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

flatart a écrit:

JanBros a écrit:

...and of course the speed of sound only matters for the length of the pipe, as it only influences the timing of the wave, not the strength of the wave.

So you are saying that given a certain pipe length, if you change the diameters of the pipe you are just changing the strength of pressure waves: bigger diameters means stronger waves, so higher max torque value but narrower curve; smaller diameters means weaker waves, so lower max torque value but wider curve. But the RPM of max torque will be more or less Always the same.

Janbros did not say that; you did. But it's not as simple as that. Getting all pipe diameters right is much more complex than getting the lengths right.
For example, if you enlarge the maximum diameter of a pipe, the suction caused by the diffuser will increase. But this means that there will be less energy left in the exhaust gas, so the stuffing pulse caused by the reflector will be weaker despite the fact that the maximum diameter of the reflector has increased too.
Changing the maximum diameters of the header has an even stronger influence. The header forms a connection between the cylinder volume and the pipe volume. Together they form a Helmholtz resonator, and if you alter the header diameters, this will strongly alter the resonator's frequency. A narrower header lowers this frequency; a wider header raises it.

The Helmholtz frequency and the wave frequency together determine the pipe frequency, so the rpm of maximum torque will not remain the same if you change the diameters. You can build a short, slender pipe and a long, fat pipe that have the same frequency. But you need to find the happy medium that gives the strongest pressure fluctuations. That was the objective when I developed my FOS exhaust concept.

flatart a écrit:

I never built an exhaust, I'm very basic in welding, it's just a hobby and if I have to build 10 pipes before having the good one it would take probably one year, that's why I posted that stupid exercise. However you confirmed that it will take even more than One year.

This exercise was not stupid at all. If nothing else, it convinced me that you have a good, analytical approach and I think it improved your understanding of what goes on in a two-stroke engine.
Building an exhaust is one of the simplest things you can do when trying to improve a two-stroke. It is much simpler than building a cylinder or a set of crankcases, and you will learn from every pipe you make, no matter how badly it performs.
I made my first expansion pipe when I was 14 years old. It sounded great, and a boy across the street wanted to buy it for his moped. He only got his way after the pipe fell off my bike, which suddenly had more top speed .

JanBros a écrit:

something Flatart said made me thinking :

flatart a écrit:

expected power is 25 HP, I wrote 50HP because of air cooled engine and however this parameter is used only for tail restrictor (D5) that it's the last thing I care about the pipe cause you can modify it in 5 minutes

When he does this, he keeps the same length but obviously changes the angle of the rear cone, which deviates the pipe from it's concept. So I suppose when dealing with "crap aircooled engines " it is better to calculate normally and than make the final part of the baffle a changeable peace to  determine to correct stinger-size and so keeping the baffle angle but changing the total length minimal?

Yes, that is better. Not only for 'crap aircooled engines' but for any engine. Exchangeable restrictors like the red part in the drawing below offer a nice opportunity for experimenting.
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flatart a écrit:

expected power is 25 HP, I wrote 50HP because of air cooled engine and however this parameter is used only for tail restrictor (D5) that it's the last thing I care about the pipe cause you can modify it in 5 minutes

That tail restrictor is supposed to be venturi, with radius and correct exit angle, so I doubt you will modify it in 5 minutes

It doesn't need to be "2 times larger", you should use factor of 3.1 in Frits's formula for water cooled and 3.3 for air cooled. If you are still concerned that it is too small, you can always use Blairs suggestions: 0.6 times exhaust port area for BMEP 11 engines, 0.65 for BMEP 9 and 0.7 for BMEP 8.

op76 a écrit:

flatart a écrit:

expected power is 25 HP, I wrote 50HP because of air cooled engine and however this parameter is used only for tail restrictor (D5) that it's the last thing I care about the pipe cause you can modify it in 5 minutes

That tail restrictor is supposed to be venturi, with radius and correct exit angle, so I doubt you will modify it in 5 minutes [smiley][Vous devez être inscrit et connecté pour voir ce lien]

It doesn't need to be "2 times larger", you should use factor of 3.1 in Frits's formula for water cooled and 3.3 for air cooled. If you are still concerned that it is too small, you can always use Blairs suggestions: 0.6 times exhaust port area for BMEP 11 engines, 0.65 for BMEP 9 and 0.7 for BMEP 8.

