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

It is indeed !

About the hump in the exhaust channel, did you have a formula to adapt it to any channel size/displacement?

carlovitch1 a écrit:

It is indeed !

About the hump in the exhaust channel, did you have a formula to adapt it to any channel size/displacement?

So, no, I didn't use a formula for it, I don't know any.
I paid attention to two points. I wanted to extend the piston crown radius as far as possible into the exhaust duct. And that automatically gives the hump height and length for most ducts. But that only works with compromise. And secondly, I wanted to realize only about 60% of the respective cylinder capacity in the exhaust channel volume, without the volume of the additional exhaust channels. Yes, that sounds very unacademic, but it is reality.
I hope that this concretisation are understandable. If this seems very vague, non-specific to you, I can only agree. I have worked out the realization of this hump with this form, therefore only with a relatively large amount of testing, worked out. But perhaps these two points will help you. These adaptation constructions all have one thing in common, they have to prove themselves in reality.
After this small step, which led to positive results on the exhaust side, the nights became even long afterwards. It was obvious to me at the time that I would not be able to find scientifically founded solutions for a useful increase in performance with my means. But the task was clearly defined for me, a comprehensive package of, torque, peak power in combination with drivability.

Thanks for your reply, Cara, trial and error is often longer (unless you are lucky) but gives as good results as formulas.
So if I understand correct for a 125cc cylinder your target is a 75cc volume.

I have little trouble figuring out the actual shape, does it mean the radius of the hump is similar to the one of the piston crown? Maybe just a handwritten sketch could clarify? Sorry I'm a kind of visual guy.

carlovitch1 a écrit:

Thanks for your reply, Cara, trial and error is often longer (unless you are lucky) but gives as good results as formulas.
So if I understand correct for a 125cc cylinder your target is a 75cc volume.

I have little trouble figuring out the actual shape, does it mean the radius of the hump is similar to the one of the piston crown? Maybe just a handwritten sketch could clarify? Sorry I'm a kind of visual guy.

I'm afraid I can't send a sketch, photo, the horror, HOW do you do that????

You have to click on "héberger une image" the 4th icon on the left of youtube. Then you look for the image on your pc by clicling on "choisir des images" and browsing in you pc after that, and once it is selected you click on "tout envoyer", once done you will see your image on the left of the small window and several links, you select the second copy and paste in your message.
Then if you want to check "previsualiser", then "envoyer" when it is OK.

carlovitch1 a écrit:

Thanks for your reply, Cara, trial and error is often longer (unless you are lucky) but gives as good results as formulas.
So if I understand correct for a 125cc cylinder your target is a 75cc volume.

I have little trouble figuring out the actual shape, does it mean the radius of the hump is similar to the one of the piston crown? Maybe just a handwritten sketch could clarify? Sorry I'm a kind of visual guy.

Thank you for your help and instructions for a “simple and complicated” handling of picture sharing.
In the meantime, I have signed up for a senior course on how to complicate and complicate communication with a PC.
I've actually been -only- devoted to the care and growing process of my plants lately and occasionally, just to pass the time, popped into – “Pit-lane”-.
I have had to deal with many formulas in my professional life and am therefore _not at all against formulas, but we are talking about -two-strokes- here. There have been several books published on two-strokes for years that were full of formulae.  If you wanted to correct all the errors in these formulae, you would need two lifetimes. I prefer not to say that they were all wrong, but many of them simply lacked the time-stroke-specific basic requirements, and that's why you usually got results that couldn't be used in reality.
The best formula is of no use if the user of the formulae cannot put the two-stroker into practice as an overall concept, with very many influences and parameters of physics, mechanics and nature, the specific application principles.
I think that without a certain imagination and a mental preload in the direction of mechanical and physical sequence in a two-stroke, despite formulas, one cannot understand the matter of two-strokes as well as the realistic solution principles.
For example, when I converted a TZ 350 to a -360° parallel twin for hill climbs, I was told by the gods and the rest of the world that it wouldn't work because of the 1, order, and, and........ . But it did work, with some modifications, of course.
At that time, after the first race run, they, fell silent.
It was now possible to accelerate out of the tight corners without grinding the clutch, which was not possible with a conventional TZ!
That was the reality, – so much for errors, formulas, and their application!
The exhaust port volume, of – about 60% —, is an approximate operation figure and is of course dependent on some parameters (different cylinders and exhaust ports), as is any change to the existing structure.
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Don't forget, this 8 mm hump was for TZ 350!
The actual hump radius does not result from a strict piston crown radius extension (is not always possible), but from the best fit of this radius. If you fit the first test piece, e.g., made of wood, in the exhaust channel, you will immediately see how the hump should look.
I think if using a 125 cm³ engine, is between 75 and 85 cm³ duct volume (without secondary ducts) and of course, take everything else into account. e.g., the intake side, the crankcase, the overflows with all parameters for the best possible cylinder filling, volume flow, compression, ignition, pre-exhaust, all duct windows optimized and aligned. And the exhaust, which of course has to harmonize with this, then you will spend a few days or weeks on the test bench or on the test track, but you will certainly have success with it.
And I know, it doesn't work completely without formulas.
So, I too have failed to find a general formula for success for a high-performance two-stroke engine, just as -Einstein- failed to find the -all-embracing world formula. Yes, either one has formulas and does not use them correctly, or one needs formulas and there are not the correct  ones. That's just the way the crazy world is.

