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

Hi frits, Thanks for your answer. Sorry but the photo and my english are terribile I try to explain my question few better.
If I increase the lenght of boost ports (see a photo with red lines) without increase the section of the total lenght of exhaust duct, the angle area and the flow will improve?
Thanks in advance!
Eric
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Eric_91 a écrit:

Hi frits, Thanks for your answer. Sorry but the photo and my english are terribile...
If I increase the lenght of boost ports (see a photo with red lines) without increase the section of the total lenght of exhaust duct, the angle area and the flow will improve?

Ah, now I understand your question Eric (and your English is better than my French; perhaps we should communicate in Italian   )
Enlarging the port area will obviously enlarge its angle.area. Then the question rises where the biggest flow restriction occurs. If it's in the port, then any port enlargement will improve the mass flow. If it's in the duct, then the maximum flow velocity in the duct will not increase, but the flow velocity will reach this maximum earlier in the blowdown phase, as soon as the mass flow through the increased angle.area equals the capacity of the duct.

Bringing an exhaust port closer to a transfer port or vice versa will always enlarge the risk of increased short-circuiting.
And take care that you do not create a short-circuiting path between the auxiliary exhaust ports and the transfer ports via the piston pin bore, or between the exhaust and the crankcase via the underside or the transfer windows of the piston.
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Thanks so much for the answer Frits! As a general rule, is better to have large angle area and large exhaust duct or large angle area and restrict duct? Thanks in advance!

Ps: I have another question for you about dyno but I will post It in apposite topic.

Eric_91 a écrit:

Thanks so much for the answer Frits! As a general rule, is better to have large angle area and large exhaust duct or large angle area and restrict duct?

The total mass of exhaust gas that can pass through the exhaust ports, must be given the opportunity to escape as quickly as possible, so it would not make sense to restrict it somewhere downstream. But care must be taken that the duct volumes are not made any bigger than strictly necessary.

Panas a écrit:

so how much power  do you get from 50cc and how much do you think one would  start making
from downsizing a rsa

HP-CC comparison distracted from Aprilia RSA measured at 27°C in 2007

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Quote from Jan Thiel August 22, 2012, 15:14:12 »
Very nice graphic Luc, my compliments!
I think it's all right.
A 50cc twin would therefore give 34.4 HP to the rear wheel.
That is, if he reaches the level of the RSA.
Must be able to, 100% sure!
A 50cc 4-cylinder would even get 40 HP at the rear wheel!

Quote from : Jan Thiel October 18, 2012 , 06:17:55
Beautiful graphics Luc ! That 'red ' abilities are 100 % definitely achievable. It will , however, most of the time, and especially money, are missing.
To meet the RSA power was used 5 days per week by 6 people and with it the aid of the drawing room, and electronic engineers. And that over about 12 years is not that simple ! We started in 1995 with 46.5 hp, and ended in 2007 with 54hp, so 0.6 to 0.7 HP per year on average. Actually seems very little , but it was a lot of work ! We made about 300-400 cylinders per year. So to get that far, there are at least 3600 cylinders made. But maybe 4800 !, I did not count them .... Of course, a lot of cylinders were equal to each other ! But we surely tested 30 different types of flushing channels, and also exhaust conduit 20 different types. Estimated that another 100 different cylinder heads and of course lots of exhaustpipes!
Overall it was pretty special, and will not arise so quickly again !

Quote from Jan Thiel November 19, 2015
Calculations with a specific program are certainly very useful!
I wish I had something at Aprilia, then I had definitely become further!

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Full explaning (sorry only for FB friends)
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What are the blue and yellow lines on the graphic? please.
Dan

Dan42 a écrit:

What are the blue and yellow lines on the graphic? please.
Dan

Dan,
These blue hp and yellow torq lines are not important here.
They belong to the 74% level of my 90cc dragracer at that time.
With main losses in bore/stroke ratio 1.35, wrong transferchannels, wrong crankshaft balance of 25% and AC.

I made only one 90cc cylinder in 2006 with little updates in the years after to about 30hp.
Equivalent to around 20hp for 50cc (with the same main losses).

Here you find this cylinder: 
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And here how it runs:
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For people who cannot see the FB note, here the extended version to 500cc. And also at a temperature of 20 ° C instead of 27 ° C
Also a Mota power graph based on exactly the original RSA data and the derived 50cc version according to the comparison graph. The second Mota power graphic RSA derivations to 250cc 400cc and 500cc.

