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

Thank You.

What kind of coating is this usually?
Titanium galls too, if I am right?

It is compromise yes, also if I put it more like B, than I loose cooling around exhaust duct.
Aprilia had valve "housing" merged with exhaust duct on top, but with thin slider, I could keep water pocket behind slider.
If I am right, it is also compromise between curvature of duct upper surface curve and cylinder curve, so you get smooth duct when valve is opened.

I don't know what kind of coating is used on the top and bottom faces of piston rings. Usually it has a bronze-like colour.
Indeed, titanium galls too. But I think titanium nitride would be suitable, and it can be applied to steel.

Frits, about your FOS-pipe. You said, that it would be good starting point, but because I really don't have possibility to make 102 pipes, how would I know what to change if:

- I want bit broader power band
- I want power band to be 500rpm lower/higher
- I want more over revs

It would be great to have even some kind of short explanation, what affects what.

Hello op76, you may find parts of your answers here:http://yam2stroke.free.fr/articles/articles-2/techno/detente/detente.htm
Sorry it's in French only but you may use google translation, at least for the table called "architecture d'un pot de moteur 2 temps" (=2 stroke engine pipe architecture), which gives you general rules to know in which direction to go to influence engine behaviour.

op76 a écrit:

Frits, about your FOS-pipe. You said, that it would be good starting point, but because I really don't have possibility to make 102 pipes, how would I know what to change if:
- I want bit broader power band
- I want power band to be 500rpm lower/higher
- I want more over revs
It would be great to have even some kind of short explanation, what affects what.

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.

ciao, first of all let me introduce myself cause I'm new in this forum: I'm Flavio from Italia, I'm 47 and my hobby is tuning old vespa piaggio, so I do exactly what Fritz hates: playing with old engines, air cooled, fixed ignition with no exhaust valve etc etc... I think that reaching 25/30HP from this old engines can be as challenging as getting 60Hp from the Aprilia RSA... and especially cheaper, saving my marriage (at the moment...)

In the last weeks I read all 5 parts of this wonderfull discussion and now I have some questions regarding "specific angle-area" concept. I know that I'm discussing with people who won world championships, but I have some doubts regarding this approach applicability.

Considering specific angle-area idea and comparing reference values from RSA, my present cylinder should deliver max torque at:
- 5750 RPM considering transfer angle area
- 4300 RPM considering blowdown angle area
- 6000 RPM considering exhaust angle area (not so important)
So this data says that max torque should go somewhere beteween 4500 and 5500 rpm and suggests that I should increase blowdown AA.

Now the reality: I get max torque at about 6200RPM, so you will say: "your exhaust is too short and it's making your engine revs higher"

But this is the main point: how should I read these numbers?
should I design my exhaust to match 4500/5500 RPM resonance in order to have max torque or power from this cylinder?
And what happens if I use a shorter exhaust to match 6500 RPM resonance?

Following specific AngleArea approach, the only way to increase blowdown AngleArea is to increase exhaust timing (no space for aux ports). Let's try from 177° to 185° to have a better compromise. Doing this I get:
- 5750 RPM considering transfer angle area
- 5850 RPM considering blowdown angle area
- 6800 RPM considering exhaust angle area (not so important)
so max torque at about 5800 RPM.

In reality after this change the engine has max torque at higher RPM (around 7000 RPM) and especially it has a significant torque loss everywhere. So reality doesn't match theory.

My modest and humble experience brings me to the conclusion that the "AngleArea approach" is valid for race engines with "proper" blowdown port area, but it has some limitations with cylinders with limited blowdown port area (no aux ports). I mean that it can bring you in the wrong direction: increasing exhaust timing. This will match transfer AngleArea and make the engine revs higher (pros) but will cost a greater torque reduction (cons) resulting in a max power reduction and less usable engine.