Glad to read that you could understand my explanation.

Thanks a lot for extra explanation and the sketch !

I agree that the only formulas that we need to use are the only ones that actually work, this is certainly why people like Jan Thiel did spend so much time testing on the dyno.

Cara, now that you have mastered the black art of image posting on Pit-Lane, I wonder:
would it be possible to clarify the text below with some sketches?

Quod et probat a écrit:

On the four-channel cylinder, I had milled a groove along the outer wall of the channel exactly next to the partition wall to the B-channel with an approx. 5 mm ball cutter inside the A overflow channel. This was approx. 3 mm deep at the duct inlet and only approx. 0.5 mm high at the flushing duct roof at the outlet and aligned in such a way that the fresh gas flow of the -A-ducts, most likely, met approx. 15–18 mm before the -C-duct.

Frits Overmars a écrit:

Cara, now that you have mastered the black art of image posting on Pit-Lane, I wonder:
would it be possible to clarify the text below with some sketches?

Quod et probat a écrit:

On the four-channel cylinder, I had milled a groove along the outer wall of the channel exactly next to the partition wall to the B-channel with an approx. 5 mm ball cutter inside the A overflow channel. This was approx. 3 mm deep at the duct inlet and only approx. 0.5 mm high at the flushing duct roof at the outlet and aligned in such a way that the fresh gas flow of the -A-ducts, most likely, met approx. 15–18 mm before the -C-duct.

Hello! Good day, Mr Frits, I have to disappoint you, I have not mastered this moronic image transmission, I just succeeded by chance after-hours despite explanations. I ask those who are responsible for this nightmare, what are you attempting to achieve with this nonsensical application for a simple image transmission? What is behind it?
Academically speaking, is it user-friendly?

So, I am attempting to clarify this intuition.

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I hope that I can pass on my inspiration understandably.

Hello Cara
Again thank you for your reply's. And nice sketches. and your not the only one who has some trouble with posting pictures.  Yes the cylinders I'm working with will need just about the same amount of work to get some good power from. Plus my cylinders have a steel liner that I want to replace with a aluminum one and then coat with nickisil. Also I only have a single exhaust port now so I'll add a 'T' port design for the extra blow down area into the sleeve. Hopefully I can get about a 4 mm thick sleeve to work, maybe 5mm. I understand most of your modifications except the one about the intake carb spacer tapering and the sharp-edged finish. Could you elaborate a little more on that? And yes I agree with you about going to 11,000 rpm and modifying the coolant path thru the cylinders.
And all of us are a little weird. It would be a pretty boring world if we were not. I also helped build a 'Twingle' bike back in the late 1960's. There were 2 brothers that were riding with the group of guys I was learning to woods ride with and the one brother was on a 90 cc Honda. He was thin and wiry and could ride well, but the other brother had to weigh about 325-350 pounds so he needed a bike with more power to carry him through the woods. So he bought a Suzuki 250 X6 street bike but of course the power was wrong for the woods so another friend read somewhere that if you turned the crank around so that both cylinders fired together it gave more torque. So we tried it and it worked. He ran over more rocks with that bike. We were amazed. He even kept the down pipes on it. Of course they were smashed but it still had good power. He out rode a lot of trail bikes with that thing. Weird? Yes! But it worked. His brother had to buy a Ossa 250 Pioneer to keep up.
So keep up your 'Thriller' We are all ears.
Your exhaust hump is also documented in a 1999 SAE paper #1999-01-3333 'The Relationship Between Port Shape and Engine Performance for Two-Stroke engine'. But you explained it much better. Someone on this site or the KIWI site showed where you could download it. Good paper but Frits went a lot farther with his scavenging concept paper as far as transfer ports are concerned.
Jeff

