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

GrahamB a écrit:: Sigh....

Join the club.
Ooops, forgot: you already did [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 4 809262

[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 4 809262

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

lodgernz a écrit:: Graham, I think your formula has an error. I believe it should be:

V= Pi * h *(R² + 2 * h²/3 +R *h * Pi /2 )

correct, I worked out the formula I posted to simplify it, and got the same result.

moadoc a écrit:: Hello my name is Andrew and I have gained much information from these pages for which I thank you all very much.
My question is Can the inlet manfold between the carb and the rotary valve be too short??
I race a PVP 251 motor which has a relatively short manifold. It could however be made 5mm shorter quite easily. Would there be an advantage in doing this? Or should I leave it alone?
I have researched these pages but could not find the answer. I aplogise if this topic has been previously covered.

Why not make it and test would be worth doing.

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

@ Jan or Frits :

have you ever experimented with the thickness of the material between combustion chamber and cooling water ? What thickness would you recommend ?
Normaly I go for 8mm.

Jan has already written about it. It depends on the quality of the material an the stiffness of the head construction, but the closer you can get to the spark plug, the better.
Of course it also depends on engine size; 8 mm would certainly be too much for a 50cc and maybe not enough for a 700cc.

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

it's for my KR1, so 125cc

Nombre de messages : 5 Localisation : Auckland Date d'inscription : 08/05/2013

Institute of TwoStrokes a écrit:

moadoc a écrit:: Hello my name is Andrew and I have gained much information from these pages for which I thank you all very much.
My question is Can the inlet manfold between the carb and the rotary valve be too short??
I race a PVP 251 motor which has a relatively short manifold. It could however be made 5mm shorter quite easily. Would there be an advantage in doing this? Or should I leave it alone?
I have researched these pages but could not find the answer. I aplogise if this topic has been previously covered.

Why not make it and test would be worth doing.

Yes I have and I will dyno shortly and let you know

lodgernz a écrit:: Graham, I think your formula has an error. I believe it should be:

V= Pi * h *(R² + 2 * h²/3 +R *h * Pi /2 )

Yes, sorry, all terms should be dimension length cubed...

Nombre de messages : 151 Localisation : usa Date d'inscription : 29/09/2013

hi frits i have a question if you dont mind. something thats stumped me for quit sometime. its been a while but ive read through all the pages by you and jan and even other pages from different members but i dont recall ever seeing much talk of cylinder pressure at kicking speeds. unless i overlooked it ?

i try not to belive much of what i read on other main stream 2t sites because ive found alot of the info is rubbish but it seems most other sites ive visited believe that cranking compression is more important than almost anything else. more important than compression ratio, more important than squish clearance, dome shape etc etc.

to be honest ive never bothered to check cranking cylinder pressure on any engine ive built because i didnt think it had much priority but i didnt have the knowledge to know for sure.

so whats your thoughts on this subject frits. when building a engine is there any importance to cranking cylinder pressure or is it just useless numbers ? thnx for any advice

brokedown a écrit:: to be honest ive never bothered to check cranking cylinder pressure on any engine

Neither have I, except when buying a second-hand car Wink

.

Nombre de messages : 22 Localisation : Perth, WA Date d'inscription : 28/12/2012

Frits & Jan,

A couple of questions:

1. In comparison to the RSA & RSW transfer ports, where C is higher than B, B is higher than A, some current KZ kart engines are the opposite. Is there any logic to this or is this some new development since the RSA era?

2. Did you ever measure fuel flow rate such that you could calculate the BSFC (gm/kWhr) at the max torque ot power engine speeds?

Thanks

Ken

Ken Seeber a écrit:: 1. In comparison to the RSA & RSW transfer ports, where C is higher than B, B is higher than A, some current KZ kart engines are the opposite. Is there any logic to this or is this some new development since the RSA era?

In the Aprilia cylinders C is not higher than B, Ken. The A-ports have a timing of 130°; B and C both have 132°.
To my knowledge there has not been any new development since the RSA era worth mentioning.
Kart cylinder internals are usually copied from Aprilia or Honda cylinders; Honda had high A-ports, lower B-ports and an even lower C-port; what we called a Dalton-staggering.
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Citation :: 2. Did you ever measure fuel flow rate such that you could calculate the BSFC (gm/kWhr) at the max torque ot power engine speeds?

The usual fuel flow measuring equipment is too slow to give reliable data on anyting but engines running at a steady rpm.
I have not yet found sensors sensors that can cope with our rate of acceleration.

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

a question to all engine builders :
- how much bigger does the viton O-ring for sealing the combustion chamber need to be ? I have a 56mm bore, can the ring begin at 58 or does it need to be further away ?
- how much % does the depth of the groove need to be for the ring? for a 2mm ring, is a groove of 1.6mm ok (20% less) ?

