[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

roost a écrit:

Hello, I would like to know your opinion on the form of the crank webs.
I'm trying to design a crankshaft that could improve the flow. Unfortunately we are working on carters that weren't designed for performance, so we try to do as best we can. Here are some pictures of what I mean:

on piaggio scooter engine is very simple solution for better flow...just turn reed valves 90°...we use it on our racing engines with very good results...we didn´t measured engine on dyno but in datarecording comparison was better response to throttle. i can´t find now photos of last version (evo3) but only photos of evo1 concept made from hebo reeds for minarelli engine...
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now we have self made pro flow reeds (without central partition) 50mm long...when they´re good prepared they have veri good life time (more than 20mth, for first try it was about 3-4mth :) ) i try make some pictures in evening...
(sory for panjabi english :) )

koenich a écrit:

frits & roots - maybe a backdraw of this design is the weakened press fit for the big end pin?

You are right. I did not look too closely at Roots' drawings but now that you mention it: the amount of crank web material around the big end pin should be at least equal to half the pin diameter. But that can be corrected within Roots' design.

Thanks Frits for your opinion.

I will make sure there is enough material around the pin - the pictures I posted were just to display what was the idea.

alcatelco; That may be enough on an 70cc engine, with a small carburettor and standard "racing" reedvalve. But with a longer stroke crankshaft & RD350 reedvalve(not as on my pics) & 35mm carburettor that part becomes quite a bottleneck. The flow also slams into the crankshaft.

we used oversized reeds (not such big as rd350 but with big flow potencial, we used long reeds about 55mm and 0,4mm thickness for bigger opening area) and 24-28 pwk carbs (50ccm cylinders)...and in final version is area behind blue marking filled by epoxi. Cylinder is 7mm higher (we used custom 2mm countersunk heads) and 85mm conrod is used...the "wave" in intake is milled out for maximum width...i think that flow potencial is enough for cylinders up to 100ccm...I try make some photos from last version (i would like to open engine on the weekend)...
now i´m working on piaggio engine with side rotary valve

Frits Overmars a écrit:

I don't fancy dealing with problems that have since been solved, like drum brakes, contact breaker points, fixed ignition timing, air cooling, sleeved cylinders, short-stroke two-strokes, short con rods, pistons with multiple rings, carburetters etc.

Hé hé... my father won some rather prestigious races in Australia before and after WW2, on a Norton International. People would occasionally ask him why he didn't get involved in vintage racing, and his response was invariably
"Why would anyone want to race one of those old s+++boxes when they could have a modern bike?"

Frits Overmars a écrit:

roost a écrit:

.... do such "chamfered" crank webs have any negative effect on the gas dynamics that happen in the carter? The carter volume shouldn't be a problem, until now, every increase in carter volume had positive effects on performance.

There may be a positive effect on inflow and there will certainly be positive effects on big end lubrication and cooling, and on mixture homogenisation. And you already discovered the positive effect of increasing the crankcase volume.

At around BDC, it looks like some of the mixture will become kind of trapped, as it can't flow to the transfers, easily.
In an ideal engine, I imagine all of the mixture in an area where it can re-fill the transfers as quickly as possible and without hindrance. So of course, for the same primary cr, that engine would have full circle crank webs and an even longer rod. But how big would you rate the difference in terms of performance compared to roosts concept?

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Haufen a écrit:

At around BDC, it looks like some of the mixture will become kind of trapped, as it can't flow to the transfers, easily. In an ideal engine, I imagine all of the mixture in an area where it can re-fill the transfers as quickly as possible and without hindrance.

Yes, in an ideal engine the complete crankcase volume should be concentrated in the transfer ducts. In the real world that is rather difficult to achieve. But what might help, certainly in Roosts concept, is a large spacing between the crank webs, so the mixture at the bottom of the crankcase can flow towards the transfers without too much hindrance.
The picture below shows an Aprilia crankshaft where the inner faces of the crank webs are flush, and 16 mm apart.

Citation :

But how big would you rate the difference in terms of performance compared to roosts concept?

Sorry, my crystal ball is still being repaired.
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Frits Overmars a écrit:

Frits Overmars a écrit:

Haufen a écrit:

At around BDC, it looks like some of the mixture will become kind of trapped, as it can't flow to the transfers, easily. In an ideal engine, I imagine all of the mixture in an area where it can re-fill the transfers as quickly as possible and without hindrance.

Yes, in an ideal engine the complete crankcase volume should be concentrated in the transfer ducts. In the real world that is rather difficult to achieve. But what might help, certainly in Roosts concept, is a large spacing between the crank webs, so the mixture at the bottom of the crankcase can flow towards the transfers without too much hindrance.
The picture below shows an Aprilia crankshaft where the inner faces of the crank webs are flush, and 16 mm apart.

Thank you for the picture. On the RSA / RSW, did you test cranks with different radiuses in the inside of the webs? Or were the edges left rather sharp to get some kind of "scavenging" in the crankcase? On roosts engine, to aid flow even more, an inclination on the corresponding parts of the webs, pointing to the inside, could be an option, I guess.

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But how big would you rate the difference in terms of performance compared to roosts concept?