This shows that these 2 different programs (Ftt calculation and Mota) show an almost identical picture to lower and higher ccm3, and thus support each other for almost 100%.
Wouldn't it be a very coincidence that the same error would be present in both programs that do the calculations on a totally different basis ??

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Frits I have another question about exhaust ducts. Sorry but Im a beginner tuner and sometimes still have stupid doubts.
For example, I have 2 auxiliary boost port that has each one an area about 50mmq. To benefit of all this area, the auxiliary exhaust duct must keep the same area as the Port for all its lenght, correct?
In other words, if I have a coin with certain diameter "D" that enter in exhaust port, It also exit from other side via exhaust duct, correct? (Im not sure Im correctly explained, maybe design can help).
Thanks in advance.
Eric
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area and shape are 2 different things. a port can have a certain shape because it is limited to that shape by the ports next to it, or because the upper part of a port contributes more to the time.area than the bottom part.
both are the main reason why you see mostly triangular auxiliary exhaust ports.
once the gases are in the duct, the shape isn't that important, as long as the transition is smooth and the area remains the same, than the velocity remains the same (maybe pure theoretical, the velocity will increase if the amount of surface area decreases ?) . but the diameter of the passing coin through the duct will change.

If the duct area is best to be the same, that's out of my league .

Thanks everybody for replies. The flow velocity in a duct (exhaust or transfer) is governed only by pressure difference between one side and other or continuity flow theoreme applies? For example for given pressure difference (from cylinder to carter ) the flow velocity through the duct is constant and if duct's area increases, increases also the flow rate?

Eric_91 a écrit:

The flow velocity in a duct (exhaust or transfer) is governed only by pressure difference between one side and other or continuity flow theoreme applies? For example for given pressure difference (from cylinder to carter ) the flow velocity through the duct is constant and if duct's area increases, increases also the flow rate?

The flow velocity in a duct is governed only by the pressure difference over the duct, if both the flow velocity and the pressure difference are constant, which is never the case in a running engine.
The initial flow velocity is zero and then increases depending on the pressure difference and the gas inertia per mm² of cross flow area. And the pressure difference varies because of the gas mass transposition from one end of the duct to the other end, and because of external influences like pipe suction and volume variations in crankcase and cylinder, and other variations listed below.
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Thanks so much! Today I have bought this book so don't worry about these questions anymore
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Frits Overmars a écrit:

Moving the power band 500 rpm up or down is easy: enter a different value for the rpm-values in the FOS-concept. But be aware that both the blowdown time.area and the transfer time.area must be suitable for the rpm-value that you choose; otherwise you'll lose power.

For a broader power band you can enter different rpm values for the X-calculation and the Lmax-calculation.
For more overrevs you can enter a higher rpm for the Lmax-calculation. But this way you'll probably not get the power that would be possible with two identical rpm-values.

Frits, what if I would want a more narrow powerrange (vario scooter), it should only work between 7500 and 9000 ?
doing the oposite would mean entering a higher rpm-number for the X-calculation than for Lmax, which feels awkward.

my very simple brain would keep Lmax at 9000 rpm and just make the belly longer and the baffle shorter/steeper.  as just making the baffle shorter would raise the rpm range of the pipe.
my "harder thinking brain" would also make the diffuser steeper and shorter . somewhere, a voice in my head tells me that to do this , the diffuser should start later and so the header should be longer and the diffuser shorter ?

for the belly, I can't realy think of anything realy. does it stay in the same position ? or in stead of making it longer like previous said, should it shift more to the back keeping it's original length so the distance of the waves (positive and negative) to each other does not change ?

JanBros a écrit:

Frits, what if I would want a more narrow powerrange (vario scooter), it should only work between 7500 and 9000 ?

Narrowing the powerband is easy enough: use a fixed-timing ignition system. Or even more drastic: use a programmable ignition and deliberately enter a curve that messes up the powerband. But somehow I suspect that you do not really want a narrower powerband; you just want more maximum power. Right, Jan?
It is often thought that a steeper end cone will give more power. But an end cone can be too steep, in which case the return pulse will turn into a shockwave before it arrives back at the cylinder. And shockwaves are not very good at shoving washed-through mixture back into the cylinder...
The end cone dimensions and indeed all dimensions in my exhaust concept are optimized for power; the resulting powerband is a byproduct. That means that you will not get more power when you change the ratios between those dimensions.

Mr Overmars,

can you look at the section " moteur", i have a question about the drawing about the cylinder head.

thanks.

