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

Zrt1200 a écrit:: With the triple ports when used in a bigger CC engine that the bore and stroke are square. Is there a general rule for maximum triple port size. If the Aprilia 125cc uses a 12mm X 13MM, Could a 250cc use twice the size of triple ports in reality??

The bore and the stroke of a 250cc engine are 1,26 times the bore and the stroke of a 125cc engine with the same bore/stroke-ratio, so a 250cc can never use ports that are twice as big as those of a 125cc. But you knew that, didn't you?

In principle any engine with the same bore/stroke ratio and the same mean piston speed as the RSA can use the same porting layout as the RSA, just scaled-up or scaled-down in accordance with the bore and stroke dimensions.

When modifying the auxiliary exhaust ports, remember that the RSA-layout requires piston pin plugs in order to avoid exhaust-to-transfer (or vice-versa) leakage; note the remark about maximum port overlap in the drawing I posted yesterday.

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Thanks again Frits. Yes I was pretty sure but thought i would ask. A upscaled RSA would be awesome project but not practical to make a one off engine. Even though it would be a lot of fun. Yes I knew about the RSA piston plugs and have tried different materials in the past for a longer lasting solution. I see Wobbly mentioned on another forum what material he is using for piston plugs. Yes I seen your Port overlap that you posted yesterday and a few pages back and I do understand what you meant by it. Thank you for pointing it out in the drawing as its a good reminder.

At Aprilia we mostly used unplugged piston pins for dyno-testing.
This was done because the welded pins made by Pankl in Austria were so expensive....
I first used plugged pin holes on the 125\1 Aprilia in 1988.
The pin hole was enlarged to a diameter bigger than transfer height.
And aluminium caps were fitted.
With a small press-fit of about 0,02.
It worked quite well for 2 races.
But the difference they made did not seem very big,
Probably because the auxiliar ports were not so big at the time....
So I abandoned the system, too soon!

Jan Thiel a écrit:: I first used plugged pin holes on the 125\1 Aprilia in 1988.

Typos happen to the best of us. Here are some pictures of Jan's 1988 machine. I was the first 125cc with a balance shaft.
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Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Frits Overmars a écrit:: [Vous devez être inscrit et connecté pour voir cette image]

is that shaft south-east of the clutch for a kick or for weirdo's who insist on changing gears with their right foot ?

JanBros a écrit:: is that shaft south-east of the clutch for a kick or for weirdo's who insist on changing gears with their right foot ?

All Garelli race engines had a left-through-right shift axle. It's what you build for a works rider who prefers shifting with his right foot. And his 13 world titles suggest that he was a bit more than just a weirdo wanted - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 34 809516

wanted - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5) - Page 34 809516

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Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Frits Overmars a écrit:: his 13 world titles suggest that he was a bit more than just a weirdo

to become a multiple-WC, you have to be some kind of weirdo, so I was right lol!

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Frits or Jan, I may have missed this while reading but what was the length of the RSA exhaust duct from the piston face to the exhaust flange?? I looked at the prints but I do not know if the cylinder bore is centered with the cylinder skirt.

Zrt1200 a écrit:: Frits or Jan, I may have missed this while reading but what was the length of the RSA exhaust duct from the piston face to the exhaust flange?? I looked at the prints but I do not know if the cylinder bore is centered with the cylinder skirt.

From the upper timing edge of the main exhaust port to the center of the flange face I measured 63 mm.
You can find that length back in the exhaust drawings.
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Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Thank you Frits. I did not think to look at the pipe drawing. Another question as I did not see asked. Do you have any idea on what kind of internal temperatures the rod big end bearing and wrist pin bearing get to durning a run on the track or dyno??

Zrt1200 a écrit:: Do you have any idea on what kind of internal temperatures the rod big end bearing and wrist pin bearing get to durning a run on the track or dyno??

I don't know Al. We would have to go by the discoloration of the parts. I've seen blue wrist pins once or twice, but the big end bearing lasts an entire season of dyno testing. What happens on the track, depends on the rider. About a century ago, Harada, Capirossi and young Valentino Rossi were riding works 250cc Aprilias. Harada and Rossi never had a problem, but Capirossi seemed to eat big ends for breakfast. The data logging reveiled that upon approaching a corner, he shifted down three gears before grabbing the front brake. By by big end...

Nombre de messages : 6502 Localisation : Limoges Date d'inscription : 19/02/2016

But Maybee his brake pads where lasting longer....

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Frits, I read this some where and it probable was in this thread. For a maximum power is there a height limit between the bottom of the spark plug and the top of the piston??

Zrt1200 a écrit:: Frits, I read this some where and it probable was in this thread. For a maximum power is there a height limit between the bottom of the spark plug and the top of the piston??

