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

Howard Gifford a écrit:: Jan what correction method did you use when measuring the horsepower?

HP was corrected for pressure, temperature and humidity.
I would have liked to correct for oxygen content too, as this varied as much as 1% some days!
But the direction told me this would be too expensive, so nothing came of it.

Even with the correction we had quite a lot more HP in winter than in summer.
With exactly the same airconditioned air temperature!

Nombre de messages : 55 Localisation : Allemagne Date d'inscription : 23/12/2011

Looking at the port layout of the RSA cylinder, I understand why it is superior to a bridged exhaust port design. There is more room for transfer ports, and there is more exhaust port area where it is needed the most, above the transfers.

Making the main exhaust port narrower, the transfers wider and the sub exhaust ports also wider looks like a promising modification to me (see drawing). If one would take this farther and farther, one would end up with Frits' FOS scavenging system.

During development of the engine, did you try to max out this modification, and what were your findings, Jan? In case you did not, would you have liked to, and which downsides do you see with a modification like this?

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We tried to raise the botom of the exhaust port, and to make the bottom narrower.
Power was a little bit better.
Making the aux exhaust ports wider still, going past the cylinder center by more than 1mm gave less power.
Bringing the transfer ports nearer to the exhaust centre port made the engine rev less and lose power as well.
This was the most 'sensitive' point in the cylinder!
Everything seemed very much to the limit and very difficult to improve any more.

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

Wow this is awesome reading,

Jan Thiel a écrit:: We tried to raise the botom of the exhaust port, and to make the bottom narrower.
Power was a little bit better.
Making the aux exhaust ports wider still, going past the cylinder center by more than 1mm gave less power.
Bringing the transfer ports nearer to the exhaust centre port made the engine rev less and lose power as well.
This was the most 'sensitive' point in the cylinder!
Everything seemed very much to the limit and very difficult to improve any more.

Jan, I often wondered how the boundary layer between the outward exhaust flow and the “A” Transfer inward flow manage to keep separated at all as there must be a large shear factor.
Am I wrong in thinking that the temperature of the charge is responsible for keep this separation?
Do you think that you disturbed this delicate balance when trying to extend the “A” transfer port closer to the exhaust port?

Thanks for your reply [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 40 771973

[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 40 771973

Was your dyno room airconditioned? That must have been a big advantage. In my dyno room the summer air temperature can vary up to 20 degrees C and that makes testing difficult to get consistentcy . Did you use the standard correction or the SAE correction method?
I have duplicated as close as I can your porting specifications on a pair of cylinders for my Rotax 440 cc engine and testing this morning has shown some significant gains. I thank you for sharing your knowledge.

Yes, our dymo room was airconditioned, and we used a standard correction.
We kept the dynoroom temperature at 27° C, as this was the average temperature found at the tracks
through the year.

GtG001 a écrit:

Jan Thiel a écrit:: We tried to raise the botom of the exhaust port, and to make the bottom narrower.
Power was a little bit better.
Making the aux exhaust ports wider still, going past the cylinder center by more than 1mm gave less power.
Bringing the transfer ports nearer to the exhaust centre port made the engine rev less and lose power as well.
This was the most 'sensitive' point in the cylinder!
Everything seemed very much to the limit and very difficult to improve any more.

Jan, I often wondered how the boundary layer between the outward exhaust flow and the “A” Transfer inward flow manage to keep separated at all as there must be a large shear factor.
Am I wrong in thinking that the temperature of the charge is responsible for keep this separation?
Do you think that you disturbed this delicate balance when trying to extend the “A” transfer port closer to the exhaust port?

Thanks for your reply [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 40 771973

When you have sufficient blowdown there is no exhaust flow anymore when the transfers open.
I suppose this happens at max. torque
After max power the blowdown becomes insufficient and exhaust gases penetrate the transfers.
You can see this when you take off the cylinder: the transer ducts become black.
Some of the fresh charge goes into the exaust, this helps piston cooling.
But when it becomes too much exhaust temperature becomes lower and you lose power and revs.
Until at a certain no. of revs the engine completely stops.

