommend Pit-Lane    and Kiwibiker
[Vous devez être inscrit et connecté pour voir ce lien]

Thank you. I definitely start to read more of your posts, now that I have found you guys.

Citation :

170° is a safe value; anything higher should be tried in little steps. A 20 mm caburettor is not too big; even a 26 mm carb would be OK, depending on the inlet timing: the lower the timing, the bigger the carb.

This was interesting. If I recall right, it was Blair or Jennings who suggested, that inlet port should be around 10-15% bigger inside cylinder than carb throat is.

But your thoughts were a lot like mine about exhaust and transfer timing. I just hesitated, because many "gurus" on our community says, that you shouldn't go beyond 195° on exhaust timing, or you are losing all torque. I must check if I can do those auxillary ports, and if not, then I go as far is needed for enough blowdown =) There was one method to calculate it on your older posts, but how would I know, how much is needed at minimum?

By the way, how much do you count on time-areas on your planning, and what method you use to calculate them (perhaps that same method like on blowdown)?

EDIT: And thanks for that exhaust update. What speed of sound should I use as base, as there is no real information about temperatures etc?

op76 a écrit:

... your thoughts were a lot like mine about exhaust and transfer timing. I just hesitated, because many "gurus" on our community says, that you shouldn't go beyond 195° on exhaust timing, or you are losing all torque. I must check if I can do those auxillary ports, and if not, then I go as far is needed for enough blowdown =) There was one method to calculate it on your older posts, but how would I know, how much is needed at minimum?

For optimum exhaust resonance (explained elsewhere on this forum in more detail) you shouldn't even go beyond 190°, but that would limit blowdown time.area and thus revs; hence the Aprilia compromise of 202° and max.power at 13.000 rpm.

Citation :

how much do you count on time-areas on your planning, and what method you use to calculate them (perhaps that same method like on blowdown)?

time.areas are paramount, although it would be even better to use time.gasflow values. But unlike time.areas, those flow values cannot easily be established. And yes, calculating transfer time.area is done using the same method as calculating the blowdown time.area.

Citation :

thanks for that exhaust update. What speed of sound should I use as base, as there is no real information about temperatures etc?

I deliberately left temperature out of the pipe equasion because many aspirant-tuners, for whom this simple exhaust concept was meant in the first place, would not have the equipment to measure it. Going directly to the speed of sound served to keep the concept as simple as possible.
Use 550 m/s as a starting value for the speed of sound in the pipe calculation. You can raise or lower the actual value produced by your engine through changing the jetting and the ignition advance.

Thanks again Frits. Do you think those time-areas would be useful, if I want to scale some bigger cylinder to 50cc...or 80cc, even that my bore-stroke ratio is not the same? What would be your approach to scaling a cylinder, or is it bad idea in a first place?

I thought it before, and my conclusion was to scale both bore and stroke separately (like 41/56mm bore=0.732 and 37.8/56 stroke=0.675, but I couldn't decide if it was right way to go. I think using timing and time-areas would be probably better way.

Also, I searched through all your posts, but I couldn't find RSA portmap from this forum. I think that search engine can't find older posts. Do you have it somewhere, and/or do you have those time-area values? Anyway, you are one inexhaustible source of information...where do you get all those photos and information of "any bike ever made"

op76 a écrit:

Thanks again Frits. Do you think those time-areas would be useful, if I want to scale some bigger cylinder to 50cc...or 80cc, even that my bore-stroke ratio is not the same? What would be your approach to scaling a cylinder, or is it bad idea in a first place?
I thought it before, and my conclusion was to scale both bore and stroke separately (like 41/56mm bore=0.732 and 37.8/56 stroke=0.675, but I couldn't decide if it was right way to go. I think using timing and time-areas would be probably better way.
Also, I searched through all your posts, but I couldn't find RSA portmap from this forum. I think that search engine can't find older posts. Do you have it somewhere, and/or do you have those time-area values? Anyway, you are one inexhaustible source of information...where do you get all those photos and information of "any bike ever made"  

Scaling is a good approach but in my book 'scaling bore and stroke separately' isn't scaling. Using timing and time.areas is indeed a better idea.
I can't say whether the Pit-Lane search engine will find older posts; I never get along with search engines anyway.
I do know that I posted the RSA time.area values, but I'm not going to search for them; I'll leave that to you.
Where do I get "all those photos and information of "any bike ever made" ? Well, I've been around for some time, dedicated most of my life to the stuff, seen a couple of factories from the inside, know some good people; it all helps

Ok, I must keep on searching. One question about your 24/7 valve. If I make cylinder with direct intake to crankcase, doesn't it matter that it is fully open when that valve pulls reed completely open? So there is 360° intake timing at high revs?

op76 a écrit:

Ok, I must keep on searching. One question about your 24/7 valve. If I make cylinder with direct intake to crankcase, doesn't it matter that it is fully open when that valve pulls reed completely open? So there is 360° intake timing at high revs?

