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

carlovitch1 a écrit:

I have another cylinder that may fit, and it has 188° exhaust timing and no significant increase in transfer angle.area. Thus the blowdown angle.area is aimed to a significant higher rpm than the transfer (somewhat 12000 rpm versus 9700). The point is that I cannot increase the transfer area or very little otherwise I will have the piston ring locating pin going inside of it. Do you think it is usable?

Probably not in combination with this piston. I suppose that the pin is located somewhere between an A-port and the B-port, and that is not optimal anyway. If you're skilful enough, you might relocate the pin to a position diametrically opposite the exhaust. Or you could get a better piston. [Vous devez être inscrit et connecté pour voir ce lien] markets 38 mm bore pistons with the pin in the optimal place.

flatart a écrit:

Frits Overmars a écrit:

The shape of the transfer ducts has no influence whatsoever on the static compression measurement of a two-stroke engine because the in-cylinder pressure is 1 bar absolute, until the exhaust port closes and the compression begins.

Yes I know, I didnt mean cylinder static pressure but pressure during combustion. This depends on static compression and on the quantity of fresh charge available for combustion. The latter depends on ducts efficiency to fill the cylinder.

The ducts' efficiency does play a role in filling the cylinder, but there is one other, much more important player: the pipe. Even a works Aprilia will not make power without its exhaust pipe. Jan Thiel once tested an engine without the pipe.
In spite of re-optimizing the ignition and the carburation for these circumstances, the pipe-less engine produced only 1/3 of its pipe-power.

ciao Frits, one question that I know you don't like too much: on your FOS exhaust model the tail restrictor diameter is related just to the crankshaft engine power, but you said that this formula can be applied only to water cooled engines and NOT to air-cooled ones.
However I think that putting a safe factor on your formula this can be valid also for air-cooled ones. I mean if desired engine power is 30HP I can use factor 2 and use 60HP on your formula to get a safe restrictor diameter.
Have you a suggestion for tail restrictor on air-cooled eng? Or do you suggest to not put a tail restrictor and go for a tailpipe-cone Junction with no restriction?

thanks

flatart a écrit:

ciao Frits, one question that I know you don't like too much: on your FOS exhaust model the tail restrictor diameter is related just to the crankshaft engine power, but you said that this formula can be applied only to water cooled engines and NOT to air-cooled ones.
However I think that putting a safe factor on your formula this can be valid also for air-cooled ones. I mean if desired engine power is 30HP I can use factor 2 and use 60HP on your formula to get a safe restrictor diameter.

I wrote: "The calculation of the tailpipe restrictor diameter is critical: you can only apply it to engines that are thermally sound. Air-cooled engines are not."
So it's not just aircooled engines. Watercooled engines with cast-iron cylinders, with shrunk-in or pressed-in sleeves, with insufficient water on the critical places, with insufficient water flow, with incorrect carburation or ignition timing, engines that are revved too high for their blowdown angle.area, are also thermally unsound.
I could use a safety factor 10, and an engine with incorrect carburation or ignition might still seize. I'd rather keep a safety factor for myself and declare that the restrictor calculation in my exhaust concept should not be used in such cases.

Citation :

Or do you suggest to not put a tail restrictor and go for a tailpipe-cone Junction with no restriction?

The wole idea of the restrictor is to have a clearly defined flow restriction, so you can use a tailpipe of much bigger diameter downstream. If you use a tailpipe without a restrictor, its diameter must be small in order to achieve good pipe action. But a tailpipe can be welded on askew, it can be kinked or dented, and it can (and will) get clogged with carbon over time. So I'd never advise a tailpipe without a restrictor.

Good evening Frits, thanks again for your answer to my question. I will try to relocate the ring pin somewhere else, so it will not hamper transfer area increase.
While you and flatart were talking about pipe tail restrictor, I'm afraid I'm did not fully understood whether I can use one or not in the pipe I will be building for my aircooled Minarelli P6. Do you only mean the formula used in your FOS pipe calculation is not applicable to thermally hazardous engines like my old vintage 50cc, or do you mean this concept is not recommended, and should not be used at all on such kind of engine? I know that using a too small restrictor will increase heat to much and may cause a risk of seizing...

