[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

Was there a point in the development where more air flow resulted in no more gains in power or is airflow through the engine a function of power output. My theory is that if the pipe is not doing its job properly pushing the mixture back into the motorfrom that increased airflow will besimply lost out the tailpipe. With increased port area increasing airflow does the baffle cone angle need to be increased tohelp push this extra flow back into the motor? |You earlier stated you tried over 200 exhaust designs. Can you post the dimensions of the pipe you found worked best? As a pipe designer and builder I am curious about finding more power with the pipe.

And by the way I would be first in line with my money to buy your book. Two strokes oil runs through the veins of more people than you think. I have every two stroke book an article ever written. Most are full of \BS or gobbledygook but your knowledge is golden and should be recorded in some form. [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked) - Page 37 101130

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked) - Page 37 101130

Howard Gifford a écrit:: Was there a point in the development where more air flow resulted in no more gains in power or is airflow through the engine a function of power output. My theory is that if the pipe is not doing its job properly pushing the mixture back into the motorfrom that increased airflow will besimply lost out the tailpipe.

Even if increased airflow increases the scavenging losses, it will also help in purifying the cylinder contents.
And even fresh mixture that directly short-circuits from the transfer ports into the exhaust ports, helps in cooling the piston.

Citation :: With increased port area increasing airflow does the baffle cone angle need to be increased to help push this extra flow back into the motor?

No, the baffle cone angle is already optimal; you cannot increase it at will. If you make it too steep, the returning pulse will turn into a shock wave that is very uneffective in pushing anything back into the cylinder.

Citation :: You earlier stated you tried over 200 exhaust designs. Can you post the dimensions of the pipe you found worked best? As a pipe designer and builder I am curious about finding more power with the pipe.

Jan, what do you think of this request?

Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

I have seen some engines using copper combustion chambers to help reduce detonation. Whats your thoughts on this, did you try it on the rsa and if so what was the results

Yes, a bridged exhaust was tried.
Of course!
2HP less.
Power is mainly determined by blowdown, which is better with a triple port.
A bridged exhaust is wider at the bottom.
Which means the transfers have to be smaller.
This gives less power.

Howard Gifford a écrit:: Was there a point in the development where more air flow resulted in no more gains in power or is airflow through the engine a function of power output. My theory is that if the pipe is not doing its job properly pushing the mixture back into the motorfrom that increased airflow will besimply lost out the tailpipe. With increased port area increasing airflow does the baffle cone angle need to be increased tohelp push this extra flow back into the motor? |You earlier stated you tried over 200 exhaust designs. Can you post the dimensions of the pipe you found worked best? As a pipe designer and builder I am curious about finding more power with the pipe.

And by the way I would be first in line with my money to buy your book. Two strokes oil runs through the veins of more people than you think. I have every two stroke book an article ever written. Most are full of \BS or gobbledygook but your knowledge is golden and should be recorded in some form.

I thought of doing it before I retired to Thailand.
No one wanted to edit it, so nothing came of it.
Then I gave away my tuning books, papers and notes.
So now there is no way I can do it anymore.
I have no photo's, designs or graph's, nothing.

Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

Jan Thiel a écrit:: Yes, a bridged exhaust was tried.
Of course!
2HP less.
Power is mainly determined by blowdown, which is better with a triple port.
A bridged exhaust is wider at the bottom.
Which means the transfers have to be smaller.
This gives less power.

Ok That makes sense.
I didnt think about it from that point of view, the exhaust port width at the bottom

In the end we reduced the width of the bottom of the exhaust port, and also raised the bottom.
That was a little bit better.
Then I stopped working.

Frits,

If you wish you can publish the exhausts sizes at this forum,no problem!

Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

Jan Thiel a écrit:: In the end we reduced the width of the bottom of the exhaust port, and also raised the bottom.
That was a little bit better.
Then I stopped working.

Im guessing that most of the work has been done by the time the piston gets to the bottom, By raising the bottom of the exhaust you would have had the exhaust port slightly higher than the piston at bdc ??

wax a écrit:

Jan Thiel a écrit:: In the end we reduced the width of the bottom of the exhaust port, and also raised the bottom.
That was a little bit better.
Then I stopped working.

Im guessing that most of the work has been done by the time the piston gets to the bottom, By raising the bottom of the exhaust you would have had the exhaust port slightly higher than the piston at bdc ??

Really most of the work must have been done when the transfers open!

We raised the bottom of the exhaust to about 2mm above the piston in BDC.

Once a copper/bronze head insert was tried.
It made no difference to detonation.
Full throttle-detonation ended when we made the transfers as wide as possible.
Thus improving piston cooling.
The problem of part-throttle detonation remained.
Part-throttle causes pre-ignition due to the remaining burned gases.