I didnt say 2 times larger but I used double power, this enlarged tail diameter from 16mm to 21mm (more or less, I dont remember exactly).
In my example above I made an errore, I wrote 17mm (maybe I used 30hp)

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flatart a écrit:

I didnt say 2 times larger but I used double power, this enlarged tail diameter from 16mm to 21mm (more or less, I dont remember exactly).
In my example above I made an errore, I wrote 17mm (maybe I used 30hp)

but as said above : that way you change the angle of the baffle.

JanBros a écrit:

flatart a écrit:

I didnt say 2 times larger but I used double power, this enlarged tail diameter from 16mm to 21mm (more or less, I dont remember exactly).
In my example above I made an errore, I wrote 17mm (maybe I used 30hp)

but as said above : that way you change the angle of the baffle.

Ok but if I cant use tail restrictor of water cooled engine, something must bè changed: or Power parameter and so baffle angle changes, or I keep the angle and I shorten the pipe

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Don't worry about angles. My exhaust concept does not stipulate any angles at all, so there is nothing to 'keep'.

flatart a écrit:

I designed a pipe with correct diameters for 6000 RPM, but that results in a 7000 RPM pipe resonance. I could say that 7000 RPM resonance of the pipe is the result of:
- 6000 RPM resonance coming from pipe diameters
- 8000 RPM resonance coming from pipe length

As I wrote yesterday, the Helmholtz frequency of the pipe is determined by the diameters. But not only by the diameters; the lengths play a role too. For example, if the header has the diameters that you calculated, but it is shorter than it should be, then the Helmholtz frequency will be higher than the 6000 rpm that you used for the diameter calculation.
You can only say that there is a 6000 rpm resonance coming from the diameters, if the lengths are also suited for 6000 rpm.

The First part of FOS exhaust is in fact the cylinder exhaust duct. So D1 is the diameter of exhaust duct and I can't enlarge It (I should do too complicate and critical modifications).
Now FOS model says it should be 37mm, but exhaust duct diameter is only 33.
How does this restriction impact?
Can I ignore this restriction in pipe design?

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flatart a écrit:

The First part of FOS exhaust is in fact the cylinder exhaust duct. So D1 is the diameter of exhaust duct...  Now FOS model says it should be 37mm, but exhaust duct diameter is only 33. Can I ignore this restriction in pipe design?

No, you cannot ignore this. But how did you find this 37 mm diameter?
Using the values you posted:
bore=62 mm ; stroke=57mm
exhaust timing= 193°
max.power rpm = 6350
speed of sound = 550 m/s
I find:
X = 31,5 mm ; Lmax = 1471,1 mm
For diameter D1 to become 37 mm, the length of the exhaust duct in the cylinder, from piston to D1, should be 242,6 mm.
I find that hard to believe...

My data Will be:
Timing 177°
Cubic capacity 182 (62x60)
RPM 6000
Sound speed 500
Lcyl 70

I get 36 (35.8) I made a mistake. However my duct is 33, not so big

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flatart a écrit:

My data Will be:
Timing 177°
Cubic capacity 182 (62x60)
RPM 6000
Sound speed 500
Lcyl 70
I get 36 (35.8) I made a mistake. However my duct is 33, not so big

Yes, I get Lmax=1298 ; X=33,85 ; D1=35,8. The big values for X and D1 are mainly caused by the modest exhaust timing.
But do you think it is realistic to aim for max.power at 6000 rpm with only 177° exhaust timing and the accompanying small blowdown angle.area? And even if you get the blowdown angle.area and all pipe dimensions right, the weak exhaust pulse will impede decent power.