Ciao Frits another question about FOS model. I remember that once you suggested to calculate X and Lmax using different RPM if we want to have a wider power band (at cost of less max Power).
In this case I would shorten/lengthen central cylindrical part ("8%" one). Am I right? Thanks

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carlovitch1 a écrit:

While you and flatart were talking about pipe tail restrictor, I'm afraid I'm did not fully understood whether I can use one or not in the pipe I will be building for my aircooled Minarelli P6. Do you only mean the formula used in your FOS pipe calculation is not applicable to thermally hazardous engines like my old vintage 50cc, or do you mean this concept is not recommended, and should not be used at all on such kind of engine? I know that using a too small restrictor will increase heat to much and may cause a risk of seizing...    

As I wrote above, I'd never advise a tailpipe without a restrictor, Carlo. The restrictor calculation in my exhaust concept is applicable if you want optimum power, but not every engine can take the heat, literally speaking. So you will have to experiment at your own risk.

flatart a écrit:

Ciao Frits another question about FOS model. I remember that once you suggested to calculate X and Lmax using different RPM if we want to have a wider power band (at cost of less max Power).
In this case I would shorten/lengthen central cylindrical part ("8%" one). Am I right?

Flavio, the length percentages in the concept should not be altered. If you change the length of only one pipe element,
their relationship is disturbed. If you wish to spread the powerband, choose an rpm-value for Lmax and calculate all lengths, then choose a different rpm-value for X and calculate all diameters.

Crystal clear.
Is it better to use X or Lmax for lower RPM value?
Have you experienced with max difference between X-RPM and Lmax-RPM?

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OK, thanks Frits, this clarifies my understanding. I will try with less restriction.

flatart a écrit:

Crystal clear. Is it better to use X or Lmax for lower RPM value? Have you experienced with max difference between X-RPM and Lmax-RPM?

I haven't performed any such experiments; I concentrated on developing an exhaust concept that would deliver both decent power and a decent power band. If you need an even wider power band, you could start with a lower Lmax-rpm.

Frits Overmars a écrit:

I haven't performed any such experiments; I concentrated on developing an exhaust concept that would deliver both decent power and a decent power band. If you need an even wider power band, you could start with a lower Lmax-rpm.

Ciao Frits, now I have just one missing variable in the "FOS equation": speed of sound. I know that there is no perfect value and this should be found experimenting with different pipes, but I've never built pipes so far so I need just a starting point value so that the first pipe won't be too bad.
For RSA I calculated a speed of sound of 620 m/s, considering 202° of exhaust and 803mm of length.
I guess that for my poor air-cooled 175cc engine the temperature will be quite lower and so the speed of sound.
500m/s can be a good starting value in your opinion?

flatart a écrit:

Ciao Frits, now I have just one missing variable in the "FOS equation": speed of sound. I know that there is no perfect value and this should be found experimenting with different pipes, but I've never built pipes so far so I need just a starting point value so that the first pipe won't be too bad.
For RSA I calculated a speed of sound of 620 m/s, considering 202° of exhaust and 803mm of length.
I guess that for my poor air-cooled 175cc engine the temperature will be quite lower and so the speed of sound.
500m/s can be a good starting value in your opinion?

Yes.

for the first time Frits explained me the missing piece in pipe design puzzle: all models available on the net consider only exhaust timing, desired RPM for max power, cubic capacity and speed of sound. that's all.
So in theory I can take any 125cc clylinder, raise to 202/132° exhaust and ports timing and I can use RSA exhaust and get an engine with max power at 13000 RPM. WOW! Great!
The problem is that this is just half of the story: the missing piece is specific port and blowdown AngleArea. If we compare these values to RSA ones we know when our engine begins to rev in "apnoea" so we have a reasonable RPM value for pipe design. But the story is not finished yet, because even if you have same RSA angle area, but ports flows are not oriented in the right way, the engine will never rev at 13000RPM.
this is really important when we experiment with pipes, to find the "perfect" one.

example: let's suppose we have an engine that, given ports and blowdown angle area, should deliver its max torque at 6000 RPM. Let's suppose that dyno-test says that this engine delivers its max torque at 7000 RPM. Now you could think that the pipe is "1000 RPM shorter". Wrong! Because this is just half of the story, we can't consider only pipe geometry. In fact the pipe is much shorter, probably it's tuned for 8000 RPM. 7000 RPM is just in the middle beteween the cylinder angleArea-capacity (6000 RPM) and pipe length frequency (8000 RPM).

now the umpteenth question for Frits: when designing a new pipe do you use theoretical RPM given by specific angle area to calculate Lmax and X values? does it make sense to use a higher RPM value?

