[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

oh s..t..
Im so preoccupied with ports, mixtures,flows etc that i totaly forgot about the coolant...
And it is something I have do quite often.
That is why you (khow who)should have written that book...

Mic a écrit:

@GrahamB -> Yes the RSA250 used the same cylinders as the RSA125. But the 250 had side mounted rotary valves like the RSW250 and RSW125.

Yes, that's why I asked vs RSW125... but then I hadn't realised about the change of crankshaft rotation!

But, umm, dumb question: wouldn't the losses from running one crank backwards be compensated by running the other one forwards, and the result end up the same?

What I was really wondering about was whether there was a loss from the curved lower pipe compared to the straight upper one, but a) I don't suppose it was possible to measure that directly (maybe on an inertial dyno with rapid sampling??) and b) I had forgotten one crank turned backwards

PS: thanks for the link: I had seen it before but my Italian is not so good. I had picked up that the gearbox was re-arranged to be vertical, but not that the primary drive swapped...

@GrahamB Both RSW250 and RSA250 have 2 counter rotating crankshaft's. The RSA250 has both crankshaft rotating in the opposite direction compared to the RSW because of the new gearbox layout. With reversed rotation you changes pressure side on the cylinder wall during the down stroke. I think this is what caused the powerloss.

The best direct compare would be between last generation RSW125 and RSW250 both with APE cylinders. Maybe Jan or Frits knows the power of these to models?

Of course we tried the top and bottom exhaust pipes of the 250.
Usually the top pipe was the worst, is was not exactly straight, mainly at the rear.
Later this was improved.

I do not really know the 250's power as 250 engines were never tested.
All development work was done on a single cylinder 125.
Even the influence of reversed crankshaft rotation was tested on a 125!
And with the pistonpin offset to the other side!
I do not think a 250 would have given double the power of a 125.
Because the crankshafts were coupled by gears, which gives a power loss.
Also the bottom 'engine's inlet trakt was not exactly the same as the upper one.
Cylinders for 250 and 125 were always exactly the same, as were the exhaust dimensions.

The RSA 125 was only slightly better than the RSW, certainly less than we expected!
Maxmum power was almost exactly the same, the RSA was a bit better at 11000 rpm.
But please consider that the RSA was built at DERBI, to substitute their existing reed valve engine.
And to beat Aprilia! At Derbi we did not want to make an Aprilia copy of course!
It only became an Aprilia engine after Aprilia was bought by Piaggio.
Many people at Aprilia did not like it because it was not made by them!
I think it's main advantage was that it permitted a better airbox!
But the necessary inclined position of the carburettor caused a lot of problems!

Hi Jan, Frits and anyone else that may be able to help.

I hope that you do not mind that this post is not strictly on topic.

I am looking for either a cylinder (in any condition), or engineering drawings of a cylinder, from the Aprilia RSW500 V-Twin (250cc cylinder) as raced by Harada and McWilliams, and also to know the bore and stroke of the engine.

This is to help me in the design of a 250cc single cylinder engine for Superkart use.

I have had no luck so far and just wondered if anyone could help. I am following this topic with interest as we also race a 250 twin superkart with a Canadian BRC engine, which use a lot of Aprilia parts and technology.

Sorry to intrude into this topic, but I thought that it might give me the best chance of finding the parts/information that I need.

Many thanks for your attention.

Best Regards

Ian Harrison
Viper Racing UK
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+44 (0)161 343 2009

Ian, you might find your joy here:
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Thank you Graham

Best Regards

Ian

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The rather wide divider between the entry of the A and B ports at the cylinder base seems to be pretty unique. What would normally be an obstruction of flow, is beneficial on this engine (so I read on the internet).

I am wondering: why?

• When developing the engine, did you investigate how it works / what it does?

• Did you build a cylinder with the same area at the beginning of the ports but with a small / conventional divider as drawn on the right half of the picture??

Hello Ian,

If you want to make a single cylinder 250 engine, which I think is not a good idea, you'd better make a
bigger version of the 125 Aprilia cylinder!
The 500/2 was a short stroke, a very bad thing for a 2stroke, and it gave little power.
No parts or drawings are available anymore, everything was sold to bike collectors.

