[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

jfn2 a écrit:

Hello:
Reading back a few pages,pg 14, about what Ken Seeber said about reducing shortcircuting. After some research this is what I tried and it seems to work fairly well. The engine is a KTM 85 SX. I drilled into the transfer port duct (the one closest to the exhaust port and installed a hose fitting near the top of the port. I then installed a one-way valve into the hose. I did this on both sides of the cylinder. I then ran a hose around to a tee fitting and connected both sides together. I then fitted a metering valve to the tee and left it open to atm.
Results....  A pretty big improvement in low end performance and some in the mid range. I did not see a difference in top end. I had to increase the low end jet about 3 sizes and changed the needle position. No change in the main jet. Engine revs very quickly. This bike is used in hill climbing and has won a few championships in it's class. Some theory is that at low throttle opening and low load ( low velocity) the transfers have more time to load up with atm air and when the transfers open mostly atm air is shortcircuted out the exhaust. When the throttle is opened and load goes up the velocity increases and now the transfer ducts are full of F/A mix. So the performance gains seem to be 'under the curve' so to speak. Total cost was about $30 US. Pretty cheap and keep it simple.
I'm sure there is a optinum size for the metering valve. I have some pics but don't know how to show them. I tried but it didn't work.

Did you test it before changing carb.

Yes I did. That's how I knew to jet higher. Then try a larger air jet and try more fuel, and so on. Like I said, I'm sure there is a limit.

Oke Thanks ,then it make sence

Jan or Frits (or anybody else ocourse) :

rod ratio's of 2.2 (or more ?) are normal (quote  from Jan) , but is this also true for motors with read valves ? I suppose the longer rod in itself isn't so much an issue, more the increased (too big ?) carnkcase volume for readvalve induction ?
I'ts not for a racebike, just a tuned street bike not pushed to it's limit's. I do not care much about bottom-end power (it's a 250 2-stroke after all), as long as it can flow with normal traffic it's ok for me.

rod ratio would be 2.27

jfn2 a écrit:

what I did was to drill a hole into the transfer duct (transfer closest to exhaust port) near the top of the port and install a hose fitting. I then installed a one-way check valve. I did this to each side of the cylinder. Then I ran a hose from the check valve around to a tee connecting it with the other side. I then installed a metering valve into the tee for both sides. The metering valve is open to the atmoshere.

Something like this, Jeff ? 
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JanBros a écrit:

rod ratio's of 2.2 (or more ?) are normal (quote  from Jan) , but is this also true for motors with read valves ? I suppose the longer rod in itself isn't so much an issue, more the increased (too big ?) carnkcase volume for readvalve induction ? I'ts not for a racebike, just a tuned street bike not pushed to it's limit's. I do not care much about bottom-end power (it's a 250 2-stroke after all), as long as it can flow with normal traffic it's ok for me. rod ratio would be 2.27

Yes Jan, some people say that things are different for a reedvalve engine.  But keep in mind that none of those people have ever succeeded in extracting as much power from such an engine as Jan Thiel did, when he briefly worked at the Derbi reedvalve GP125 engine, before he went on to develop the RSA.
I would say that a rod ratio of 2,27 is fine. But I am curious about your engine; 250 cc two-strokes usually have a much lower rod ratio.

Frits, I only have KR1's if you don't remember (what would be perfectly normal)

I only want to stop the barrels from cracking arround the studholes ( [Vous devez être inscrit et connecté pour voir ce lien] )

I am still curious as to why the KR1 webs couldn't be made to press the big end pin in the centre webs to. originaly they have integral pins, and everybody tell's me they are to thin, but the web on the right side has the same dimensions and needs to pass all the torgue the engine makes to the primary gear and that one is "pressed in" ?

only problem I see is that there wouldn't be much metal between the big end hole and the centre shaft hole. therefor I thought of turning down the big end pin on 1 side to 19.5 (and maybe weld if for more strength). It would give me the same material/hole ratio as a standard RD/Banshee crank which should be more than plenty as those cranks are being stroked. Ever done that ?