I mean that maybe running the engine a little "out of breath" will give a max torque loss, but the engine will rev higher and so the global payoff for power (RPM x torque) might be positive.

flatart a écrit:

now the umpteenth question for Frits: when designing a new pipe do you use theoretical RPM given by specific angle area to calculate Lmax and X values? does it make sense to use a higher RPM value?
I mean that maybe running the engine a little "out of breath" will give a max torque loss, but the engine will rev higher and so the global payoff for power (RPM x torque) might be positive.

When designing a pipe I use the rpm permitted by the angle.areas. The engine should develop maximum torque at this rpm.
It will be revved higher anyway, until the power drops off below the value that can be had at the same riding speed in the next gear.
This power drop-off can be quite steep when the engine runs out of blowdown time.area, because then the exhaust gases will start using the transfer ports as exhaust ports, and the remaining effective transfer time.area will also drop off steeply.
You can fine-tune this approach by varying the speed of sound in the exhaust concept, and by varying the ignition timing on the test bench.

So Frits, if I can make my 50cc cylinder STA allowing max torque at 9,000 revs (BD&TR), and my machine is meant for top speed/drag racing. That STA should be achieved with timings around 132/196. How would you proceed from here? I guess that engine might produce about 9hp at 9500, but I have absolutely no idea, what kind of torque curve I could get with FOS-pipe. If my gear up revs would be around 9500rpm, I would need to have descent torque up from 7000, because 3rd and 4th gear have such long ratios.

Do you see any point to make pipe producing max torque at 12-13,000rpm? Then I would need useful torque range from 9,800-13,200.

And one completely different question. There once was some talk about Langtuning 17HP 50cc racing cylinder, which has piston controlled intake and only 2 transfer ports. Do you happen to have any pictures of it (or some other data), as I can't seem to find anything about that cylinder?

op76 a écrit:

So Frits, if I can make my 50cc cylinder STA allowing max torque at 9,000 revs (BD&TR), and my machine is meant for top speed/drag racing. That STA should be achieved with timings around 132/196.

A  50 cc cylinder with those timings ought to reach its maximum BMEP at about 14.000 rpm, unless you have extremely narrow port windows. If you really want to have maximum BMEP at 9500 rpm, it would be better to make the ports lower and wider. But why would you? 9500 rpm is incredibly low for a 50 cc engine.

Citation :

I have absolutely no idea, what kind of torque curve I could get with FOS-pipe.

Neither have I. The torque curve shape does not exclusively depend on the pipe, but also on many other factors.
For example, I have no idea how your inlet system, your transfer ducts and ports, your combustion chamber and your ignition curve look like.

Citation :

Do you see any point to make pipe producing max torque at 12-13,000rpm?

Not in combination with time.areas that are optimized for 9500 rpm; then the torque curve will completely collapse before it even reaches 13000 rpm.

Citation :

There once was some talk about Langtuning 17HP 50cc racing cylinder, which has piston controlled intake and only 2 transfer ports.
Do you happen to have any pictures of it (or some other data), as I can't seem to find anything about that cylinder?

I have hundreds of pictures and data of most Langtuning cylinders, but I am not at liberty to post them here.
You can ask your questions direct via [Vous devez être inscrit et connecté pour voir ce lien]

Frits Overmars a écrit:

A  50 cc cylinder with those timings ought to reach its maximum BMEP at about 14.000 rpm, unless you have extremely narrow port windows. If you really want to have maximum BMEP at 9500 rpm, it would be better to make the ports lower and wider. But why would you? 9500 rpm is incredibly low for a 50 cc engine.

Because there are quite low amount of material on those cylinders for porting, and with the short 37.8mm stroke, you cannot get STA reaching those 14000rpms...probably impossible for even Jan Thiel  

And how is it with just about every street bike ever made? Even MX-bikes. I don't think many of them reach those STA-levels of RSA? There are still quite a few of them, which has redline around 13000. Have I missed something (probably)?

Frits Overmars a écrit:

Neither have I. The torque curve shape does not exclusively depend on the pipe, but also on many other factors.
For example, I have no idea how your inlet system, your transfer ducts and ports, your combustion chamber and your ignition curve look like.