JanBros a écrit:: a question to all engine builders :
- how much bigger does the viton O-ring for sealing the combustion chamber need to be ? I have a 56mm bore, can the ring begin at 58 or does it need to be further away ?
- how much % does the depth of the groove need to be for the ring? for a 2mm ring, is a groove of 1.6mm ok (20% less) ?

I have seen many cylinder/head adptations to oring. And seen those with less than 3/2,5mm from water or bore to oring grove craking many times (Both on alloy and iron cylinders)
So, if you have a 56 bore and want it reliable better start it at least at 61mm

Had used Viton oring's of 2mm, no problem...reliable

Nombre de messages : 151 Localisation : usa Date d'inscription : 29/09/2013

hey frits on this it says a helmholtz resonator is used on some engines to remove the need for a reed valve. what engine might they be talking about ?

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The O-ring itself alone can't seal in direction explosion/ water.
To obtain a reliable mounting, you must meet a metal/metal contact of 4 mm minimum for this bore.
As everyone knows, the rubber is incompressible, it is just deformable.
This explains the cracks observed sometime on the water side wall.
0.3/0.5 mm compression for a torus diameter 2 mm is correct
An O-ring groove must be calculated by volume.
The filling ratio of the groove by the seal ring must always be less than 100%, 98% is OK.
To preserve the cylinder, now we place the O-ring into the cylinder head. [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 4 809262

It is cheaper to repair a head than a cylinder.
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Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

had my ring beginning at 62, but was mainly asking about this because had someone else interested in my KR1 head but he has a big bore 300cc version.
so he can not use it.

surface area of 2mm ring = 3.14mm²
surface area 2x1.6 = 3.2mm²

so I was spot on Very Happy

TNX guys !

JanBros a écrit:: a question to all engine builders :
- how much bigger does the viton O-ring for sealing the combustion chamber need to be ? I have a 56mm bore, can the ring begin at 58 or does it need to be further away ?
- how much % does the depth of the groove need to be for the ring? for a 2mm ring, is a groove of 1.6mm ok (20% less) ?

Try this site link as it gives the correct groove dimensions for the size of 'O'ring
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Nombre de messages : 56 Localisation : Austria Date d'inscription : 02/03/2014

The modena kk1 kart engine's cylinder is tilted backwards a bit so that the reedcase can be aimed upwards straight into the transfers. But does this give a (considerable) gain? Because if the crankcase is big enough the engine doesn't have to breath directly out of the reeds or does it?

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

last question before I'm of to my lath : do you make the height of the inserts equal to the cut-out in the head, or make the insert a tiny bit higher to make sure it is pressed profoundly on the cylnders and if so, how much higher, 5/100's ?

to make it more clear : I'm talking about the height of the outer ring of the insert on which the head presses down for sealing.

Right, 0.05 mm higher

Hi Jan,
You mention in an earlier section that the divider between the auxiliary exhaust ports and the main exhaust port should be as short as possible – where you able to put a reason on why this is? Do you think it is related to heat transfer to the mixture going through the exhaust port?
Regards
Allan.

Nombre de messages : 151 Localisation : usa Date d'inscription : 29/09/2013

hi frits i have a question. in this photo you see the crank wheels has 4 large holes.

if you plugged the holes and added a slightly longer rod and spacer plate under the cylinder, would the mixture volume that previously occupied the 4 holes now be located where the spacer plate is (which is much closer to the transfer opening) when the piston starts to open the transfer windows ? i hope that makes sense. so you would be eliminating volume of them holes and adding volume to a higher location where the spacer is and near transfer opening. does this sound correct and if so would that meen more mixture can now make it to the cylinder since it will travel a smaller distance ? [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 4 809262

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Nombre de messages : 86 Localisation : Australia Date d'inscription : 05/05/2012

Brokedown, I'm intrigued & would like to hear the opinion of Frits.
My view on this. Your assuming that those holes spinning about would have something going in & out of them the same as the cylinder does ( empty. Fill, empty ) I'm sure you can regard this Volume as non existent from a crankcase volume perspective, the centrifugal forces wouldn't allow entry. The disturbance about these holes may actually assist in the lubrication of the big end, but if substituting for a heavier rod then these may be of some use if filled to achieve a more conducive balance factor

brokedown a écrit:: in this photo you see the crank wheels has 4 large holes.
if you plugged the holes and added a slightly longer rod and spacer plate under the cylinder, would the mixture volume that previously occupied the 4 holes now be located where the spacer plate is (which is much closer to the transfer opening) when the piston starts to open the transfer windows ?

That is correct Brokedown.

Citation :: would that mean more mixture can now make it to the cylinder since it will travel a smaller distance ?

The column of mixture would be less tortuous and shorter, so the available pressure difference between cylinder and crankcase could accelerate it to a higher flow velocity and 'pull the mixture apart' less; its density would remain higher, so you would get more mass into the cylinder.

» [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)
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