I wonder if someone had the same experience I described

Last but not least: grazie, grazie e ancora grazie a Fritz and Jan

ciao

Ciao Flavio, maybe you can give a bit more info about the engine you are working on (bore x stroke, induction type, angles...).

flatart a écrit:

Considering specific angle-area idea and comparing reference values from RSA, my present cylinder should deliver max torque at:
- 5750 RPM considering transfer angle area
- 4300 RPM considering blowdown angle area
So this data says that max torque should go somewhere beteween 4500 and 5500 rpm and suggests that I should increase blowdown AA.

Now the reality: I get max torque at about 6200RPM, so you will say: "your exhaust is too short and it's making your engine rev higher".
The only way to increase blowdown AngleArea is to increase exhaust timing (no space for aux ports).
Let's try from 177° to 185° to have a better compromise. Doing this I get:
- 5750 RPM considering transfer angle area
- 5850 RPM considering blowdown angle area
so max torque at about 5800 RPM.
In reality after this change the engine has max torque at higher RPM (around 7000 RPM) and especially it has a significant torque loss everywhere. So reality doesn't match theory.

Benvenuto Flavio. I liked reading your first post: it is quite analytical and thorough.
Your list of things that I hate is almost complete; there is only one item still missing: writing Fritz instead of Frits .

You already predicted what I am going to say: "Your exhaust pipe is too short".
For your second example, with max torque at about 5800 rpm, you only mention changing the exhaust timing,
so I must assume that you did not change the pipe. This means that the pipe is still too short, and increasing the exhaust timing from 177° to 185° makes this even worse.
Now the pipe forces the engine to run at an rpm that is too high for the blowdown angle.area and too high for the transfer angle.area. So no wonder there is a loss of torque. Reality matches theory: at low revs cylinder filling suffers because of the wrongly-timed pipe pulses, that have become even stronger because of the higher exhaust timing, and at high revs it suffers because of the insufficient angle.areas.

By the way: can your inlet angle.area handle those 5800 rpm?

carlovitch1 a écrit:

Ciao Flavio, maybe you can give a bit more info about the engine you are working on (bore x stroke, induction type, angles...).

ciao Carlo:
bore 62 x stroke 57 (but soon I will go for a 62 x 60)
port A timing 123°, port B e C timing 119°
exhaust timing 177° (and then 185)
reeds intake VForce4
DEll'orto PHBH 30
fixed ignition 17,5° BTDC
compression ratio 12 and squish 1,05mm

Frits Overmars a écrit:

Benvenuto Flavio. I liked reading your first post: it is quite analytical and thorough.
Your list of things that I hate is almost complete; there is only one item still missing: writing Fritz instead of Frits .

You already predicted what I am going to say: "Your exhaust pipe is too short".
For your second example, with max torque at about 5800 rpm, you only mention changing the exhaust timing,
so I must assume that you did not change the pipe. This means that the pipe is still too short, and increasing the exhaust timing from 177° to 185° makes this even worse.
Now the pipe forces the engine to run at an rpm that is too high for the blowdown angle.area and too high for the transfer angle.area. So no wonder there is a loss of torque. Reality matches theory: at low revs cylinder filling suffers because of the wrongly-timed pipe pulses, that have become even stronger because of the higher exhaust timing, and at high revs it suffers because of the insufficient angle.areas.

By the way: can your inlet angle.area handle those 5800 rpm?

ciao FritS, sorry for my unforgivable mistake.

Yes, I just changed the exhaust and nothing else.

I've to read again and again your answer, but I think I got your point. So I played a little bit with my personal excel sheet that calculate angle.area etc... and I figured out the following compromise:
- raise up the cylinder with 1mm gasket >> this will raise up port timing to about 129° (A) and 124° (B and C) without touching transfer ports (too critical process)
- raise the exhaust with a file up to 192/193°

With these modifications I get 6350 as max torque RPM both for Blowdown AA and Transfer AA (Bingo!).

After these mods I should test again the exhaust and check RPM value for max torque. This will give me an idea of how much the exhasut must be lengthen.