It's piston port and 20mm carb. I'm going to start with nearly maximum allowed width and 160° timing. Combustion chamber is going to be "similar" to RSA, and because of air cooling, probably 1:11 compression at max. Ignition is harder, as there are not very precise documentation. It's around 17° to 6000rpm, 25° @ 8500 and 20° @ 11000. Above that it seems that no one knows   You can adjust stator position about +/- 5° if needed.

So, I know this is hardly optimal, but this is what I need to deal with.

EDIT. And I need to add, that fastest cylinder of this type had 6.5 rear wheel horsepower, and made it top speed of 93km/h @10,500rpm.

Ciao Frits, there is something that you explained in this forum that I took for granted but honestly I didnt understand. When you use specific angle area to estimate RPM of max torque, you always consider transfer-Ports-AA and Blowdown-AA and you ignore exhaust-AA. This is something that I dont really understand.
Let's have an extreme example: think of a cylinder where the exhaust-AA is equal to double blowdown-AA, so the bottom edge of exhaust port is at the same height of top edge of transfer port. In your opinion this cylinder should have same rev capacity of a "normal" cylinder, so the same scavenging capcity regarding hot combustion gas.
Even if I have never tested this, it's really hard for me to believe.
Especially if we talk about cyilinders you dont like (but that I use), with no exhaust boost ports: after blowdown phase I guess there are still hot gas to be scavenged thanks to fresh charge coming from the crankcase that pushes the hot gas towards the exhaust.
Can you please explain us this concept?
grazie ciao

flatart a écrit:

think of a cylinder where... the bottom edge of exhaust port is at the same height of top edge of transfer port. In your opinion this cylinder should have same rev capacity of a "normal" cylinder, so the same scavenging capacity regarding hot combustion gas.

Such a cylinder could develop maximum torque at the same rpm as a 'normal' cylinder, to use your words. And maybe even better because there may be less mixture loss through short-circuiting. But at higher revs things will be very different. Remember what I wrote only two days ago?

Frits Overmars a écrit:

I use the rpm permitted by the angle.areas. The engine should develop maximum torque at this rpm. It will be revved higher anyway, until the power drops off... This power drop-off can be quite steep when the engine runs out of blowdown time.area, because then the exhaust gases will start using the transfer ports as exhaust ports, and the remaining effective transfer time.area will also drop off steeply.

Exhaust gas in a cylinder that runs out of blowdown time.area, will use the transfer ports as exhaust ports, but of course it will also use the exhaust port area below the top edge of the transfer ports. But if there is no exhaust port area at all below the top edge of the transfer ports, even more exhaust gas will enter the transfer ducts, so the power drop-off will be very steep and the overrev capacity will be mediocre.

flatart a écrit:

after blowdown phase I guess there are still hot gas to be scavenged thanks to fresh charge coming from the crankcase that pushes the hot gas towards the exhaust.

Of course it's a matter of pressure differences, but for a better mental picture I prefer to say that those hot gases are sucked out of the cylinder by the pipe, and the blowdown area is usually big enough for this action.
We must also take into account that the hot exhaust gases that are still in the cylinder, will shrink considerably as soon as they come into contact with the cool fresh mixture.       PS: I like your questions

I have questions about the right way to proceed in building a pipe especially how to interpret dyno-test results to correct and build next pipe.
Let's suppose I have to design a pipe for my engine. Looking at my Transfer AngleArea and Blowdown AngleArea I should go for 6000RPM for max torque.
So these are input parameters of your model, obviously speed of sound is the great question mark and I start with 500 m/s:
speed sound: 500
exhaust timing: 178
cil capacity: 182
RPM max power: 6.000
Power crank (x 2 for cooled engine): 50

And this is the result of FOS mnodel:
Lmax 1,305
X 33,7
D1: 35
D2: 46
D3: 118
D4: 118
D5: 17
tail diameter: 27

Now suppose that I build exaclty this pipe but dyno test says that max torque is at 6500RPM.

6500 = (6000+7000)/2

where 6000 are RPM coming from angleArea capacity and 7000 are RPM coming from the pipe.
So now I know that my pipe is too short because the real speed of sound is higher of what I supposed in my design.

Now I have a lot of doubts regarding how to proceed for my next pipe design.

The simplest thing to do is to find the speed of sound that gives me Lmax=1,305m with RPM=7000: it's 586 m/s.

But this is not correct: this is correct only if we consider pipe length, but it's not correct for pipe diameters. Pipe diameters in FOS model dont depend on speed of sound (and this is a great news because it means they are correct at first try...)

