[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 2) (Locked)

Frits,

I have cut 2 heads with the profile you posted, modified for the TZ piston. I will adjust the volume for 14:1 and give the pump fuel a try.

Thanks very much for your responses.

Michael

cristogrr a écrit:

Did you use some superbenzene additive to boost the octane for commercial gases(cheaper)? , it's not trackable on spectro analysers...

Benzene is highly carcinogenic and with today's knowledge I would consider using it in a fuel as downright criminal.

Jan or Frits,

1. Does this piston edge(red line) which looks distant from the piston pin unlike most modern 2t pistons(blue line) has big flow effect to the transfers on RSA?
2. General question, when piston is in TDC, does it make any good effect on mixture suction if all transfer ports are opened "under" the piston?
Thanks

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After more observation, maybe the answer for the first question is that holes for piston pin are very high because of 120 mm conrod so that edge looks distant compared to most other 125 cc pistons.

Since you are just talking about different fuels and compression ratios anyway, I have got a question.

What is the maximum compression ratio one should use at an AIRcooled engine with normal 98 octane petrol from a german gas station, called "Super Plus" here?

Let's say I use 16:1. My guess is, that the engine will have more power the first few times it runs in powerband, but then it will get so hot that the power drops lower than it would be with a lower compression ratio.

So where is the limit for most power, even if the engine is hot? 12:1?

I know that this is also depending on other factors like ignition und cooling surface and so on, but I think there is a approximate value, isn't it?

And while I wrote "ignition" the next question came up to me. I think the ignition advance shouldn't be as little as at a watercooled engine, too, to prevent the engine from getting too hot, right? So, is there also a limit?

Regards,
Daniel

There are no limits, only tendencies, Daniel. What an engine will tolerate, depends for a large part on its cooling. And there can be a lot of difference between good and bad cooling. I have seen watercooled engines that were worse than a good aircooler.
The permissible compression ratio is limited by detonation, and the chance of detonation depends on the temperature and purity of the mixture at the beginning of compression, on the cylinder, head and plug temperatures, and on the compression ratio. A compression ratio of 20:1 won't be a problem in a well-cooled engine that does not make any power. But the more power an engine produces, the lower will be the permissible compression ratio, all other things being equal (which of course they never are).

You are right in saying that more ignition advance can make an engine run cooler. The funny thing is that the same is true for high compression ratios. Assuming that you stay away from detonation risks, a high compression ratio equals a high expansion ratio which will extract more heat from the burnt gases before they enter the exhaust. Hot gas in a cylinder is relatively stationary; it does not move around a lot and heat transfer from the gas to the cylinder walls is limited. But when that gas starts flowing through the exhaust duct, its velocity becomes much larger and so will the heat transfer.

While I am on the subject: there exists an ineradicable misconcention that a high compression ratio is necessary for power. In a foulstroke that may be true, but a two-stroke largely depends on exhaust gas energy for its power and the more the exhaust gas expands in the cylinder, the less energy will be left for exhaust resonance.

I may have told this before, but it won't hurt to repeat it: Jan Thiel once performed a series of tests in order to establish the true importance of the exhaust system. At that time a good Aprilia RSW125 produced 52 gearbox-HP. Then the complete exhaust system was removed and carburation and ignition were carefully re-opimized for the pipe-less engine. The best result was 17,5 HP. Conclusion: a good pipe can triple the power!

Frits Overmars a écrit:

You are right in saying that more ignition advance can make an engine run cooler. The funny thing is that the same is true for high compression ratios.

I hadn't thought of this... in particular it would suggest that trapped compression ratio is a really bad measure of of the state of tune of a motor !

GrahamB a écrit:

Frits Overmars a écrit:

You are right in saying that more ignition advance can make an engine run cooler. The funny thing is that the same is true for high compression ratios.

I hadn't thought of this... in particular it would suggest that trapped compression ratio is a really bad measure of of the state of tune of a motor !

Right you are!

