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

JanBros a écrit:

Has it ever been tried to campfer/round of the piston edges in front of the transfer ports ? as the piston edge is almost all the time in front of the transfers, shouldn't the inflow off the gasses benefit from a rounded edge as oposed to a straight edge that should cause turbulence where it is not needed ?
If your answer is "go ahead and try it" , how much material would you recommend to remain above the top ring at least (I would try it on a 50cc moped)

A radius on the piston edge gives a huge flow improvement. But since it also changes the timing, further modifications will be necessary. Go ahead and try it, Jan.
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will do.
as it's for moped wars, and a good second hand cylinder costs only 10 euro, I'm definitly going to try out lot's of things .

Is detonation no problem with such a shape?
Or does the shape needs to be transfered to the head also?

Sanderhoutman a écrit:

Is detonation no problem with such a shape? Or does the shape needs to be transfered to the head also?

No, yes.

didn't thought about Sanderhoutman's remark.

easiest solution then is to round off the entire piston circumference and also round of the entire edge of the combustion chamber I suppose?

JanBros a écrit:

easiest solution then is to round off the entire piston circumference and also round of the entire edge of the combustion chamber I suppose?

Yes

Hi Jan, hi Frits
I tried something like that, but not with a radius but with a 30° angle phase.
I changed the squish band, for having the same profil like piston squish height and volume like before.
One other advantage is mabe the larger squishband due to the 2 angles.
With the same timing i see on dyno an advantage in mid-toprange power. Bottom power did not change.
Frits allow me this question, how to calculate this please,
"aft vector together with the axial vector will give you a resultant that will lean towards the rear of the cylinder, or point straight up towards the head"
This resultant should be very interesting comparing different scavenging concepts, no?

Frits,

The subject of the piston crown edge raises a question and that is relating to the use of Dykes rings. The engines in the CIK KF (with a cast iron bore) class universally use Dykes rings, whereas in the higher power KZ (plated bore) engines, the more conventiol flat rail ring is used (as used in the Aprilias). The upside of a Dykes ring might be sharper timing, the downside being a hotter crown edge. Is there some other logic to the use of Dukes rings?

Also, while you are there, is there any advantage in having a piston pin off the bore centreline? Seemingly most CIK motors have no offset, and maybe that answers the question, but it would be interesting to know if offsets in either direction were ever tried in the development of the RSW / RSA?

Thanking you in anticipation young fella.

Vortex a écrit:

Frits allow me this question, how to calculate this please, "aft vector together with the axial vector will give you a resultant that will lean towards the rear of the cylinder, or point straight up towards the head". This resultant should be very interesting comparing different scavenging concepts, no?

Yes. Finding the length and direction of the resultant of two vectors is simple enough. The difficulty lies in establishing the length of the axial vector, because the colliding radial vectors create a high-pressure zone which enhances the axial flow. And you will need Computational Fluid Dynamics to calculate that.

Ken Seeber a écrit:

The subject of the piston crown edge raises a question and that is relating to the use of Dykes rings. The engines in the CIK KF (with a cast iron bore) class universally use Dykes rings, whereas in the higher power KZ (plated bore) engines, the more conventiol flat rail ring is used (as used in the Aprilias). The upside of a Dykes ring might be sharper timing, the downside being a hotter crown edge. Is there some other logic to the use of Dukes rings?

The Dykes ring has a circular volume above it, unless you either heighten its vertical rail, which makes the ring heavier and more prone to torsional vibrations, or unless you make the piston material above the ring groove very thin, which creates risks of insufficient mechanical strength and ditto heat transport. And this circular volume provokes detonation. And yes, you can adapt the cylinder head so it fills this circular volume. But that would give a sharp edge in the head which, you guessed it, provokes detonation.
The KF engines (without gearbox) need a wide power band, resulting in a low mean effective pressure, so they are not as prone to detonation as the six-speed KZ engines that are more highly tuned.

Citation :

Is there any advantage in having a piston pin off the bore centreline? Seemingly most CIK motors have no offset, and maybe that answers the question, but it would be interesting to know if offsets in either direction were ever tried in the development of the RSW / RSA?

