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

Eric_91 a écrit:

With Narrower exhaust duct im expect less gas expansion so higher exhaust temp so more Max Power but less bottom end, correct?

A narrower exhaust duct will provoke less turbulence, hence more remaining energy in the exhaust gas; in this respect you may be right. But I also expect a different, stronger effect: the narrower duct will lower the Helmholtz frequency of the exhaust system, so more bottom end. Try and let us know, Eric.

Good afternoon everyone, I have another question for Frits or Jan.
I've seen in a magazine a strange mod applied to the dome of a head, see these grooves:
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The tuner claims it improves atomization of the fuel, then most likely its combustion, and prevents from detonation.
What is your opinion on such thing?

Ce serait pas une culasse lagarigue... inventeur aussi du piston gonflable?

carlovitch1 a écrit:

I've seen in a magazine a strange mod applied to the dome of a head, see these grooves:
The tuner claims it improves atomization of the fuel, then most likely its combustion, and prevents from detonation. What is your opinion on such thing?

It is 100% Bullshit, just like internal golf ball-dimpling and turbo-crankshafts.
I have seen a power comparison of such a cylinder head, before and after the modification.
The after-version was fractionally better but the tuner 'forgot' to mention that he had also raised the compression ratio.
A cylinder head with the raised compression ratio and without the grooves would have been a lot cheaper and perform a lot better, but the tuner was not going to say that to his customers...

Thanks for your reply, Frits !

carlovitch1 a écrit:

Good afternoon everyone, I have another question for Frits or Jan.
I've seen in a magazine a strange mod applied to the dome of a head, see these grooves:
[Vous devez être inscrit et connecté pour voir cette image]

The tuner claims it improves atomization of the fuel, then most likely its combustion, and prevents from detonation.
What is your opinion on such thing?

Big jump in the impression ratio along with a corresponding decrease in you bank balance

One question about exhaust for Frits.

What inside diameter had the tailpipes of the RSW 125/250?

With the FOS formula for the tailpipe i calculate an inside diameter of approx 29-29,4mm depending of the assumed rpm of max. power. That sounds quite big.
On the drawing of the "tubo 102" the restrictor is labeled 23,3mm. Jan Thiel wrote here in the forum that the RSA had a tailpipe inside diameter of 26mm.
There´s a big difference between 26mm and 29,4mm.

As I´m about to construct a pair of exhaust with the "tubo 102"-design I´m really interested in your suggestion about the "right" tailpipe diameter.
The max power rpm of the engine is about 12.800-13.000 rpm and the cylinder capacity is 124,9ccm.
Thank you in advance for an answer.
Best regards,
Bernd

Döllinger a écrit:

With the FOS formula for the tailpipe i calculate an inside diameter of approx 29-29,4mm depending of the assumed rpm of max. power. That sounds quite big. On the drawing of the "tubo 102" the restrictor is labeled 23,3mm. Jan Thiel wrote here in the forum that the RSA had a tailpipe inside diameter of 26mm. There´s a big difference between 26mm and 29,4mm.

The flow difference between a 26 mm tube and a 29,4 mm tube would be huge, but the 23,3 mm restrictor (diameter D5 in the drawing below) at the entrance of this tube should annihilate any flow difference. And in order to make sure that the restrictor,
and not the tube diameter D6 downstream of the restrictor is decisive, I advised a D6 diameter that is markedly bigger than D5.

The difference between perforated silencer tubes with 26 mm or 29,4 mm diameter is not big when it comes to silencing,
but in my experience the bigger diameter, with obviously a bigger wall surface area, gives slightly better silencing.
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Hi Frits,
I am puzzling over what happens in the rear cone, or reflector, of a typical race exhaust pipe. Would I be correct in imagining that the reducing cone forms a pressure gradient with the diameter adjacent the pipe middle diameter at the lowest and the highest pressure at the tailpipe junction. Is the tailpipe entry point also at the highest temperature in the length of the rear cone? My real question concerns the behavior of reflected waves, can waves from the tailpipe area travel faster and have a larger amplitude than those from the larger diameter and can they overtake and collide with the slower waves and, what are the possible consequences of that collision?
A lot of questions I know but any explanation you can offer is gratefully received, I am also sure there must be many readers just as in the dark about these things as I am, and rear cones don`t seem to get the same analysis as other parts of an exhaust pipe. Many thanks in advance.
Cheers, Melvyn.

melvyn trevor a écrit:

Hi Frits,
            I am puzzling over what happens in the rear cone, or reflector, of a typical race exhaust pipe. Would I be correct in imagining that the reducing cone forms a pressure gradient with the diameter adjacent the pipe middle diameter at the lowest and the highest pressure at the tailpipe junction. Is the tailpipe entry point also at the highest temperature in the length of the rear cone? My real question concerns the behavior of reflected waves, can waves from the tailpipe area travel faster and have a larger amplitude than those from the larger diameter and can they overtake and collide with the slower waves and, what are the possible consequences of that collision?
A lot of questions I know but any explanation you can offer is gratefully received, I am also sure there must be many readers just as in the dark about these things as I am, and rear cones don`t seem to get the same analysis as other parts of an exhaust pipe. Many thanks in advance.
Cheers, Melvyn.