I hadn't thought of this... in particular it would suggest that trapped compression ratio is a really bad measure of of the state of tune of a motor ![/quote]
What is trapped compression ratio?
The Europen method ((engine capacity+head capacity)/head capacity) ora Japanese method?
About a thermally sound engine:is this a water cooled engine, with proper jetting, ignition timing and head parameters, right?
Then, working on a thermally sound engine, we can put one wrong parameter (main jet, ignition advance, or squish), without an engine failure..or not?
PS Frits, when you have some time, could you give a look at my last drawings?

Filandro a écrit:

What is trapped compression ratio? The Europen method ((engine capacity+head capacity)/head capacity) or a Japanese method?

'Trapped' is the Japanese method: total volume above the exhaust port / combustion volume.

Citation :

About a thermally sound engine: is this a water cooled engine, with proper jetting, ignition timing and head parameters, right?

About right. Watercooling is a condition, but it is not a guarantee. Like I said, I have seen watercooled engines with bad cooling.

Citation :

Then, working on a thermally sound engine, we can put one wrong parameter (main jet, ignition advance, or squish), without an engine failure..or not?

Within limits: yes.

Citation :

PS Frits, when you have some time, could you give a look at my last drawings?

I looked at them the moment you showed them, but I cannot find the time to comment on them.

Thanks a lot!
O meglio: grazie mille, ancora una volta.

Thank you Frits! Your explanation helps me a lot!

hello

a question to Frits

can you please tell me how the engine characteristics changes if i make the end cone as in the drawing the red line? the exhaust has first an end cone angle of 15.5 °, and I change: first part 15 ° to 16 °. the diameter of the middle section, tailpip, and the length remains the same.

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a change like this is always a volume change. what effect has the volume to the exhaust characteristics??

thanks

manuel

i would of thought the more volume "held" would be more energy stocked,

on the other hand, the more volume you hold for longer (time) (mass/momentum/change of direction/saturation the more the energy could disipate, almost like a shock absorber,

if my idea is total rubbish put me right please,

Manuel Rainer a écrit:

a question to Frits
can you please tell me how the engine characteristics changes if i make the end cone as in the drawing the red line? the exhaust has first an end cone angle of 15.5 °, and I change: first part 15 ° to 16 °. the diameter of the middle section, tailpip, and the length remains the same.
a change like this is always a volume change. what effect has the volume to the exhaust characteristicsl

I am not fond of questions like these, Manuel, because there is never enough information. For example, a larger pipe volume will lower the Helmholtz frequency and thus lower the resonance rpm values for maximum torque and maximum power. But if that pipe volume becomes too large, the maximum torque value will drop too. And I have no way of knowing whether that volume will be too large, because you do not give any information about the engine (and please don't bother).
The shallow first stage and steeper second stage of the reflector are a similar situation. A steeper reflector will send a shorter pulse with a higher amplitude and a shorter duration back to the engine. Does the engine need this? I have no idea.
Furthermore, because the second reflector is steeper than the first one, the pulse from the second reflector will move faster than the first pulse. It may even overtake the first pulse and combine with it into a steep single pulse. And if that pulse it too steep, it will turn into a shock wave which is very inefficient in pushing washed-through mixture back into the cylinder.

Frits Overmars a écrit:

Manuel Rainer a écrit:

a question to Frits
can you please tell me how the engine characteristics changes if i make the end cone as in the drawing the red line? the exhaust has first an end cone angle of 15.5 °, and I change: first part 15 ° to 16 °. the diameter of the middle section, tailpip, and the length remains the same.
a change like this is always a volume change. what effect has the volume to the exhaust characteristicsl

I am not fond of questions like these, Manuel, because there is never enough information. For example, a larger pipe volume will lower the Helmholtz frequency and thus lower the resonance rpm values for maximum torque and maximum power. But if that pipe volume becomes too large, the maximum torque value will drop too. And I have no way of knowing whether that volume will be too large, because you do not give any information about the engine (and please don't bother).
The shallow first stage and steeper second stage of the reflector are a similar situation. A steeper reflector will send a shorter pulse with a higher amplitude and a shorter duration back to the engine. Does the engine need this? I have no idea.
Furthermore, because the second reflector is steeper than the first one, the pulse from the second reflector will move faster than the first pulse. It may even overtake the first pulse and combine with it into a steep single pulse. And if that pulse it too steep, it will turn into a shock wave which is very inefficient in pushing washed-through mixture back into the cylinder.