Looking just at the bare numbers (192° and 129°), this conclusions upset my convictions. I just wonder:
- what would happen at low revs (<5000)
- how wide could be the power band considering only 4 gears transmission
- how this engine will rev after max power

but I guess that these problems could be handled with the right exhaust design and/or having four gears more close one to the other (vespa after-market is a crazy world, we can have everything we need...)

I have to think a lot....

Grazie Frits, always illuminating

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.

Ok, thanks Frits. There is still this, that how do I know what are the right time.areas? I have tried to find this information without success. Only figures I can find are those RSA's.

If I want to have 50cc (B 41mm x S 37,8mm) peaking at 12000rpm, what would be the correct time.areas? It's also piston port controlled intake.

flatart a écrit:

- 5750 RPM considering transfer angle area
- 5850 RPM considering blowdown angle area
so max torque at about 5800 RPM.
In reality after this change the engine has max torque at higher RPM (around 7000 RPM).
.......
- raise up port timing to about 129° (A) and 124° (B and C)
- raise the exhaust with a file up to 192/193°.
With these modifications I get 6350 as max torque RPM both for Blowdown AA and Transfer AA (Bingo!)

Your 'Bingo!' suggest that you are happy because this value of 6350 rpm is closer to the 7000 rpm that the pipe was enforcing upon the engine. But is the pipe really optimal for 7000 rpm??
The following is not an exact calculation, but a rough indication of how you should be looking at the situation:
7000 rpm is an average of the 5800 rpm preferred by the cylinder, and about 8200 rpm, enforced by the pipe.
In other words, the pipe is much too short, even for the new situation.

Citation :

After these mods I should test again the exhaust and check RPM value for max torque. This will give me an idea of how much the exhaust must be lengthened.

You could do it this way, but it would take many iterations in order to arrive at a final result. I suggest that you build a new pipe (it does not need to fit the scooter perfectly, that can be done later) and test it.
Below is my simple FOS exhaust concept once more; that should quickly get you in the correct time zone.
[Vous devez être inscrit et connecté pour voir cette image]

Hello! One question, what's the meaning of "crankshaft_HP" and how can I determine it ?

Thanks again Frits, I wish I was 20 years younger. All that knowledge was not available when I was racing, it took me years of patient spying listening and investigating to collect few clues for my engine. Thanks to your contribution I now look far more experienced that I really am!!!

op76 a écrit:

thanks Frits. There is still this, that how do I know what are the right time.areas? I have tried to find this information without success. Only figures I can find are those RSA's.
If I want to have 50cc (B 41mm x S 37,8mm) peaking at 12000rpm, what would be the correct time.areas? It's also piston port controlled intake.

The explanation you need is somewhere in this thread. But it is way past midnight here, so I am not going to search for it now (I never seem to get along with search engines anyway); I'll leave that to you.

EDIT: I decided to give it one go, and to my surprise I found what I was looking for!
This is what I did: in the Google search window I entered:
specific time.area site:[Vous devez être inscrit et connecté pour voir ce lien]
And this is what I got:
[Vous devez être inscrit et connecté pour voir ce lien]

Now I can go to sleep peacefully

alextatic a écrit:

Hello! One question, what's the meaning of "crankshaft_HP" and how can I determine it ?
Thanks again Frits, I wish I was 20 years younger. All that knowledge was not available when I was racing, it took me years of patient spying listening and investigating to collect few clues for my engine. Thanks to your contribution I now look far more experienced that I really am!!!

My pleasure Alex. Crankshaft_HP is HorsePower at the crankshaft. You can determine it either by wishful thinking,
or by using two-stroke simulation software such as EngMod2T, by measuring directly at the crankshaft (that's the only real way), or by measuring it some other way (at the gearbox exit shaft, or at the chain, or at the rear wheel) and correcting the measured value with the transmission losses.
And before you ask: these transmission losses can be measured too, at special test stands, rare as hen's teeth.
But you can approach them by assuming a transmission loss of about 2,5% per transmission step. A step is a set of gears, or a chain, or a wheel on a dyno drum. The latter transmission loss percentage is dependent on drum diameter, wheel load and tire pressure. This loss is converted to heat, which heats up the tire, which changes the tire pressure.
In other words: if you wish to measure accurately, don't do it via the tire.
 