Sorry, my crystal ball is still being repaired.

Too bad, so is mine. At first, it started showing japanese signs and double displacement figures from time to time, and then, rather quickly, nasty stuff like valves and camshafst appeared rather frequently. Of course, to prevent it from damage, I immediately unplugged it. Must be some kind of trojan.

hi frits i returned for more questions   . i think mostly everybody here prepares their own cylinders but a few of us like myself are junior tuners preparing cylinders the best we can with our limited knowledge and maybe they and me have some things not very clear. you and jan mentioned a few times about keeping the exh duct volume small as possible (without impairing flow of course). with your bicycle pump comparison it  was easier to visualize because i imagine the duct is the hose on the bicycle pump and it all makes perfect sense. ok so i understand to keep the duct small as possible. and since there seems to be alot of importance in keeping the volume small i plan to use yours and jans advice

but heres whats not very clear and there seems to be no good answer that i can find, but one other person asked the same question and still i cant find any answer. regarding the size/area relationship between the duct entrance (cylinder bore) and duct exit (beginning of exh flange), which one is supposed to be bigger ? or is it only small importance which one is bigger ?   if the duct exit is supposed to be bigger than the entrance and we modify it in such a way, then certainly the duct volume will increase. so for junior tuners is it best to not fool around with the exit until we have more knowledge ?  thnx

brokedown a écrit:

...regarding the size/area relationship between the duct entrance (cylinder bore) and duct exit (beginning of exh flange), which one is supposed to be bigger ? or is it only small importance which one is bigger ?   if the duct exit is supposed to be bigger than the entrance and we modify it in such a way, then certainly the duct volume will increase. so for junior tuners is it best to not fool around with the exit until we have more knowledge ?

I am sure I have answered this before; I just don't remember in which forum it was, so here we go once more.
Do not look at the total area of the exhaust ports in the cylinder; look at the total blowdown area. From there on you can keep the duct's cross sectional area constant or you can let it diverge slightly (never more than 3° included angle).

Ahhh Frits if only we could find a reliable crystal ball repair shop or a reliable pipe program that worked universally on every motor for every type of racing.

Frits Overmars a écrit:

I am sure I have answered this before; I just don't remember in which forum it was, so here we go once more.
Do not look at the total area of the exhaust ports in the cylinder; look at the total blowdown area. From there on you can keep the duct's cross sectional area constant or you can let it diverge slightly (never more than 3° included angle).

hi frits thnx for the response   . i searched 150pages in this forum and didnt find that answer becasue i didnt want to ask you multiple same questions . but your answer today makes it very clear about the question i asked. thnx again and i hope nobody bothers again with that question   

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These have been around forever, I've personally never tried one... but thought I'd ask here if anyone ever has. The concept is to try and make the crank into a "turbo" to help pressurize the crankcase.

Ever try that Frits?

williamsmotowerx a écrit:

These have been around forever, I've personally never tried one... but thought I'd ask here if anyone ever has. The concept is to try and make the crank into a "turbo" to help pressurize the crankcase. Ever try that Frits?

No. I've seen a lot of these brainwaves; grooved crank webs like you posted, serrated crankshaft circumferences, the lot.
It might work if:
A: crankshaft rpm were ten times higher than what it is now
and
B: the mixture were supplied through the center of the crankshaft, which would require hollow shafts with huge diameters.

williamsmotowerx a écrit:

These have been around forever, I've personally never tried one... but thought I'd ask here if anyone ever has. The concept is to try and make the crank into a "turbo" to help pressurize the crankcase.

alot of products today are bullshit just to steal your money. if that grooved crankshaft was so good then rsa125 would certainly use it !   

brokedown a écrit:

alot of products today are bullshit just to steal your money.

True.

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if that grooved crankshaft was so good then rsa125 would certainly use it !

Probably. But you have to be careful with such views, Brokedown. A hundred years ago people were saying: "Everything has already been tried". And it still isn't true; the RSA125 is state of the art but it is by no means the end of possible development.

Frits Overmars a écrit:

But you have to be careful with such views, Brokedown. A hundred years ago people were saying: "Everything has already been tried". And it still isn't true; the RSA125 is state of the art but it is by no means the end of possible development.

i understand. always keep open mind   

hey frits something thats about the same topic. i believe jan said he raised the exh floor and it showed promising results but then he retired. do you know if anyone has investigated this further ? if raising the floor improves blowdown flow without needing to raise the roof any higher, then transfers could be alittle higher possibly. if so, then maybe the rsa125 could manage a couple more HP then its current state of tune ?? perhaps close to 60hp ? what do you think ?

also i see this product of pulse induction. i understand the basic concept i think, but i dont comprehend the full concept. but i guess it capsures the back flow pressure resonance and puts it back into the carb ?  so i wonder if this is a legitimate concept or just bullshit ? the manufacturer claims it 'pumps up the power'    .  but then why dont they have any proof with dyno paper or something ????  have a look frits  

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brokedown a écrit:

hey frits something thats about the same topic. i believe jan said he raised the exh floor and it showed promising results but then he retired. do you know if anyone has investigated this further ? if raising the floor improves blowdown flow without needing to raise the roof any higher, then transfers could be alittle higher possibly. if so, then maybe the rsa125 could manage a couple more HP then its current state of tune ?? perhaps close to 60hp ? what do you think ?