Pagi, I assume that you are referring to the text below:

pagi a écrit:

Mr Overmars, could you please, drawn me a combustion chamber flat toroidal shape when you have time!!!!(and if you want!!!) for a engine 800cc-twin cylinder, 2 stroke for sure!! ( because i find....Z3 very hight....around 23.502mm...).
So, my engine have: bore = 82mm, stoke 74mm, i need a domes 30cc ( not the combustion chamber so without the piston, only the dome), squish= 1.00mm,deck hight 0.08 mm between squish band and head gasket (picture) ,squish band area 50 per cent, the piston crown shape is flat, but with 2.05° angle ( picture)
thanks in advance for you help

I did see your question. But I receive similar questions every day and answering all of them would require more time than I have available; I'm sorry.

ok, i understand.....no problem!!!

Frits Overmars a écrit:

you just want more maximum power. Right, Jan?    
It is often thought that a steeper end cone will give more power. But an end cone can be too steep, in which case the return pulse will turn into a shockwave before it arrives back at the cylinder. And shockwaves are not very good at shoving washed-through mixture back into the cylinder...
The end cone dimensions and indeed all dimensions in my exhaust concept are optimized for power; the resulting powerband is a byproduct. That means that you will not get more power when you change the ratios between those dimensions.

yes please

first of all : tnx for sharing your FOS concept. once you've figured out the temp to enter, I've not had a pipe that did not do what it was disigned for. but I feel there is more to be gained, especialy for me as I do not need a wide power band. Just by trying out an old pipe from a cylinder with to high timings, my current engine revved 500rpm more, and lost some "bottom end" but that's  before my clutch engages so it does not matter.
That pipe had about the same Lmax but a slightly bigger belly and a shorter  end cone (it was made with other software - Cocker, don't know if you know it ?, but that one has the "flaw" that diameters get bigger with longer timings). I can only use this pipe on the dyno, no longer on the bike so I want to make a new one with a shorter end cone to try out.

So from there my curiosity if you had some guidelines ;-)

Good evening JanBros, and for our info, which temperature did you use?

only 500°C

but it is only for a 50cc forced-air-cooled moped with not much tuning as there is no point in revving much further than an original. it is for moped-MX.

OK, thanks ! I'm also into aircooled 50cc MX but not mopeds, vintage bikes, I have some bikes of the beginning of the 80's like Aprilia MX50 1981, with Minarelli P6 engine.

My own estimation was 760 K (487°C), we are not that far away.

Hallo,

Is there a good aftermarket solution of fitting a "powerjet" fuel cut-off valve on a standard carburetor?
The engine is a 250 PVP tandem with 2x 42mm carburetors. All the setup-parts (not the slide) are Dellorto.
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Mikuni offers an electric powerjet with a solednoid.
As the carburetor isn't prepared for a cut-off valve the seat for the solenoid piston has to be outside.
There are a few solenoids to buy from e.g. TZ 5ke or Honda RS ect.. But in all cases the seat is inside the body of the carburetor.
Another important thing would be the size of the "jet".
Anybody here got a good idea or even did such a modification?

Someone got the RSW 250 rotary valve timing? Is opening 51,4mm BTDC and closing 28,8mm after TDC a good value nowadays?
Seams very similar the old 256 timing.

Best regards,
Bernd

Döllinger a écrit:

Is there a good aftermarket solution of fitting a "powerjet" fuel cut-off valve on a standard carburetor? The engine is a 250 PVP tandem with 2x 42mm carburetors. All the setup-parts (not the slide) are Dellorto. [Vous devez être inscrit et connecté pour voir ce lien]
Mikuni offers an electric powerjet with a solednoid.
As the carburetor isn't prepared for a cut-off valve the seat for the solenoid piston has to be outside.
There are a few solenoids to buy from e.g. TZ 5ke or Honda RS ect..  But in all cases the seat is inside the body of the carburetor.
Another important thing would be the size of the "jet". Anybody here got a good idea or even did such a modification?
Someone got the RSW 250 rotary valve timing? Is opening 51,4mm BTDC and closing 28,8mm after TDC a good value nowadays? Seams very similar the old 256 timing.

Aprilia used Keihin powerjets on their Dellorto carburetors. Powerjet size was 120 but you must consider that these powerjets were modulated, not just turned on-off.
The best rotary valve timing was: opening 142,5° before TDC, closing 88° after TDC. Experiments were also performed with 152,5° / 90°.
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