Really most of the work must have been done when the transfers open!

We raised the bottom of the exhaust to about 2mm above the piston in BDC.

[/quote]

Hi Jan,
Did you do any more tests to see how far you could raise the exhaust port floor?
Had you thought about creating a transfer port under the exhaust port by raising the centre of the exhaust port floor?

Yes, I did that in 1974, without succes!

At Aprilia I had a cylinder cast, in 2007, with the underside of the exhaust raised to the level of the upper
transfer port height.
It was my intention to lower this until the power became good.
But when I realized that I would be retired when the cylinder would be plated I abandoned it, and had it
CNC machined to 'normal'
A pity!

Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

Jan Thiel a écrit:: Yes, I did that in 1974, without succes!

At Aprilia I had a cylinder cast, in 2007, with the underside of the exhaust raised to the level of the upper
transfer port height.
It was my intention to lower this until the power became good.
But when I realized that I would be retired when the cylinder would be plated I abandoned it, and had it
CNC machined to 'normal'
A pity!

How dare they.

Jan Thiel a écrit:: Yes, I did that in 1974, without succes!

At Aprilia I had a cylinder cast, in 2007, with the underside of the exhaust raised to the level of the upper
transfer port height.
It was my intention to lower this until the power became good.
But when I realized that I would be retired when the cylinder would be plated I abandoned it, and had it
CNC machined to 'normal'
A pity!

Jan, in adding the transfer under the exhaust port, were you looking at adding to the main column of fresh charge or allowing the suction of the exhaust to take it out the exhaust port to be pushed back later?

I was looking to add to the main transfer flow.
But the engine gave no power at all and did not rev.
I think the flow of this extra transfer port made a 180° turn, straight into the exhaust!
Maybe this cooled the exhaust gases so that the pipe could not work properly!
Probably the blowdown was not finished when it opened.
It was a little bit lower than the main transfers, and its direction was towards the opposite cylinder wall.

Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

I am jealous it would be a dream of mine to work with a great team like aprillia and to be able to spend the time looking for extra power, you get to spend there money. So much better

Having read all of the postings on this topic (gp125-caracteristiques-aprilia-rsa) I get the feeling that it is hard to beat a RSA style rotary valve motor but the KTM has answered the challenge with a Reed Valve motor. As there is very little information about on this motor being posted, can I be bold enough without offending you and ask you if you know how they were able to do this?

Yes, no problem.
Their cilinder and exhaust pipe were copied from Honda.
Their fairing and gearbox were designed by Porsche
Instead of a powerjet they used an injector in the crankcase.
In their last year they had a very good rider, Marc Marquez, but he never won.
When Marquez changed to Aprilia he nearly always won!

Jan Thiel a écrit:: Yes, no problem.
Their cilinder and exhaust pipe were copied from Honda.
Their fairing and gearbox were designed by Porsche
Instead of a powerjet they used an injector in the crankcase.
In their last year they had a very good rider, Marc Marquez, but he never won.
When Marquez changed to Aprilia he nearly always won!

The HP quoted in the media for the KTM 125GP is 55HP, do you think this is at the gearbox or crankshaft?
Also, they are quoted as using a 39mm Keihin Carburetor but this seems a little small compared to the Aprilia; do you think it is correct or do reed valve motors work better on a smaller Carburetor?

many thanks for your time.

The flow capacity is limited by the reedvalve, not the carburettor.
So it makes no sense using a carburettor bigger than 39.
We tried, of course, but it was not better

55 KTM HP is surely at the crankshaft.

Jan Thiel a écrit:: Frits, if you wish you can publish the exhausts sizes at this forum, no problem!

OK, here we go, for all two-stroke addicts: the latest exhaust pipes from Aprilia, Honda, Derbi and KTM.

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Nombre de messages : 60 Localisation : australia Date d'inscription : 01/06/2012

And we all right click, save image as.

Thank you Frits. Im sure there are a lot of snips and torches being used this week as everyone tries these designs. It is very intersting to me that the Aprilia pipe has no parallel center section. This goes against the norm. Was it found that a tapered center carried the negative wave longer thus evacuating more charge through the motor? Also the last section of the difuser cone is very long and has a relatively steep angle. This would indicate a very high suction from this pipe. I am curious as to the name 102Tubo? What does this signify?

Frits Overmars a écrit:

Jan Thiel a écrit:: Frits, if you wish you can publish the exhausts sizes at this forum, no problem!

OK, here we go, for all two-stroke addicts: the latest exhaust pipes from Aprilia, Honda, Derbi and KTM.

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked) - Page 37 241515

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked) - Page 37 199739

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked) - Page 37 1993206895

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