Nombre de messages : 112 Localisation : Germany Date d'inscription : 07/02/2012

guys this is incredible! jan and frits, you shared so much information, so much knowledge and a lot of stuff with us - big thank you from my side!!!

I was reading the posts since first day and now I would like to ask a question I was thinking a lot about:

Jan, Frits or to whoever feels to give an answer - what are your thoughts on primary compression/crankcase volume on TDC?

From what I learned till now, we are not looking for a very low crankcase volume/a very high primary compression. But, is there any point when the crankcase volume gets to big? Or sth like a thumb rule to say crankcase volume should be factor x bigger than capacity of the cylinder?

Nombre de messages : 55 Localisation : Germany Date d'inscription : 20/02/2012

I am not 100% sure but as far as I know Frits or Jan once said, that the factor of the Aprilia RSA is 5,2. I am tuning Simson engines and I mostly use conrod lengths that are about 2,0-2,2 x Stroke (what should be okay) and I mostly reached factors about ~4,9. That's why I guess that it is kind of hard to make it "too big", so I would advise to make it as big as possible.

By the way, what is the conrod length of an Aprilia RSA engine? Sorry if it was already mentioned in this thread.

Regards

Nombre de messages : 112 Localisation : Germany Date d'inscription : 07/02/2012

i think they said sth like 625cc - what gives exactly a factor of 5. I am dealing with the Derbi EBS engine which has a 90mm conrod, with 39,7mm stroke this results in a factor of 90:39,7=2,27.

The main reason why I am asking is that everyone who is doing crankcase work keeps telling me on a Derbi 50cc engine only small machining should be done - otherwise the crankcase volume will get way to high. At the moment I am really really gently flowing the inlet tract and the top end of the crankcase so that i do not lose any more crankcase volume. But this is not what I would consider a good flow...

Nombre de messages : 62 Localisation : Denmark Date d'inscription : 12/02/2012

Daniel A. a écrit:: I am not 100% sure but as far as I know Frits or Jan once said, that the factor of the Aprilia RSA is 5,2. I am tuning Simson engines and I mostly use conrod lengths that are about 2,0-2,2 x Stroke (what should be okay) and I mostly reached factors about ~4,9. That's why I guess that it is kind of hard to make it "too big", so I would advise to make it as big as possible.

By the way, what is the conrod length of an Aprilia RSA engine? Sorry if it was already mentioned in this thread.

Regards

120mm. 5mm. longer than the RSW125

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

Talking about cranks, I would like to ask a question mr. Thiel or mr. Overmars.
Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance? I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance. It's a small 70cc engine, the crankshaft webs diameter is 71mm and crankcase is 75.
Thanks.

Nombre de messages : 112 Localisation : Germany Date d'inscription : 07/02/2012

roost a écrit:: Talking about cranks, I would like to ask a question mr. Thiel or mr. Overmars.
Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance? I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance. It's a small 70cc engine, the crankshaft webs diameter is 71mm and crankcase is 75.
Thanks.

with smaller webs you are also increasing the crankcase volume, so we are back on my question [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 40 809262

[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 40 809262

could also be somehow a proof that at a certain point the crankcase volume gets too big!?

Nombre de messages : 28 Localisation : Slovenia Date d'inscription : 19/03/2012

Koenich: indeed, before the crankcase was enlarged I tested to increase the crankcase volume by rising the reed valve and I have measured a good gain in performance. The crankcase was then machined and the gain in volume was only half of the volume gain by rising the reedvalve. After the rebuild I measured substantially lower performance.

Nombre de messages : 112 Localisation : Germany Date d'inscription : 07/02/2012

roost a écrit:: Koenich: indeed, before the crankcase was enlarged I tested to increase the crankcase volume by rising the reed valve and I have measured a good gain in performance. The crankcase was then machined and the gain in volume was only half of the volume gain by rising the reedvalve. After the rebuild I measured substantially lower performance.

hm strange - maybe it is because while rising the reed valves you also got another inlet tract length. anyway it would be nice to see some thoughts of someone who knows and is not guessing around [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 40 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 40 241515

btw - are you the roost exhaust guy from slovenia? [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 40 809262

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