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

I can not find right post at this moment, but there was talk about problems of lubrification when throttle is closed and proposed solutions with more or less success (problems of detonation, variable exhaust..).

Did (and does it makes sense) anyone tried to change mixture ratio? Today, with electronics and injectors could be possible to remain with open throttle (fully or partly) but make mixture "less flammable".

We always take account that mixture is ideal (which is best way), but what if we in some moment need less than perfect mixture?

One step further, and to put into calculation - oil. If oil can be separately dosed and injected, fast enough, maybe it is possible? Of course,there is (very high) price of fouled plugs, but I am not talking about that much wrong mixture, just altering it enough to reduce power little bit, with no cost for lubrification..

Just thinking..

Hello Jan and Friends!

I see! Thanks a lot!

Well, regretfully it has to be carry out like this, as you can see on the drawing ’C’.

So, according to your opinion the version C is it better, apart from the fact that on drawing C the auxiliary port’s flow area is bigger than the same area on drawing A, meanwhile their passages have the same size?

It’s a little bit strange for me what you are saying, because if the auxiliary port is totally opened the capacity of the ports has to be the same, because of the same size of their passages, isn’t it?

From this point of view it is hard to define the difference between what we consider port or passage?

It’s also true, that in practice the port is opening gradually, that’s why if we take the drawing C the flow area is going to be bigger on it for a while, exactly untill the point when the port’s flow area reaches the same size as the area of the passage has.

And could you please tell me what’s up with the drawing D? As you can see on this pic the width of the port is not X, but smaller, although because of the direction of the port, its flow area is the same as on the drawing A.

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Jan Thiel a écrit:

Solution A is the worst, and B the best.
But sometimes C is the only possibility, due to the cylinder stud holes.
You can try to make them a little bit higher internally.
So that you maintain the same surface.
But this may be limited due to the transfers.
Sometimes it is really better to have a central bridge.
Surely auxiliary ports gives the best HP!
But only if you can make them as big as necessary!

Hello Everybody!

Just an off topic question: What do you know about the Frits's FOS cylinder? I saw that a test version has just come out. Any information?

B.

Frits will answer himself if he has more informations

Hi

Can somoene post a photo of one RSA or RSW piston upside down?

The reason is, does the botton of the piston squirt match the cylinder squirt at bdc?

The botton of piston squirt is rounded or just knife edge like the pistons I have on the table?

Should't this improve flow?

Jan Thiel a écrit:

Plating and replating was always done by the same factory, called Tecnol.

I guess this one, Jan?

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So MAPE has bought Tecnol.
I did not know this!
MAPE makes crankshafts.

romeuh80 a écrit:

Hi

Can somoene post a photo of one RSA or RSW piston upside down? Sorry, I do not have foto's

The reason is, does the botton of the piston squirt match the cylinder squirt at bdc? No

The botton of piston squirt is rounded or just knife edge like the pistons I have on the table?

Should't this improve flow?

It is knife edged.
Of course I tried a round edge, thinking it would improve flow.
But HP diminished a little bit!

It seemed logical that at leats does not affect power, but diminished!?....
Experience says it all...

Thanks

Jan Thiel a écrit:

We tried from 40C to 90C, every 5C more caused 0,5HP power loss!

Jan, I can't find it anywhere but you find an optimum water temp to run at? Did I read/remember it correctly the RSA water pump is 130l/m, the ideal was lots of water moving rapidly ? Was a thermostat used?

Found today 10deg increase in water temp cost about 2-2.5HP

@ Howard Gifford, Frits Overmars & el castor

I've got a little question regarding the Brisk spark plugs.

Which do you now think should one choose best for a 2 stroke engine?

Howard seems to have used an Iridium plug (without real improvements), while el castor used a Multi Spark Plug ("ZS" designation) that seems to give good improvements. What I am wondering about, because the big "head" of such a plug theoretically throws a "shadow" where burnt gases are hard to flush away (I hope you can understand how it is meant)

Brisk offers so many different kinds of plugs that I don't look right through

I can't say because I did not test them, Daniel. Ask Jan Schäffer of Langtuning.

Hi everyone. I have some questions about exhaust systems.

Usually exhaust midsection is a continuos section, it's internal diamenter is constant. But the Aprilia RSA exhaust, as posted on this thread earlier has no straight forward midsection, instead there is 2 small sections diverging from 117 to 125.5 and 125.5 to 127.8.

This means that straigh forward midsection is a compromise?
It's strange how the Honda Pipe is mutch more simple and had not changed mutch over the years (the recomended changes in exhaust of 96' Honda RS125 are very near from the one frits posted).

Should higher compression ratio engine get a bigger midsection diameter? Or ajusting the timing correct's the problem? If you could understand how I am thinking.

Thanks

Mr Frits has said that both the midsection and the 2-stage baffle are "relics" from an older design philosophy. And that a straight belly/single stage baffle would work just as well, given there is time for testing & finding the best dimensions.

I cannot help you with the belly Dia, because I don't know. I think of it as a medium of fine-tuning the pipe volume, its length and the baffle cone's angle; not a 'real' pipe part.
Thus, I believe it's not directly connected to Comp ratio or timing -as is stinger nozzle for example.
Higher Comp might reduce the EGT a little, 'lengthening' the pipe, while retarding the timing will do the opposite. Again, belly Dia seems irrelevant -better wait for Frits to correct me though.