Hallo Jeff!
I wonder how an adult person comes up with such an absurd idea, with these procedures—posting pictures-. I believe that the person responsible for this is a – hater of human beings !!!!
— That is a winning idea with the aluminium bush and Nikasil coating.
You should do all the necessary welding beforehand! For example, reinforce the cylinder base mounting with an additional higher aluminium round body and weld it to the cylinder outer wall, so that a (reasonably) distortion-free cylinder mounting is possible. This is of great importance when increasing the power.
Please pay attention to the aluminium alloy of the liner and whether it is old enough before you use it!
Five mm is better, but there must still be enough cylinder material to avoid possible tearing of the cylinder when pressing it in.
The aluminium combination needs some time afterwards to release the joint stresses. Allow the cylinder to rest for a while before mating it with the old and rested liner. Resting time and also heat treatment after pressing in is an important step, among others, for long use and little piston friction.
You should finish configuring the channel guides and openings before pressing in. All the reworking with an angle grinder-cutter is very tedious and time-consuming, as well as dangerous for the ready-to-use cylinder coating.
And make the coating company aware of this,
that as little as possible increased Nikasil deposits -in- the channels, but do not allow the channel windows to be debarred.
You should also consider fixing the bushing to prevent twisting.
Think again, if you are already doing this advantageous conversion, why not make a safe outlet zone with three outlets? A centre bar in the outlet duct is a massive problem area and this centre bar will give you a headache. E.g., thermal problems, already because the bushing wall for a centre bar then provides -only 5 mm for it.
I prefer not to lecture you, but a three-channel outlet system is far superior to a centre bar outlet. You can, for example, favourably influence the outlet times with the side outlets and, of course, increase the total window area for the outlet.
To the—taper in the intake tract-.
It is actually quite simple.
Between the carburettor and the cylinder, put a piece of plastic (temperature insulating) about 15 mm long (depending on the rpm peak), which has the same contours as the flange on the cylinder and on the other side as the rubber flange for the carburettor.
So, a piece of synthetic between the cylinder and the carburettor.
If you look from the carburettor side towards the cylinder, you will see the synthetic intermediate piece at the beginning is about 39 or 40 mm, like the diameter of the carburettor. This then tapers in a slightly rising radius (not sloping) towards the cylinder. The radius does not end at the end of the plastic piece! But at the strongest point of the taper and that is at approx. 35–36 mm diameter and at the end of the 15 mm. And thereafter, the intake duct,! without a smooth transition! Continues as before with a diameter of approx. 40 mm in the direction of the cylinder.
If you now look from the cylinder side towards the carburettor, you will see a taper from approx. 40 mm to 35-36 mm, i.e., a -sharp-edged ring, which is the rebound ring surface. Under no circumstances apply any rounding, radius, this would only facilitate the backflow towards the carburettor!
This ring surface gets in the way of the fresh gases flowing back from the crankcase and reduces to some extent the loss of fresh gases that want to escape again through the carburettor.
And it positively influences the carburettor setting as well as the fresh gas volume in the crankcase. Furthermore, a small return wave is created in the crankcase, which in turn has a positive effect on fresh gas recharging. For this purpose, it is optimal if the crankcase is relatively large, as this wave moves back and forth relatively quickly (often), so there should be enough space for the reloading effect. You should take this into account and not neglect it. And you would be well advised to reduce the compression ratio and optimize the ignition for this.
I made a few different exhausts for the – TZ 350 parallel twin. The ultimate exhaust was a voluminous single exhaust. Of course, it had a bifurcated oval pipe exhaust manifold that provided optimized flow characteristics. This exhaust was powerful, and the bike had an unprecedented ground clearance in the curves. The power delivery was tremendous, it was Thor's hammer. I didn't know until this point how long arms could become.
? The -SAE papers from 1999 ??, I used the hump 21 years before that!!! I have ONLY given the findings of my exhaust channel hump!
And if you now think that all this is degenerating into a monologue, then you, Jeff, are to blame because you have encouraged my atrophied grey two-stroke brain cells to become active again.
Have a good trip, greetings Cara

The thriller continues

Frits Overmars a écrit:

The thriller continues

I am pleased if I have been able to contribute something to the – amusement – of others, but I prefer not to be responsible for trivial literature here.

Hi Cara, thanks for your experience sharing and explanations.

Do you have a picture or a drawing of your single exhaust for the TZ350?

carlovitch1 a écrit:

Hi Cara, thanks for your experience sharing and explanations.

Do you have a picture or a drawing of your single exhaust for the TZ350?

Hello!
I have already written–Jeff-, this is a bit further back.