Good night indeed! here it's much earlier in the day! Thanks for your superfast reply.

Frits Overmars a écrit:

EDIT: I decided to give it one go, and to my surprise I found what I was looking for!
This is what I did: in the Google search window I entered:
specific time.area site:[Vous devez être inscrit et connecté pour voir ce lien]
And this is what I got:
[Vous devez être inscrit et connecté pour voir ce lien]

Now I can go to sleep peacefully

You are better using search engines than I, but this is still the same information I already had. It's quite tricky for me to explain, as English is not my native language, and I'm not too good at it...

I take one example:

We have this 74cc Malossi "racing cylinder" for our Suzuki moped engines. It has 50mm bore and 37,8mm stroke. Stock timings are about 179/120 and 174 for piston controlled intake. It has angle areas of about 28250/2535, which should give us Spec.time-area of about 27,69 for transfers and 2,48 for blowdown. So this would mean, that our engine should be out of breath at 3560rpm because of the BD (or 5230 TR).

Now this cylinder has dyno tested with very large variety of pipes from very choked stock pipe to many after market racing and DIY pipes, and it usually has useful revs to at least 11,000 regardless of pipe...and even 13,000 with some DIY pipes. Rear wheel HP is about 7-9, and it peaks between 8,500-11,000 rpm with all those pipes.

So how this is possible? I know this cylinder is very bad for getting a lot of power, but I'm just wondering how should I use this spec.time.area to estimate my cylinder, and to what rev range I would need to make my own pipe? Of course I will be porting the h*ll out of that cylinder, but I'm trying to figure this out first

And what should I use for intake spec.time.area?

Your cylinder produces 7-9 hp at 11000 rpm with a blowdown time.area that is only 1/3 of what it should be for 11000 rpm (11000 / 3560 = 3,09), and the same is true for the power it produces at 11000 rpm; it makes perfect sense.

I have no actual data for the time.area of piston-controlled intake systems because I have not occupied myself with such systems, and we can't use the value of a rotary inlet system because the pressure history is quite different.
By the way, your English is excellent   .

Thanks again. I swear I thought the same. So, can we say, that these time.areas are better for predicting power than rpm? What should we use for the latter then?

I would instantly buy Engmod2t if I only had money

EDIT. And for what I thought about those intake time.areas, shouldn't it be at least as much as transfers?

Hi Frits,

As we are about the angle.area topic, when I go to the page you found, on which you explain those of the RSA (thanks for finding it !), there's a point I do not understand: we have 1302 mm² area and an angle.area of 110315°mm². If I divide this 110315 by 1302, I obtain 84,7°. As the transfer timing is 130°, shouldn't we havee 65°? Obviously, I'm missing something.

carlovitch1 a écrit:

Hi Frits,

As we are about the angle.area topic, when I go to the page you found, on which you explain those of the RSA (thanks for finding it !), there's a point I do not understand: we have 1302 mm² area and an angle.area of 110315°mm². If I divide this 110315 by 1302, I obtain 84,7°. As the transfer timing is 130°, shouldn't we havee 65°? Obviously, I'm missing something.

I used search-method provided by Frits , and found this:

Frits Overmars a écrit:

I see where you problem lies, Senso. Angle.area is not simply the total blowdown area multiplied by the total blowdown angle. It is the sum of a lot of small area steps, multiplied by the time during which each of these steps is open.

For example, let us assume that the exhaust port is a simple rectangle, 40 mm wide, that it opens 1 mm further for each degree of crankshaft rotation, and that the total blowdown angle is 30°.
Then the first degree of exhaust opening will open an area of 40 mm x 1 mm = 40 mm², and this area will be open during the whole 30° of blowdown period. That first area thus has an angle.area of 40 x 30 = 1200°mm².