Yes, that is the intention.
24/7 is an indication of opening hours. You can see it on shops that are open 24 hours a day, 7 days a week.

This confuses me...what I've read before, there are some guidelines to "correct" time-areas, and if you have intake time-area this large, wouldn't it be impossible to get all other things right? Or is it just, that your transfers and exhaust need to be "at the same level", and have enough blowdown...and intake should be as "big as it gets"?

op76 a écrit:

This confuses me...what I've read before, there are some guidelines to "correct" time-areas, and if you have intake time-area this large, wouldn't it be impossible to get all other things right? Or is it just, that your transfers and exhaust need to be "at the same level", and have enough blowdown...and intake should be as "big as it gets"?

The intake should not be 'as big as it gets'. The frequency of the Helmholtz system consisting of crankcase volume, inlet tract diameter and inlet tract length should be such that it resonates in step with both the exhaust pipe frequency and the engine rpm. But this has all been covered here before.

I tried to read that again, and I kind of understand the concept...but I don't really...because why do you need to restrict your intake timing so much with piston controlled port then But no worries, maybe that all comes to me, when I read this whole thread again.

It's too bad, that this topic has all these zero information posts (like mine), it get's too heavy to read. If only you could filter it somehow, that only good posts are shown. There'd need to be some sort of ranking system. Your answers are really gold, as Jan's.

Do you have any formula for your FOS exhaust to estimate temperature? I tried one of Blair's formulas, and that gave temperature about 850°C and speed of sound around 670m/s for RSA, so that is not very accurate I guess.

EDIT.

Now I found this:

Citation :

The picture below shows the Aprilia RSA125's blowdown angle.area and transfer angle.area. If you divide these values by the 124,8 cc cubic capacity plus the 8,6 cc combustion volume, and by the 12.500 rpm at which this engines develops its maximum torque, you will find the optimum specific time.areas for blowdown (8,72°mm² per cc per 1000 rpm) and transfer (66,16°mm² per cc per 1000 rpm) .

So what is your way to measure that area, is it just port width as a chord, or do you measure it actual effective width, which takes account x and y angles?

If I try to scale these values to my 80cc (B 52mm/S 37.8mm), and want it to have maximum torque at 12500, what would be your estimated output? I know that because of short stroke it should have lot more revs, but I want it to last longer.

And how much do you think there is difference between piston port, reed valve, rotary valve or 24/7?

I don't come back to blame you if you estimation will be wrong...I just wanted to have some kind of enlightened guess =)

op76 a écrit:

Do you have any formula for your FOS exhaust to estimate temperature? I tried one of Blair's formulas, and that gave temperature about 850°C and speed of sound around 670m/s for RSA, so that is not very accurate I guess.

Some time ago I wrote: "The 'FOS exhaust concept' is only meant to help beginning tuners on their way. A lot of important factors, like compression ratio, ignition timing, type of fuel, carburetter diameter, crankcase volume and angle.areas, are not taken into account. Instead of all those factors that I left out, I included one variable, the speed of sound. Starting with 550 m/s will get you in the right ballpark, after which you can vary this value according to your findings."
You don't need any temperature data when using that exhaust concept. Besides, the actual exhaust gas temperature varies with jetting, with ignition timing, with revs, with power, and even along the pipe length. If you wish to take all those factors into account, I advise obtaining the EngMod2T software.

Citation :

If I try to scale these values to my 80cc (B 52mm/S 37.8mm), and want it to have maximum torque at 12500, what would be your estimated output? I know that because of short stroke it should have lot more revs.
And how much do you think there is difference between piston port, reed valve, rotary valve or 24/7?

Estimating the power of an engine of which I now nothing more than the bore and stroke is beyond my skills. And a short-stroke two-stroke is handicapped by its limited angle.areas so it is less willing to rev, compared to a square or long-stroke engine.