It amazes me to hear that most CIK engines have no piston offset, because both the Aprilia RSW and RSA engines have 1 mm offset in the direction of the exhaust, and most kart engine manufacturers seem to mimic Aprilia. Jan Thiel once tested the influence of this offset; offset in either direction gave about the same result and no offset at all was a tiny bit worse. Personally I wouldn't bother.

Citation :

Thanking you in anticipation young fella.

You're welcome oldtimer .

Frits,

It's the old fella again !

Just to clarify, are you saying that the Dykes rings provide better sealing (and therefore engine performance) over a wider rpm range than could be provided with a flat rail ring, hence the use in the KFs ?.

Thanks

Ken Seeber a écrit:

Frits,

It's the old fella again !

Just to clarify, are you saying that the Dykes rings provide better sealing (and therefore engine performance) over a wider rpm range than could be provided with a flat rail ring, hence the use in the KFs ?.  

Thanks

Hi,
Please allow me to give my opinion,
The actual KF engines use Dykes because the sealing against the cylinderliner is much better at low revs. Those KF engines used on karttracks must have a very bright powerband, beginning at 6000 rpm going up in the limiter at 15000 rpm. I tested both solutions on dyno and i saw a clearly disadvantage coming from the I- ring in the range from 6000- 10000 rpm.

Ken Seeber a écrit:

are you saying that the Dykes rings provide better sealing (and therefore engine performance) over a wider rpm range than could be provided with a flat rail ring, hence the use in the KFs ?

Ken, I cannot comment on the sealing of a Dykes ring because I haven't used one in ages. They may give sharper timing control but for my purposes this does not outweigh their detonation disadvantage. Recent experience favours a thin flat ring, fitted high on the piston in a really tight groove.

Recent experience favours a thin flat ring, fitted high on the piston in a really tight groove.

Bonsoir Frits

On my Kawasaki KR 350 I need to use other pistons as the genuine one are unobtainable . What is the minimum hight between the top of the piston and the ring ? If the piston have two grooves where do I fit the ring top or bottom ? What is a tight groove ? I have two kinds of 1 mm thick rings but one is wider , what is the best ?

Many thanks Jean-Paul

Mestre a écrit:

Bonsoir Frits,  On my Kawasaki KR 350 I need to use other pistons as the genuine one are unobtainable . What is the minimum hight between the top of the piston and the ring ? If the piston have two grooves where do I fit the ring top or bottom ? What is a tight groove ? I have two kinds of 1 mm thick rings but one is wider , what is the best?

Bonsoir Jean-Paul,
The minimum hight of piston material above the ring groove depends on the quality of the piston material, the quality of the cooling and the power produced by the engine; there is no simple answer without much more information.
Your second question is easier: if there are two ring grooves, put the ring in the top groove (like I mentioned above: high on the piston).
A tight groove is a groove where the ring will just fit in without forcing it, but also without any axial clearance.
The last question is once again unanswerable because 'wide' and 'narrow' are no distinct dimensions. If the narrow ring is 6 mm wide, the wide ring will be unusable; if the wide ring is 2 mm wide, the narrow ring will be unusable.

Dear Frits

Bonjour de Normandie

I will say you more , my bike is a Kawasaki factory tandem twin rotary valves same as Mang and Ballington 64 mmx 54.4 mm I think it is around 80 HP . I have find casting pistons from Vertex with flat top and I wish to make a domed top so the distance from piston top to ring will be shorter .Is 1 mm to small ? One ring is 2.60mm wide and the other is 2.40 mm .
Merci Bonne journée Jean-Paul

Mestre a écrit:

my bike is a Kawasaki factory  tandem twin rotary valves same as Mang and Ballington 64 mmx 54.4 mm I think it is around 80 HP . I have find casting pistons from Vertex with flat top and I wish to make a domed top so the distance from piston top to ring will be shorter .Is 1 mm to small ? One ring is 2.60mm wide and the other is 2.40 mm .