Yes Frits, you got it.
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but criteria 1993.

JanBros a écrit:

only problem I see is that there wouldn't be much metal between the big end hole and the centre shaft hole. therefor I thought of turning down the big end pin on 1 side to 19.5 (and maybe weld if for more strength).

if i understand you correct you would end up with a stepped pin of some sort ? i was going to try that idea one time, and even machined the pin but i got scared and never used it. everyone i talked to said it had a higher chance of failure. i still have the pin sitting in the garage shelf. i ended up boring the crank wheel hole bigger so i could fit a longer rod with 2mm bigger diameter crank pin, everything is holding up fine so far.

im going from old memory but somebody around here gave some good advice about the proper distance between the outer crankpin edge and outer crankwheel edge. seems like they said the distance between crankpin edge and crank wheel edge should be atleast equivalent to the crankpin diameter, or maybe the distance should be atleast half the pin diameter, i cant remember exactly. im sure frits knows for sure

brokedown a écrit:

if i understand you correct you would end up with a stepped pin of some sort ?

yes.

if it was a single I would have just tried it. But as it's a twin, it's always double costs, for trying and when it goes wrong

Dear all,

this year I want to switch from the reading mode into discussion. :-)
I am from Germany and I love Two Stroke since over 30 years.
Since I have a dyno I had the chance to make a couple of thousand dyno runs.
I love reading 2 stroke literature including the old crap and software simulation.
It´s important to me to understand how the parts and changes work together!

So now let´s switch to business:

@Frits
I found some details in the web to the Profi MB 40 Engine.
You did some development to the A and B ports. What I could see on the picture, the A port got a wide flat "kicker". You also changed the flow direction of the B port, so that it looks much more like the RSA porting.

A) Did you generally change your mind to the radial flow concept?

B) Is that flow concept of the MB40 SL universal, and would it also work on a 200ccm engine with 10000-12000 RPMs? If not, what flow directions would you change?

C) I found in that topic a screenshot from your Scavenge Port Direction Program where you describe a 50ccm concept. What is the Parameter alphaUp, pa? I count 3 Ports but 4 AlpaUps?
What does "width" mean. Is the result a factor or a length in inch?

D) Why is the piston in the MB 40 asymmetric?

E) The inner wall of the RSA Duct has a tangential contour to the bore. The RB40 has a radial shape. Did you test a tangential shape on the MB40?

F) Why does the B-Port have a twist now? What I understand from your discussions is, that a twist hase influence on the kinetic of the flow.

F) What I can see, the RSA has no"kickers" on the A Port, but it has a rounded corner through the complete duct. Is there a big difference between a kicker and that round duct edge?

I would very much appreciate some answers and input.

Hello & Happy 2015 All.
The RSW & RSA power valve, this is only within the main exhaust post & not the sub ports. Can someone explain how messy the flows within the cylinder becomes exactly. I assume this layout is fine for use within these racing cylinders as wide open throttle & high rpm all flows it becomes all sorted out.
Is this where a pulsed power jet really becomes advantages ?

A pulsed powerjet will always be an advantage, even on a street bike!

Would someone please explain what a 'pulsed powerjet ' is? Thank you.

Polinizei a écrit:

@Frits I found some details in the web to the Profi MB 40 Engine. You did some development to the A and B ports. What I could see on the picture, the A port got a wide flat "kicker".

I can't see it; I never designed any cylinder with a kicker.
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Citation :

Is that flow concept of the MB40 SL universal, and would it also work on a 200ccm engine with 10000-12000 RPMs?

After I published to MB40 flow concept, I published the 'FOS scavenging concept' which is supposed to be universal. Here it is once again.
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Citation :

I found in that topic a screenshot from your Scavenge Port Direction Program where you describe a 50ccm concept. What is the Parameter alphaUp, pa? I count 3 Ports but 4 AlpaUps? What does "width" mean. Is the result a factor or a length in inch?