-- Unfortunately, I no longer have any documentation (drawings, photos) or test stand records because I had no idea that this might be of interest again. So, some (long) time ago, I disposed of several kilos of paper.
I have found some notes, but unfortunately, I can no longer correctly attribute them all. --

So, it would be untrustworthy of me to pass on inaccurate measurements for an exhaust, when every millimetre counts for a high-performance racing engine exhaust, and especially for this unconventional engine variant. I would be pleased to help you, but without sufficient documentation, it is unfortunately not possible. And there was more than just -one- exhaust system made.
Besides, there will be considerable differences between our engines, I think.
Greetings Cara

Thanks for your reply, Cara, I don't need sizes, just wanted to make sure my understanding was correct: you manufactured a single pipe for the TZ350, which as everyone knows is a twin. By single pipe do you mean one pipe per cylinder or a 2 in one pipe?

carlovitch1 a écrit:

Thanks for your reply, Cara, I don't need sizes, just wanted to make sure my understanding was correct: you manufactured a single pipe for the TZ350, which as everyone knows is a twin. By single pipe do you mean one pipe per cylinder or a 2 in one pipe?  

Yes, a -two in one –exhaust system for the TZ 350.
But these TZ's 350 were converted to 360° parallel twin, and so I made some -two in one-exhaust systems for them.

OK, thanks for your confirmation, Cara.

I'm surprised that such 2 in 1 pipe could work well on a race bike, I would be afraid of some potential flow pertubations from one cylinder to the other, even if the crankshaft is set to 360°.
I would be curious to see what Frits thinks about it.

At which section of the pipe were the 2 joining, I suppose somewhere in the elbow just after the exhaust ducts?

I’m not sure  but something  like this exist on the Konig ?

carlovitch1 a écrit:

OK, thanks for your confirmation, Cara.

I'm surprised that such 2 in 1 pipe could work well on a race bike, I would be afraid of some potential flow pertubations from one cylinder to the other, even if the crankshaft is set to 360°.
I would be curious to see what Frits thinks about it.  

At which section of the pipe were the 2 joining, I suppose somewhere in the elbow just after the exhaust ducts?

With a 360° parallel twin, the same flows are in each of the two cylinders at the same time.
Where should flow obstacles arise?
Since the cylinder exhaust pipes are about 105 mm apart in the centre, the manifold merging is not a big problem.
I developed this configuration mainly for hill climbing, as torque in tight corners, accelerating out, is more important than peak power.
And again, just with the exhaust system, it's not done, there were still many ingredients and the concept as a whole worked.
But with a lot of testing beforehand.

Adco a écrit:

I’m not sure  but something  like this exist on the Konig ?

König is a similar but ultimately different engine concept.

Quod et probat a écrit:

With a 360° parallel twin, the same flows are in each of the two cylinders at the same time.
Where should flow obstacles arise?

Maybe at the joining area between the 2, in the comeback wave ?

carlovitch1 a écrit:

Quod et probat a écrit:

With a 360° parallel twin, the same flows are in each of the two cylinders at the same time.
Where should flow obstacles arise?

Maybe at the joining area between the 2, in the comeback wave ?

You – could – be right.
But remember that the gas dynamics in the two cylinders are approximately the same and therefore also affect the return wave. Since the pressure drop in the two cylinders is also approximately the same, the return wave will not have much time to make a wrong decision.
Because of this merging, I naturally made different configurations.
Unfortunately, I was unable to check this division- whether it was true or not.
I can understand your doubts, but it worked already -over 40 years ago!
Unfortunately, I can no longer --certify-- it.[/i]

I do not put your word in doubt, Cara, it's just that I'm curious, I just try to understand how and why it worked in spite of theoretical potential issues.

So, I suppose you can tell us where was the merging of the 2 pipes (approximate distance from the exhaust flange) and if you had a special shape to minimize losses.

carlovitch1 a écrit:

I do not put your word in doubt, Cara, it's just that I'm curious, I just try to understand how and why it worked in spite of theoretical potential issues.

So, I suppose you can tell us where was the merging of the 2 pipes (approximate distance from the exhaust flange) and if you had a special shape to minimize losses.

Have no problems with your and all other questions, I think it's good when there is so much interest.
Maybe I have worded something wrong.
Please don't misunderstand me, I would also prefer to still have archival records about it.
I repeat myself when I stress again that without exact data—about the dimension(s) – it is unserious to simply pass on measurements like that.
But since you are only curious, I hope I don't have to fear that you will curse me after countless exhaust variations.
So, the connection point was just after the exhaust flange and inside the short pieces of manifold that started with a small opening angle. After this connection point, the first short diffuser stage started immediately with a larger opening angle.
Question.
What theoretical issues do you know that should be relevant in this arrangement? And which -losses- should be minimized?

I had tested an oval manifold, which was a little more difficult to bring together and also had a slightly different characteristic.
Many of my fabrications had a -special shape-that resulted from the fact that I was working with existing engines. With new designs, I would certainly have saved myself a lot of work and time.