1 mm is definitely too small, Jean-Paul. The Aprilia RSA piston has 3,4 mm material above the ring groove and that is a 54 mm piston; a 64 mm piston will have a higher thermal load.
Can't you use Yamaha TZ350 pistons? Those are proper racing pistons with only one ring groove and they have the same 64 mm bore as your Kawa.
The ring widths of 2,4 mm versus 2,6 mm do not make much difference. But for reliability it is important that the ring groove is deeper than the ring width, and for power it is important that this difference is very small, like 0,05 to 0,1 mm.

1 mm is definitely too small, Jean-Paul. The Aprilia RSA piston has 3,4 mm material above the ring groove and that is a 54 mm piston; a 64 mm piston will have a higher thermal load.
Can't you use Yamaha TZ350 pistons? Those are proper racing pistons with only one ring groove and they have the same 64 mm bore as your Kawa.
The ring widths of 2,4 mm versus 2,6 mm do not make much difference. But for reliability it is important that the ring groove is deeper than the ring width, and for power it is important that this difference is very small, like 0,05 to 0,1 mm.[/quote]

Merci Frits

No I can not use the TZ one as the KR piston is 8 mm lower between the piston pin and the crown , and I have fitted TZ conrods 110 mm instead of the Kawa105 mm.

Bonne journée Jean-Paul

You can make a batch at you own specs from a few companies , it would be easier and certainly safer

Hi Jan,
Did you ever try a tapered transfer windows on the “B” transfer port – that is a port opening which increases in width with duration? I was thinking it might allow a greater duration overall on the port.

Regards
Allan

STA are changing , nothing related to duration

Mestre a écrit:

1 mm is definitely too small, Jean-Paul. The Aprilia RSA piston has 3,4 mm material above the ring groove and that is a 54 mm piston; a 64 mm piston will have a higher thermal load.
Can't you use Yamaha TZ350 pistons? Those are proper racing pistons with only one ring groove and they have the same 64 mm bore as your Kawa.
The ring widths of 2,4 mm versus 2,6 mm do not make much difference. But for reliability it is important that the ring groove is deeper than the ring width, and for power it is important that this difference is very small, like 0,05 to 0,1 mm.

Merci Frits

No I can not use the TZ one as the KR piston is 8 mm lower between the piston pin and the crown , and I have fitted TZ conrods 110 mm instead of the  Kawa105 mm.

Bonne journée   Jean-Paul[/quote]
Mestre,Could you not use the TZ piston with a 10mm cylinder base spacer?You may need longer cylinder studs,but they would be cheaper than custom pistons..Crankcase volume would increase also.

If you could find some 115mm rods then it was just a matter of making 2mm spacers to put under the cylinder and all would play nicelly.

Mestre a écrit:

Dear Frits

Bonjour de Normandie

I will say you more , my bike is a Kawasaki factory  tandem twin rotary valves same as Mang and Ballington 64 mmx 54.4 mm I think it is around 80 HP . I have find casting pistons from Vertex with flat top and I wish to make a domed top so the distance from piston top to ring will be shorter .Is 1 mm to small ? One ring is 2.60mm wide and the other is 2.40 mm .
Merci Bonne journée    Jean-Paul

i think theres a couple different 64mm yamaha racing pistons. i have one with 15* dome angle. top edge of the wristpin to top edge of the piston is 38mm. wristpin diam is 16mm. top edge of the ring to top edge of piston is about 3.3mm. i dont have the rings but the top ring groove is about 1.25mm tall. bottom ring groove is about 1.5mm tall. groove depth is about 3.3mm. not sure if they have this piston in single ring version. also it has 2 sqaure inlet windows

there is also a 64mm piston from ktm but i dont know anything about it [Vous devez être inscrit et connecté pour voir ce lien]

GtG001 a écrit:

Hi Jan,
Did you ever try a tapered transfer windows on the “B” transfer port – that is a port opening which increases in width with duration? I was thinking it might allow a greater duration overall on the port.

Regards
Allan

if you have the space for the wide bottom of the B-transfer, why would you not use it at the top. making it smaller is a waste of space, your time-area will decrease and to compensate yoi would need a higher transfer timing loosing blowdown.