3 main doubts regarding your answer:
1) with 193° Im afraid of loosing too much torque at low RPM and I have only 4 gears
2) raising the exhaust from 177 to 193 is always possibile, not the contrary
3) do you know how difficult would bè to mount an exhaust 1,5m long+ silencer on a vespa?! Need to keep It shorter

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Just to give you an idea. This is 1m long more or less and it's done by a professional

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Frits would you educate me somewhat please.
Looking at the RSW 125 exhaust pipe, the bend of around 25* is made very close to the cylinder and abrupt as opposed to a longer gentle bend.
Is there a reason / advantage / disadvantage with having  a short more abrupt Bend with 2 stroke exhausts ?
Following picture is of two aprilia pipes, the centre one has 02 / 98 stamped into the mounting brace. 
The tacked together pipe is with a larger bend radius

The tacked-together pipe may have Aprilia dimensions but I don't think it's a genuine Aprilia pipe.
The Aprilia framebuilders are to blame for the bend close to the cylinder; the picture below shows what I mean.
A longer, gentle bend would require more degrees of bending than the actual pipe, or it would collide with the crossbeam that carries the upper shock attachment. This stronger bending would result in an upward-pointing pipe, which would also require a second bend, counteracting the first one, in order to stay below saddle height.
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Yes the tacked one is not aprilia but my own. The bend angle is 4* further on a 250mm CLR, it fits perfectly in my 91 125cc bike without a 2nd bend back to clear the seat. 
However I’m still interested in your specific explanation upon long radius bends Vs short as the aprilia has with regards to what effects both have in your experiences, pressure pulse differences, flow & possible attention to the stinger Venturi diameter, measures that were taken to correct 
Just a general thought provoking & broader understanding for all of the would be if we could be audience that follows these GREAT educational writings within these thus far 5 part tutorials

You sure know how to sweet-talk, RAW  .
It will come as no surprise that I'd prefer a completely straight exhaust pipe. And if there has to be a bend, it should have the biggest possible radius. But in reality the differences are often unclear; in some cases a curved pipe can even perform better than a straight one; just compare the RSW pipe with the RSA pipe.

The pipe as fitted to the bike usually gave slightly more power than the straight pipes used for testing.
Bends in pipes must always be tested very carefully, as they can destroy power quite a lot..
Of course RSW pipes were tested on the RSA, and vice versa.....
The RSA pipe proved to be a bit worse, and caused some more detonation.
I think due to the necessary longer 26mm tailpipe...
Without this tailpipe there was no detonation at all!
The pipe under the engine heated the crankcase.
So a carbon shield was made to prevent this.
A rear exhaust would gave been better of course.
But that was quite impossible as the carburetor was already there....

Query. 
The RSA pipe design details 0.7mm thick material. 
The picture detailing the particular titanium material to use is 0.6mm thick. 
Was the 0.6mm tested as an alternative 
What sort of differing results were obtained between the two differing material thicknesses if it was trialed 
What life span were the Ti pipes used for & how long until fatigue resulted from normal non crash usage conditions typically

That is a lovely looking exhaust, the curvature to each segment I have not worked out how to do. Maintaining a true round pipe throughout is awkward enough [Vous devez être inscrit et connecté pour voir cette image]

RAW a écrit:

That is a lovely looking exhaust, the curvature to each segment I have not worked out how to do.

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

RAW a écrit:

Maintaining a true round pipe throughout is awkward enough

Awkward yes, but not necessay, and not even optimal. The centrifual force acting upon the mass, flowing through a bend, causes a pressure difference between the inner and outer walls, and this difference causes a secondary flow, square to the main flow. The resulting turbulence causes a pressure loss, hence a decrease of the flow that would be possible through a straight pipe with the same cross flow area.
How can we reduce this secoundary flow? By reducing the (outer bend radius/inner bend radius) ratio. In other words: by bringing the inner bend wall closer to the outer bend wall. In still other words: by flattening the bend's height and increasing its width.
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