It's not irrelevant, Vagelis. but it's not critical either.

Institute of TwoStrokes a écrit:

Jan Thiel a écrit:

We tried from 40C to 90C, every 5C more caused 0,5HP power loss!

Jan, I can't find it anywhere but you find an optimum water temp to run at? Did I read/remember it correctly the RSA water pump is 130l/m, the ideal was lots of water moving rapidly ? Was a thermostat used?

Found today 10deg increase in water temp cost about 2-2.5HP

No, a thermostat was never used.
On the dyno the lower the temperature, the more HP!
I do not know the lower limit, if something 'strange' happens if you go too low.
But at a certain piont you might get fuel evaporation problems.
At Aprilia HP still improved going from 45° to 40°.
And at Bultaco I used tap water for cooling, about 20°, with good results and without any problem.
Lots of water moving rapidly was clearly the best.
Some people believe cooling the exhaust duct 'too much' would cost power.
This proved to be nonsense.
The more we cooled the more HP!
Reducing wall thickness also proved to improve HP.
The limit is mechanical reliability.
Which makes me think that fixing the cylinder to the crankcase at its bottom is all wrong!
The combustion forces should not stress the cylinder!

Jan Thiel a écrit:

No, a thermostat was never used.
On the dyno the lower the temperature, the more HP!
I do not know the lower limit, if something 'strange' happens if you go too low.
But at a certain piont you might get fuel evaporation problems.
At Aprilia HP still improved going from 45° to 40°.
And at Bultaco I used tap water for cooling, about 20°, with good results and without any problem.
Lots of water moving rapidly was clearly the best.
Some people believe cooling the exhaust duct 'too much' would cost power.
This proved to be nonsense.
The more we cooled the more HP!
Reducing wall thickness also proved to improve HP.
The limit is mechanical reliability.
Which makes me think that fixing the cylinder to the crankcase at its bottom is all wrong!
The combustion forces should not stress the cylinder!

Mr Jan, what do you mean with "Reducing wall thickness also proved to improve HP"???
i did not understand???

This is my cylinder section ... I designed with a thickness of 7 mm ... I was wrong?

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Stefano,

Compliments with your very nice design!
I just meant to say that reducing wall thickness improved power.
And especially in the hottest places of course.
Like the top of the cylinder, and around the exhaust duct.
The limit, of course, is mechanical strength.
And, in your case, maybe cooling capacity!

Jan Thiel a écrit:

Stefan,

Compliments with your very nice design!
I just meant to say that reducing wall thickness improved power.
And especially in the hottest places of course.
Like the top of the cylinder, and around the exhaust duct.
The limit, of course, is mechanical strength.
And, in your case, maybe cooling capacity!

Hello Jan
I am Stefano Brutti ... your facebook friend!
In this project, I can decrease the thickness with a milling cutter in small steps... by water flow...

Yes, I know who you are Stefano!

Try reducing the thickness and to test the power afterwards!
But take care of deformation!

Jan Thiel a écrit:

Yes, I know who you are Stefano!

Try reducing the thickness and to test the power afterwards!
But take care of deformation!

We have sucessfully tested coolant temperatures as low as 50 degrees F (10C) on the dyno . Colder than this seems to cause a misfire with our race fuel possibly from incomplete vaporization and we run the risk of what we call a "cold seizure" caused by the piston heating up and expanding faster than the cylinder. Our fuel is 116 octane r+m/2. The difference between 10C and 50C coolant temperatures is around 4HP
The difference from 10C to 80C is 8HP on our 440 cc motor so you can see there is a big advantage to having lots of cooling. We also have the advantage of racing in winter temperatures that dip to -30C at times and we cool our race sled with ice so it is easy to keep it cool, however under sustained running the coolant out of the motor temperature stabilizes at around 80F (26.6C) . We run an auxillary electric pump to boost the flow and also cool the motor down to below 10C whenever there is a restart. One of our problems is that we have difficulty starting the motor cold if the coolant temperature gets below -10C. We have a handy "squirt bottle " with regular 87 octane that helps us start a real cold motor otherwise we keep the race sled inside a heated trailer. My advise is cool the motor as much as you can and run as much water through the motor as possible. There is no such thing as "overcooling" in summer weather temperatures.

I think what Frits is saying (correct me if I am wrong)is that it is more important to optimize the shapes and lengths of the cones to match your motor requirements than worry about the center section diameter. The center section (belly) is needed to attach the two cones together and time the reflected wave and the diameter is derived from the design of the cones. Larger center sections mean more agressive cone designs which usually have more top power but give up powerband and are used with high BMEP engines. I suspect the designer of Tubo-120 wanted a longer , more agressive reflected wave therefore he designed the baffle cone larger and the result was a slightly tapered mid section. However two stroke exhaust design is still progressing through empherical testing (although not at the level that Aprillia did with the RSA). Successful designs are then analyzed and reverse engineered to come up with math equations to explain why they work. Math only derived designs usually flop!

This is news overhere, I always thought that cylinder temperature would have an Ideal... 60º to 80º.