I mean an engine like the RSA. in the drawing of the exhaust the end cone is first 13.83 ° and then 15.48 °. what is the difference in charakteristics when I have the same exhaust only with a one-piece end cone with the same diameters and length.

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the question is only that i understand what is the effect of this modification in an exhaust

thanks

Manuel Rainer a écrit:

I mean an engine like the RSA. in the drawing of the exhaust the end cone is first 13.83 ° and then 15.48 °. what is the difference in charakteristics when I have the same exhaust only with a one-piece end cone with the same diameters and length.
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the question is only that i understand what is the effect of this modification in an exhaust

A single stage reflector with the same initial and final diameters and the same total length would reduce the pipe volume somewhat. It would also mean that the initial part of the reflector would become steeper than it is in the original Aprilia pipe. That could either raise max. torque rpm and max. power rpm, or it could cause the onset of a shockwave. The increased rpm values would be too high for the cylinder's angle.areas. And a shockwave would be even worse.
Probably the effect of the multiple reflectors can also be achieved with a single reflector, but it would require a lot of testing. After all, that was the way Jan Thiel developed the RSA to what it is now: by years of hard work..

Good evening, I shall like knowing what value of speed of sound you use for your calculation? 340.29? Because gives his me a strangely short exhaust

Thank you in advance

And sorry for my bad english

Polo les gazs a écrit:

Good evening, I shall like knowing what value of speed of sound you use for your calculation? 340.29? Because gives his me a strangely short exhaust

I do not work with the speed of sound. In the first place, that speed varies with engine rpm, engine power, mixture strength, ignition timing and compression ratio. Then it varies per crank degree during each engine revolution. Then it varies along every position in the exhaust pipe. And finally the positive pulses in an exhaust pipe are supersonic (= faster than the speed of sound) and the negative pulses are subsonic.

The speed of sound in my simple exhaust concept is merely an approximation, meant to help beginning tuners on their way, as I have already mentioned a couple of times. You will have to experiment a bit, to find out which value gives the best result for a particular engine.

Thank you for your answer and your explanations

[/quote]A single stage reflector with the same initial and final diameters and the same total length would reduce the pipe volume somewhat. It would also mean that the initial part of the reflector would become steeper than it is in the original Aprilia pipe. That could either raise max. torque rpm and max. power rpm, or it could cause the onset of a shockwave. The increased rpm values would be too high for the cylinder's angle.areas. And a shockwave would be even worse.
Probably the effect of the multiple reflectors can also be achieved with a single reflector, but it would require a lot of testing. After all, that was the way Jan Thiel developed the RSA to what it is now: by years of hard work..[/quote]

hello

So a volume increase by a 2 stage reflector represents an increase of the Helmholtz resonance. and that means that the resonance rpm decreases in deeper rpm`s. the same you can reach with a longer exhaust.

At the same volume increase have the both versions exactly the same effect on the rpm?

Is it true that a smaller angle (at the end cone and diffuser) causes a broader but shallower engine band and a steeper angle more power but only a short useful rpm band?

Thanks Frits for answering my questions

Manuel

I wonder if we aren't hammering Frits a little too much with questions.
After all, if he comes in here it is for his pleasure to talk about things that excite him, which is possibly not the basics of gas dynamics and chamber design
(although I just learnt more in the last few pages than I did in a year of cutting and welding)

Merry Christmas Jan and Frits - thank you for the gracious sharing of your knowledge on this forum.

I hope you take some joy from the many appreciative readers you have on this forum

GrahamB a écrit:

I wonder if we aren't hammering Frits a little too much with questions.
After all, if he comes in here it is for his pleasure to talk about things that excite him, which is possibly not the basics of gas dynamics and chamber design (although I just learnt more in the last few pages than I did in a year of cutting and welding)

Thanks Graham, for covering my back and for your compliment.

GtG001 a écrit:

Merry Christmas Jan and Frits - thank you for the gracious sharing of your knowledge on this forum.
I hope you take some joy from the many appreciative readers you have on this forum

Merry Christmas to you all. And yes, Jan and I do take great joy in your appreciation.

Do Jan and Frits willl be "closed" for Christmas vacation?



Thanks so much for all.