Haufen a écrit:

[Vous devez être inscrit et connecté pour voir cette image]
The rather wide divider between the entry of the A and B ports at the cylinder base seems to be pretty unique. What would normally be an obstruction of flow, is beneficial on this engine (so I read on the internet).

I am wondering: why?

• When developing the engine, did you investigate how it works / what it does?

• Did you build a cylinder with the same area at the beginning of the ports but with a small / conventional divider as drawn on the right half of the picture??

The reason I tried this was that I wanted to change the flow direction.
With the small divider, as it was before, it was directed at the C-port.
And I wanted it to be directed to the cylinder centre.
So I tried to fill the duct with epoxy, and it proved to work.
Of course this test was repeated several times, it always worked.
All modifications were always tested at least 3 times.
This test was done in 1997.
It gave a little bit more power, and more maximum revs.
After that we modified our casting models.

Hi Jan,
Very interesting - are you saying that it is not good enough to just aim the port opening in the direction you want; in this case towards the centre of the cylinder, but the whole of the stream should be moving towards this direction from the start – more mass motion? Wouldn’t this introduce a swirl motion when leaving the port or was that the purpose from the start?

Thanks for your answer
Regards
Allan

Allan,

There was no special purpose, it was just something I wanted to try.
Maybe limiting the entrance of the B-ducts reinforced the flow of the A-ducts.

Jan Thiel a écrit:

The reason I tried this was that I wanted to change the flow direction.
With the small divider, as it was before, it was directed at the C-port.
And I wanted it to be directed to the cylinder centre.
So I tried to fill the duct with epoxy, and it proved to work.
Of course this test was repeated several times, it always worked.
All modifications were always tested at least 3 times.
This test was done in 1997.
It gave a little bit more power, and more maximum revs.
After that we modified our casting models.

Thanks Jan, very interesting! After the test, did you also test to epoxy the red area in the transfer part of the cases? This way, the good flow direction remains the same, but the obstruction caused by the big divider is eliminated.

Like this:

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No, I did never try this, as I did not want to reduce crankcase volume!
What I did try were different radiusses on the divider.
They made absolutely no diference.

Thanks Jan, i've got another question regarding the exhaust port, this time. You wrote:

Jan Thiel a écrit:

I tried to keep them separated by making the 'dividing wall' longer.
I did this mainly to keep it from cracking!
It resulted in power loss and a lot less max. revs
So I had to make the 'dividing wall' as short as possible.
Sometimes it cracked when a piston seized, then it was impossible to repair.
Enlarging the auxiliaries gave a big iprovement!
This was done in 1998 I think.

Would this be a dividing wall already too long?

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Do you remember which length of the dividing wall you ended up with, on the RSA cylinder?

That's Calvin's cheetah :)

Hi Jan

Thank you very much for your reply.

I thought that might be your answer, but I did not know the full "story" of the RSW500 or know it's bore x stroke dimensions.

The single cylinder 250cc category is there by regulation to use 250 motocross engines (like the Honda CR250) in Superkarting (particularly in the UK).

As these production engines are now dissapearing, there may be some people (like me!!) who will start to produce engines specifically for this category, just as Francis Payart (who you see on this forum) produces his very succesful FPE engine for the twin cylinder category.

There is one manufacturer already and his cylinder is modelled closely on the Honda 500cc V-twin GP engine.

However, the bridged exhaust is not my favourite design and from your comments on this forum it seems that more power and perhaps reliability can be had from the Aprilia type exhaust port design and perhaps a 68 x 68.5mm configuration

Many thanks for your valued input.

Best Regards

Ian

Hi Jan. It is intersting hearing about your testing and I love your direct answers.
You said in your last answer you did not want to reduce crancase volume. When designing the RSA engine did you test different crankcase volumes and if so what crankcase compression ratio did you find worked best?
Also do you think the RSA engine could be successfully scaled up and get similar power / cc?

Hello everybody.

I'm astounded to read answers of geniuses as Mr. Jan Thiel and Frits Overmars, I'm very grateful that you answer the questions that many people admire.


On the question of Howard Gifford, I read somewhere in this forum that the RSA engine has 650ml volume in the crankcase with the piston at TDC.