Define the bottom of the spark plug. Is it the thread? Or the part nearest the piston dome, i.e. the mass electrode?
I prefer the spark to be about halfway between piston and head, preferably with the electrode gap exposed as much as possible to the flow of the fresh mixture, as in the modified spark plugs that I showed here some time ago.

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Hi Frits. I am talking about the part nearest the piston dome. I am think I read somewhere that you want to keep that distance around 3-5mm??

Nombre de messages : 5 Localisation : usa Date d'inscription : 06/12/2016

Hi Frits, Jan and others, Does anyone know how the use of methanol fuel rather than gasoline fuel affects expansion chamber dimensions for a well developed engine similar to the RSA 125?

Zrt1200 a écrit:: Hi Frits. I am talking about the part nearest the piston dome. I am think I read somewhere that you want to keep that distance around 3-5mm??

I don't think I ever wrote that. Maybe Wobbly did?

Virgil Tripp a écrit:: Hi Frits, Jan and others, Does anyone know how the use of methanol fuel rather than gasoline fuel affects expansion chamber dimensions for a well developed engine similar to the RSA 125?

We never bothered because methanol is not allowed in our main branch of motorsport. But as methanol could give a clear increase in power, it would at least require a bigger end restrictor diameter.

Nombre de messages : 12 Localisation : USA Date d'inscription : 30/09/2019

Thanks Frits. When you guys got to the track did you test your race gas RVP?? (RVP = Reid Vapor Pressure) Here in the USA and Canada we are finding a lot of the VP & Sunoco race gas has only 1-2PSI RVP where it should be higher. We have found that a low RVP requires a much richer A/F to avoid detonation and we end up loosing HP. A lot of us serious racers have now built a small portable RVP tester so we can check our fuel at home or the track. If our fuel is dead we can purchase isopentane and add it to the race gas to bring it back to life and get the RVP back into alignment. Another thing we have found is keeping your fuel in a refrigerator at home and transporting it in a big cooler full of ice will help preserve the RVP.

A low Reid Vapor Pressure is an indication that some of the volatile fuel components have left the building, which leads to a lower flame speed. Leaving fuel containers open and/or in the sun is a major cause, as is the ineradicable habit of determining fuel consumption by draining the tank contents after each training. Apparently just taking the tank off and weighing it is too simple...
I think RVP checking it is a very good idea. But I seem to remember that the factory teams left that to the fuel supplier, who was responsible for it and who had the proper equipment.

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Hi, Frits

You have provided us blowdown and transfers STA's from Aprilia, but not intake values? Engmod2t seems to use very different unit for STA, so I wonder if I can assume, that intake should have STA at least 2 times of transfers to have it in balance?

I believe this picture is from Engmod2t, and even with intake STA two times as big as transfers it shows it as a "bottleneck":
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Do you have any idea, what they use in Engmod simulation to get this hp approximation?

op76 a écrit:: Hi Frits, you have provided us blowdown and transfers STA's from Aprilia, but not intake values? Engmod2t seems to use very different unit for STA, so I wonder if I can assume, that intake should have STA at least 2 times of transfers to have it in balance? Do you have any idea, what they use in Engmod simulation to get this hp approximation?

I'd say this is a question for professor "EngMod2T" Neels Van Niekerk. Neels, are you there?

Yes Frits I am here.

STA is an indication of what combination of time and area is required for a specific engine capacity to achieve a certain level of performance. The target values were determined by measuring successful engines and taking an average of their values to use as target. Over the years these values drifted as pipes became more efficient and scavenging and combustion was developed. There is no exact values but indications.

What is done in EngMod2T (Dat2T) is just the reverse, it looks at what the STA value is and interpolate between existing values to loosely predict the power the port is sized for.

Nombre de messages : 32 Localisation : Suomi Date d'inscription : 26/08/2018

Vannik a écrit:: What is done in EngMod2T (Dat2T) is just the reverse, it looks at what the STA value is and interpolate between existing values to loosely predict the power the port is sized for.

Thanks Vannik, now it seems even more weird for me, that Engmod predicts that inlet to produce so little horsepower...but that is of course irrelevant, and even more so, if it don't have effect for simulation. That STA unit seems to be sec-mm²/cm³, and you are using Jennings method to measure mean effective area?

Frits, as far as I have understood, you are using chordal width and open height of the port for angle area, getting area of xx mm² for each degree separately, and sum all degrees together, getting angle area unit of °mm², and then again STA units of °mm²/cm³/1000rpm. Is there some good reason for using this instead of what Jennings did, because at least in manual measuring, this seems much bigger job?

No I am not using Jennings' method. I use numerical integration. Jennings' method only gives the correct value if the port is rectangular with no corner radiuses and the port does not have an axial angle. It was good for its time.

Read the description by Blair, his values are dated but his maths is sound.

The oldest reference to STA I could find was on two-stroke diesel engine scavenging by PH Schweitzer, 1949. He shows the integrals as well.

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