So this means that I designed a pipe with correct diameters for 6000 RPM, but that results in a 7000 RPM resonance so for sure it's too short because speed of sound is faster but I dont know how much faster.

I could say (very strong assumption and maybe stupid assumption) 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
This means that speed of sound would be even faster 670m/s (!!!).

in one word: help

flatart a écrit:

with 500 m/s:
speed sound: 500
exhaust timing: 178
cil capacity: 182
RPM max power: 6.000
Power crank (x 2 for cooled engine): 50
...
But this is not correct: this is correct only if we consider pipe length, but it's not correct for pipe diameters. Pipe diameters in FOS model dont depend on speed of sound (and this is a great news because it means they are correct at first try...)

no, your diameters are wrong from the first try, as expecting 50BHP @ 6000 with only 178° exh timing from 182cc  ain't very realistic.

And again : it is only a concept.maybe bigger or smaller diameters are better. but that's up to you to find out.

and off course  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.

JanBros a écrit:

no, your diameters are wrong from the first try, as expecting 50BHP @ 6000 with only 178° exh timing from 182cc  ain't very realistic.

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

JanBros a écrit:

and off course  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.

flatart a écrit:

Let's suppose I have to design a pipe for my engine. Looking at my Transfer AngleArea and Blowdown AngleArea I should go for 6000RPM for max torque. So these are input parameters of your model, obviously speed of sound is the great question mark and I start with 500 m/s. Now suppose that I build exaclty this pipe but dyno test says that max torque is at 6500RPM.
6500 = (6000+7000)/2
where 6000 are RPM coming from angleArea capacity and 7000 are RPM coming from the pipe.
So now I know that my pipe is too short because the real speed of sound is higher of what I supposed in my design.
The simplest thing to do is to find the speed of sound that gives me Lmax=1,305m with RPM=7000: it's 586 m/s.
This is correct if we consider pipe length, but it's not correct for pipe diameters. Pipe diameters in FOS model dont depend on speed of sound. So this means that I designed a pipe with correct diameters for 6000 RPM, but that results in a 7000 RPM resonance so for sure it's too short because speed of sound is faster but I dont know how much faster.
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
This means that speed of sound would be even faster 670m/s (!!!).
in one word: help

Flavio, the problem is in your average-calculations:
6500=(6000+7000)/2
and
7000=(6000+8000)/2
The calculations themselves are of course correct, but they should not be applied here. But I am to blame that you did use them here. Remember this quote from 9 days ago?

Frits Overmars a écrit:

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.

I called it a rough indication because it shows in which direction the pipe effect works, but it does not represent the exact strength of this effect. I used a simple average-calculation as an example but in reality the pipe effect is not quite as strong as the above indication might suggest; the pipe "preference" in the quoted example would lie closer to 7000 rpm than to 8200 rpm.
One other thing, that I already pointed out in the text accompanying the exhaust concept:

Citation :

Many important factors, like compression ratio, ignition timing, type of fuel, carburetter diameter, crankcase volume and angle.areas, are not taken into account. Instead of all those factors that I left out, I included one variable, the speed of sound.

So if your dyno test suggests that you should use a different value for the speed of sound, it could just as well mean that you need a richer mixture or more ignition advance.
Two-strokes: they look so simple .

It's clear but I never built an exhaust, I'm very basic in welding, it's Just an hobby and if I have ti build 10pipes before having the good One it would take probably One year, thats why I posted that stupid exercize.
However you confirmed that It will take even more than One year so

Janbros gave another perspective regarding FOS model.
I would like to know if it's correct. Given Lmax, length of all' exhaust components is fixed. Then playing with X (so all diameters D of your model) you can have higher max Torque peak and narrow band (X bigger) or lower max Torque peak and wide band (X smaller).
Specific X formula you provided with FOS is the better compromise that your long experience suggests between these conflicting needs: Power and wide band. Am I wrong?

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Flatart : in my spreadsheet, there is a second page with some explanations. In there I wrote (well, not me but probably Frits or Wobbly as I keep all interesting/important posts from topics in documents) :

A bigger D max increases the volume of the pipe and shifts the optimal cylinder filling to lower rpm's. But the available energy is limited : a bigger D max uses more of the available energy for suction, meaning their is less energy left for pushing back the fresh charge that's already in the header back in the cylinder. A solution could be to use a steeper baffle cone (shorter length) to give back some amplitude .

@ Frits, 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 normaly 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?