I think the benefit Jan experienced from raising the exhaust floor was less "short-circuit" of the fresh gas going from A port directly into the exhaust. Continuously raising the floor in small increments and then find the balance between less "short-circuit" and big enough exhaust port opening.

Mic a écrit:

brokedown a écrit:

hey frits something thats about the same topic. i believe jan said he raised the exh floor and it showed promising results but then he retired. do you know if anyone has investigated this further ? if raising the floor improves blowdown flow without needing to raise the roof any higher, then transfers could be alittle higher possibly. if so, then maybe the rsa125 could manage a couple more HP then its current state of tune ?? perhaps close to 60hp ? what do you think ?

I think the benefit Jan experienced from raising the exhaust floor was less "short-circuit" of the fresh gas going from A port directly into the exhaust. Continuously raising the floor in small increments and then find the balance between less "short-circuit" and big enough exhaust port opening.

There is a difference between exhaust PORT floor raising and DUCT floor raising of course.
Raising the duct floor was one of the first things I did at Aprilia, I was already used to this when working
with 50 and 125/2 engines!
Raising the port floor was tried in 1996, and was negative.
In 2007 it was tried again, with the now far bigger auxiliary ports.
And surely for the reason Mic says!
And a 2mm higher port floor seemed promising.
I had some ctlinders cast with the port and duct floor raised a lot more, about 12mm.
But when I realized that I would not see the result before I retired I had them CNC machined to the normal size.
So who knows?

Jan Thiel a écrit:

Raising the port floor was tried in 1996, and was negative.
In 2007 it was tried again, with the now far bigger auxiliary ports.
And surely for the reason Mic says!
And a 2mm higher port floor seemed promising.
I had some ctlinders cast with the port and duct floor raised a lot more, about 12mm.
But when I realized that I would not see the result before I retired I had them CNC machined to the normal size.
So who knows?

With the port and duct floor raised 12mm. and even bigger auxiliary port we would end up with a kind of single exhaust version of Frit's FOS cylinder concept 

brokedown a écrit:

...if raising the floor improves blowdown flow without needing to raise the roof any higher, then transfers could be alittle higher possibly. if so, then maybe the rsa125 could manage a couple more HP then its current state of tune ?? perhaps close to 60hp ? what do you think ?

Gaining 10% power normally takes about three years of hard work. Gaining it all at once would be very nice but not very realistic.

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also i see this product of pulse induction...is this a legitimate concept or just bullshit ?

Maybe it needs a fuel with 20% snake oil...

Mic a écrit:

With the port and duct floor raised 12mm. and even bigger auxiliary port we would end up with a kind of single exhaust version of Frits' FOS cylinder concept.

It won't work without the symmetry, Mic. If you want to take advantage of the increased transfer angle.area then you will need to increase the blowdown angle.area too, so you need that second exhaust. And then the transfer flow will short-circuit into that exhaust unless you concentrate it in a central column.

this says 6-12% MORE power !  but i think just more snake oil   

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hey frits i found in my garage a more modern engine to work on   

the horizontal line is side piston skirt at TDC. vertical lines is the wristpin path. the aux port can be made alot larger in most all directions but you can see the problem. without piston plug there is bleeding from the transfer to the exh when the piston pin runs over them. now i see the importance of the plugs,

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now have a look here. its 72mm stroke with 125mm rod. im going to use a longer rod. i think theres a 135mm that will fit right in. and even some longer rods 140-145mm also but they use slightly larger wristpins so finding a solution to that problem might not be easy.  but anyways lets say i use the 135mm rod, ill put aproximatly a 10mm spacer under the cylinder. i thought i recall you saying that the transfer duct should decrease in cross sectional area only slightly. did you meen from entrance to exit and if so then where is the true entrance ?  at the red arrow or orange arrow in the photo ?  because the spacer will make the red arrow measurment quit large but perhaps thats ok    .  and regarding the red arrow is there any rule that says it need to be 1.2-1.5 times bigger than the exit window ? thnx for your help
and also crankcase volume will go higher    

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brokedown a écrit:

...now have a look here. its 72mm stroke with 125mm rod. im going to use a longer rod. i think theres a 135mm that will fit right in. and even some longer rods 140-145mm also but they use slightly larger wristpins so finding a solution to that problem might not be easy.

If you start changing con rods anyway, put in a proper length, like 2.2 * stroke, or at least 2* stroke.

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but anyways lets say i use the 135mm rod, ill put aproximatly a 10mm spacer under the cylinder. i thought i recall you saying that the transfer duct should decrease in cross sectional area only slightly. did you meen from entrance to exit and if so then where is the true entrance ?  at the red arrow or orange arrow in the photo ?

The real entrance should be where your red arrow is. But that is only true if the whole duct from entrance to cylinder port has a nice regular narrowing shape.

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Regarding the red arrow is there any rule that says it need to be 1.2-1.5 times bigger than the exit window ?

Not in my rulebook.