Take a look at the 'explanation of abbreviations' on the above picture. There are three coloured AlfaUps corresponding to the ports and one SigmaAlfaUp for the complete cylinder.

Citation :

Why is the piston in the MB 40 asymmetric?

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Because the exhaust port is in line with the crankshaft, not perpendicular to the crankshaft as in a motorcycle engine.
The exhaust side of the piston must be long enough to keep the exhaust closed when the piston is in TDC.
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Citation :

The inner wall of the RSA Duct has a tangential contour to the bore. The RB40 has a radial shape. Did you test a tangential shape on the MB40?

No; the RSA cylinder was designed between existing cylinder studs; with the MB40 engine I had complete freedom.

Citation :

Why does the B-Port have a twist now? What I understand from your discussions is, that a twist hase influence on the kinetic of the flow.

Same answer as above.

Citation :

What I can see, the RSA has no "kickers" on the A Port, but it has a rounded corner through the complete duct. Is there a big difference between a kicker and that round duct edge?

The RSA has no kickers anywhere. Jan Thiel commented on this not too long ago.

jfn2 a écrit:

Would someone please explain what a 'pulsed powerjet ' is? Thank you.

Someone did, Jeff. I'm afraid you'll have to read hundreds of pages. But hopefully they'll be interesting .

Oh well, OK then, because it's a new year: a pulsed powerjet can open and close fast; up to 13 times per second in the case of the RSA. It can be completely closed all of the time, or open all of the time, or open for 1/13 of a second and closed the rest of the time, or open for 2/13 sec and closed the rest of the time, or open for 3/13.... get the picture?

Hi Frits, thanks for your replies.
I'm talking about that engine in the link.
I thought it's a newer version of the above engine, based on your influence.


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I found another pic of the engine above with a big modifcation of the B-Port. Maybe it was Jan who wanted the engine to look more like a RSA Barrel :-)

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Mr. f3d wrote:

Hallo liebe Motorenfreaks,

nach vielen Nachfragen habe ich ein Bild zum besseren Verständnis gemacht, auf dem man erkennt wie der MB 40 Zylinder verändert wurde. Man sieht dass die beiden vorderen Überströmkanäle mit grauem Epoxy teilweise gefüllt wurden.
Diese Modifikation wurde bei den neusten Zylindern schon im Guß genauso gemacht.

Dies zur Info - fürs Tuning !!!

MFG Mr. f3d

Lieber Polinizei, I explained the reason for the modification of the original B-ports here: [Vous devez être inscrit et connecté pour voir ce lien]

Frits, the RSA powerjet was this the same item used at derbi upon the read valve engines & was it the valve that was fast or the electronics that drove it, how fast could the std aprilia powerjet be cycled

I can't say for sure what type of powerjet was fitted on the reedvalve Derbis and all my documentation is back home in Holland so I can't check. But Jan will certainly know.
The electronics were programmed to operate the valve at 13 Hz and maybe the valve could have been a bit faster still.
The electronics themselves could have handled much higher frequencies.

Thank you Frits

Hi,
for the transfer ports I have to design, I have limited space between the four asymmetric studs.
What is your opinion reg. that design compromise? The other possibility would be to create way smaller transfer ports.
Is there any good rule to create a transfer port with several radii, like on the RSW drawings?

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I have exactly the same problem as you.
Maybe you can move your studs?
I experience other types of fixation, i design a key(tool) for the nuts
There is a patented Maxiscoot system.

[quote="florent.doublet"]Je m'amuse également avec les outils numériques.

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C'est conçu pour minarelli am6

florent.doublet a écrit:

I experience other types of fixation, i design a key(tool) for the nuts. There is a patented Maxiscoot system.

The Maxiscoot patent drawing:[Vous devez être inscrit et connecté pour voir cette image]

Only my humble opinion, but from what I'm hearing out of France, I think I'd be looking at something more like a Maxi-shoot. All " cylinders " are not created equally.