When the crankshaft turns 1 degree further, an additional area of 40 mm² is opened. This second area will be open during 29°, so its angle.area is 40 x 29 = 1160°mm².

Repeat this calculation for each crank degree until the end of the blowdown phase, and add all the angle.area values; that will give you the total blowdown angle.area.

In reality this calculation is complicated by the fact that not every degree of crank angle gives the same port height difference, and even more by the fact that the exhaust port is not a simple rectangle. But you will understand the principle of angle.area.

Here it is: [Vous devez être inscrit et connecté pour voir ce lien]

Thanks, op76, it does make sense for me now, while more complex than what I was thinking about.

2 strokes is an art, a beautiful art of incredible nobility

op76 a écrit:

Thanks again. I swear I thought the same. So, can we say, that these time.areas are better for predicting power than rpm? What should we use for the latter then?

I agree that time.areas are better suited for predicting power than rpm, but we must remember that rpm is already included in the time.area value because rpm influences the time during which a port area is open.
If you double the rpm, it will halve the time.area of a given port. And if you stop the engine and a port happens to be open, even partially, its time.area becomes infinite (until you start the engine again).
Therefore I prefer to use angle.area; that depends only on the port dimensions and the stroke--conrod geometry.

What should we use instead of rpm? Mean piston speed. It enables us to directly compare engines with different dimensions.
For example, the Aprilia RSA has a 54,5 mm stroke and it produces its maximum power at 13.000 rpm.
During one revolution the piston moves 54,5 mm up and 54,5 mm down; that's 0,109 meter.
13.000 revolutions per minute = 13.000 / 60 revolutions per second; that's 216,667 rps.
So the piston moves a total distance of 216,667 * 0,109 meter = 23,6 meter per second.

A 50 cc engine with a 39,7 mm stroke, producing its maximum power at 15000 rpm, has a mean piston speed of
2* 0,0397 * 15.000 / 60 = 19,85 m/s. But in order to equal the development level of the Aprilia, it should produce its maximum power at 17.834 rpm and it should reach the same mean effective pressure at those revs!

Citation :

for what I thought about those intake time.areas, shouldn't it be at least as much as transfers?

Your remark certainly makes sense. But the total flow through a port not only depends on the available time.area, but also on the varying pressure difference during this time. And in this respect the inlet an transfer phases are not identical.

Toop a écrit:

2 strokes is an art, a beautiful art of incredible nobility

Frits,

in this drawing

[Vous devez être inscrit et connecté pour voir cette image]

Blowdown is continuing a bit while the transfers are already open (as it is in real life), but I wonder if this has to be incorporated into calculations or not ? How much would you say on average (you are probably going to say "can not tell you, it depends on the preasure still left in the cylinder at TO).
taking measures from the drawing, it's about 2.6°. I can manage to re-create the RSA in my Excell meeting the STA for transfer with the data from the RSA drawings , but with loosing those 2.6° of the transfers, it would be impossible.

You also said "our engines should meet the same STA's as the RSA" . Does this count for ALL/any engine or only for thermaly healthy high BMEP engines ?
I would think you'd say yes (on thermaly healthy high BMEP) , but am in doubt as less rev's gives higher STA , but less BMEP would require less A.A so levelling things a bit.
Below is an original Honda Sky cylinder (only gamble on all the data is the head volume) with the aim of producing 8 BHP at 8.000RPM (1HP for every 1000 rpm seems like a realistical target to me) :

[Vous devez être inscrit et connecté pour voir cette image]

The BMEP target's are an average of all the different ones I found, all giving about the same results, but differentiate enough so to keep it simple, for now I use an average of all of them (= Blair/Bimotion,  SoftEngine, Mota and an unknown). Here lower BMEP than RSA gives lower targets.

Can one have to much transfer T.A  ? I mean : For the Sky, I should only increase Blowdown and done. But there is still plenty of transfer real-estate available, is it bad to make use of it ? Would power drop again ?

(discard the numbers in "old", they leed to an old excell and the links are no longer relevant, need to update)