Now, my question is as follows:
On the issue of Haufen to confront the ramp of cylinder transfers duct of the crankcase. You said that it did not want to try to reduce the volume of the crankcase.
But if I had done that Haufen drawn and this volume compensated by biggest housing of the crankshaft to reduce sliding oil?

The volume would have been the same, and in addition to the cylinder still s'haguera able to reduce slippage for oil to the crankshaft.

Many thanks for their attention and sorry for my bad English.

Haufen a écrit:

Thanks Jan, i've got another question regarding the exhaust port, this time. You wrote:

Jan Thiel a écrit:

I tried to keep them separated by making the 'dividing wall' longer.
I did this mainly to keep it from cracking!
It resulted in power loss and a lot less max. revs
So I had to make the 'dividing wall' as short as possible.
Sometimes it cracked when a piston seized, then it was impossible to repair.
Enlarging the auxiliaries gave a big iprovement!
This was done in 1998 I think.

Would this be a dividing wall already too long?

[Vous devez être inscrit et connecté pour voir cette image]

Do you remember which length of the dividing wall you ended up with, on the RSA cylinder?

No, it is about right!
By making them narrower you'd probably gain power, our's were about 2,5 mm
But in case of a seizure they will crack!
This is a nice and 'safe' casting!
Very well made, with good cooling.

Ian Harrison a écrit:

Hi Jan

Thank you very much for your reply.

I thought that might be your answer, but I did not know the full "story" of the RSW500 or know it's bore x stroke dimensions.

The single cylinder 250cc category is there by regulation to use 250 motocross engines (like the Honda CR250) in Superkarting (particularly in the UK).

As these production engines are now dissapearing, there may be some people (like me!!) who will start to produce engines specifically for this category, just as Francis Payart (who you see on this forum) produces his very succesful FPE engine for the twin cylinder category.

There is one manufacturer already and his cylinder is modelled closely on the Honda 500cc V-twin GP engine.

However, the bridged exhaust is not my favourite design and from your comments on this forum it seems that more power and perhaps reliability can be had from the Aprilia type exhaust port design and perhaps a 68 x 68.5mm configuration

Many thanks for your valued input.

Best Regards

Ian

Ian, I did not know about the single cylinder class!
With a big cylinder the bridged exhaust could cause a lot of problems.
So I think an Aprilia-type exhaust would be better, and it also gives more power!
68X68,5 would be the best bore and stroke.
You could try to 'upscale' an Aprilia 125 cylinder.
A very nice and interested experiment, I am convinced it would work great!

Howard Gifford a écrit:

Hi Jan. It is intersting hearing about your testing and I love your direct answers.
You said in your last answer you did not want to reduce crancase volume. When designing the RSA engine did you test different crankcase volumes and if so what crankcase compression ratio did you find worked best?
Also do you think the RSA engine could be successfully scaled up and get similar power / cc?

Howard,

The Aprilia RSW had 650cc crankcase volume, I never measured the RSA.
It would surely have been bigger with it's 5mm longer conrod.
Scaling up the RSA would be very interesting!
But the difference with the RSW was not that big!
And a side inlet is such a LOT simpler!!

Piquer,

Your English is quite good!

There is also another reason not to make a division in the crankcase tranfers.
I think few, if any, people know exactly how the mixture enters the cylinder from the crankcase.
With the spinning crankshaft and moving conrod and piston.
So I wanted an as big as possible entry into the transfers!

Jan Thiel a écrit:

Ian, I did not know about the single cylinder class!

So I think an Aprilia-type exhaust would be better, and it also gives more power!
68X68,5 would be the best bore and stroke.
You could try to 'upscale' an Aprilia 125 cylinder.
A very nice and interested experiment, I am convinced it would work great!

Hi Jan, Many thanks for your input, I will do exactly as you suggest and let you know the results. In the meantime I will leave this Forum to stay on it's subject matter of the Apilia 125.

However, I thought you might just be interested in a photograph of the engine installation of a Honda CR250 in one of our Superkarts.
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Best Regards

Ian

Haufen:
Back in the early 70's Kawasaki motors in Artic Cat snowmobiles had transfer dividers in the carnkcase. One of the modifications was to reshape them on 75 and older engines. In 76 Artic Cat used Suzuki engines and in 77 Kawasaki built their own sleds. In 77- 82 they did not have the dividers. I have a picture but I don't know how to post it.