Hi Melvyn, the answers to your questions are yes, yes, yes, yes, yes, yes   . But I feel that your 'real' question deserves a more elaborate reaction than just six yesses.
Waves with a large amplitude can overtake waves with lower amplitudes and velocities that were generated earlier in time. If that happens, they join forces and start moving even faster.

A typical wave has more or less a sine shape, starting with a modest amplitude that becomes stronger and stronger as the wave develops towards its maximum amplitude.
The initial part of the wave moves rather slow and it gets overtaken by the subsequent stronger parts. Because of this overtaking, the resultant wave not only becomes stronger but also shorter and steeper. If the wave is underway long enough, it can develop into a very short but vicious shock wave. But shock waves are not good at moving a mass of gas, so they are of no use in an engine.

This is the reason that the shape of the reflector cone in an exhaust pipe is fairly defined. If you make it too steep, the strong reflected wave will develop into a shock wave before it arrives back at the cylinder, and it will do a rather poor job at pushing washed-through fresh mixture back into the cylinder.

Now if a strong wave can overtake a weak wave, what happens when that weak wave is a suction wave?
Suction waves move really slow compared to pressure waves, so there is a real risk that they will be overtaken. And when that happens, they do not join forces. On the contrary, they kill each other.

To give a numerical impression: the speed of sound in ambient air is a little over 300 meters per second.
Strong pressure waves in hot exhaust gas can double that. But suction waves, especially if they have a strong negative amplitude, can be as slow as 200 meters per second.

In a typical two-stroke exhaust pipe we see a header that may be either parallel or very slightly conical, then a diffuser, consisting of one or more cones, then a parallel belly, and finally a reflector, again consisting of one or more cones.
The diffuser sends a suction wave back to the cylinder. A bit later on, the reflector sends a pressure wave back to the cylinder.
If the pressure wave overtakes the suction wave, neither of them will survive. This is where the belly comes in: it creates a battle-free period of time between the two.

Above we mentioned the header. Why doesn't it have a larger angle of aperture, so it can do a better suction job?
Because that would be a waste of exhaust gas energy.
When the exhaust port starts opening, the pressure differential between the combustion gases in the cylinder and the pressure in the exhaust system is supercritical. Those combustion gases will leave the cylinder with a flow velocity of Mach 1 and extra sucking at the low-pressure end of the header cannot raise the flow velocity any further.

Besides, as long as the flow velocity in the header is so high, a suction wave would not be able to move upstream through the header towards the cylinder anyway. It would be like a bird trying to fly into a storm: it does not advance but only wears itself out. So it's better to wait for the pressure in the cylinder and the flow velocity in the header to drop somewhat before we let the diffuser start sucking.

Fantastic, that`s great Frits, a very comprehensive and expansive reply, far more an authoritative answer than can be found in a dozen of the "popular" tuning books. Thank you so much.
Regards, Melvyn

Hi Frits.
Would you please advise me about crank shaft balance rate effect to power? or purpose vibration reduce only?
What 's concept of crank shaft balnce & RSA use balance rate ratio 50%?
What 's balance rate different in term of power or anything between 40%,50%,60%
Thank you for your advice.
Regards. Poj

pojoran a écrit:

Hi Frits.
Would you please advise me about crank shaft balance rate effect to power? or purpose vibration reduce only?
What 's concept of crank shaft balnce & RSA use balance rate ratio 50%?
What 's balance rate different in term of power or anything between 40%,50%,60%.

There is no direct coherence between crankshaft balance rate and engine power, but vibrations may strongly effect the carburation and thus the power, so applying a balance shaft may give a net power profit. Reducing vibrations may also allow for a lighter frame construction.
When using a balance shaft, it is advisable to give both the crankshaft and the balance shaft balance factors of 50%.

Hello Frits,

what is timing value to openning and closed rotary valve to 125 RSA?

Thank

I think you have the answer already in this page (page 19 of the topic): [Vous devez être inscrit et connecté pour voir ce lien]

I have seen this topic, but is not really clear.

Isn't 146°/93° (before/after TDC) not clear enough?

Hi everyone,

i am searching for more pictures of a 1991 Aprilia AF1 250cc racebike. This is the only picture I was able to find of it online.
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It states, that it is a 1991 used in Modena.

Anyone got more pictures of the whole design?

Regards
Tim

Tim Ey a écrit:

i am searching for more pictures of a 1991 Aprilia AF1 250cc racebike. This is the only picture I was able to find of it online.
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It states, that it is a 1991 used in Modena. Anyone got more pictures of the whole design?

Here you go Tim.
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Bedankt Frits,
but that is not the design I mean.

I linked this one
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Regards
Tim

Isn't it the same bike but with different colours? Or you would like to see the complete graphics of the bike?

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Without Frankie seated on it.

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Merci Beaucoup!!!
<3

Want some close up ?

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