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

[quote="GtG001"]

Frits Overmars a écrit:

Jeram a écrit:

...Another concern would be the width of the exhaust port (74%) which is at the theoretical maxiumum. As the bore size increases will the negative effects (ring life) of such a large exhaust port become more apparent or will it remain the same when scaled up?

The optimum exhaust port width is 70%. If you make it wider, you will need ever larger radii in the corners to keep the pistong ring happy. Those radii take away port area where it is most needed, at the very beginning of port opening (the yellow areas in the picture below) which will reduce blowdown angle.area.
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Thank you Jan and Frits for the interesting topic.
In regards to the opening of the exhaust port top edge, if I understand your explanation; the exhaust port top edge would open completely at once if you did not have to worry about the ring being eased in and out of it – no generous radiuses in the corner – is this correct or is the gradual opening of the exhaust main port intentional to widen the power band at the expense of maximum power
Along this same line, did you have plans to add power valves to the RSA auxiliary exhaust ports so you could increase their port timing to the same as the main port?
Best Regards
Allan.

We tried to close the auxilaries to see where power improved.
It improved at 95000 rpm and below.
So we thought it was useless to put a valve on it.
We lost more than 12HP by closing the auxilaries.
Raising then would cause a lot of torque loss.
And max. power did not improve, only overrev.

teriks a écrit:

Jan Thiel a écrit:

2005bully a écrit:

In regards to the piston pin plug.... I have had found pure teflon as a good material to use. It can be easily machined with a protrousion that helps to secure it by a press fit into the wrist pins ID. Plus if any part of the plug ever rubs the cylinder wall or becomes un lodged, damage to motor parts is highly unlikely.

It will probably melt. All non-metallic plugs we tried did!

I guess PEEK was among the tested materials? If not, that would be very high on my list of possible candidates with a Tg of ~145 degrees C, and a melting point above 340 degrees C.

Jan was referring to the tests carried out at Aprilia. I continued developing my piston pin plugs after Aprilia closed its two-stroke racing department. The temperature at which a material still retains its mechanical properties, lies a lot lower than its melting point. PEEK was tested, and it was unsatisfactory. And I am not going to tell you which material was finally found adequate. I can tell you that it is horribly expensive!

Daniel A. a écrit:

Okay, it was clear for me, that f.e. higher revs require steeper angles in general, but in case of your exhaust concept the angle won't be "really" big anyway, because it's a 2-stage diffusor.
Exhaust pipes with a 3-stage diffusor most time have a relatively big angle at the third stage and I am wondering, if in this case there is a specific critical angle so that, if exceeded, the flow will get REALLY bad, what could result in a clearly noticeable powerloss.

You did not do your homework, Daniel. The steepest diffuser angle in the Aprilia pipe on the previous page is 17,2° and its maximum lenght from piston to tail restrictor is 803 mm. We know that the exhaust timing is 202° and that maximum power is produced at 13.000 rpm, so a bit of reverse engineering will tell us that the average speed of sound in that pipe was 587 m/s.
Enter those same values in my simple concept-for-beginning-tuners (which, by the way, does not have a 2-stage, but a 1-stage diffuser; the header does not count as a diffuser) and you will find that this single diffuser has an opening angle of .... 17,2° .

Anyway, try to get rid of the idea that a diffuser has to have a certain angle. In gas dynamics there is no such thing as an optimal difusser with a fixed angle; it will look more like the horn of a musical instrument.
But that might create the misimpression that an exhaust pipe is an acoustic device. It most definitely is not;
you might want to read my 'Helmholtz Blues' elsewhere in this forum to discover why acoustic rules do not apply.

Frits Overmars a écrit:

teriks a écrit:

Jan Thiel a écrit:

2005bully a écrit:

In regards to the piston pin plug.... I have had found pure teflon as a good material to use. It can be easily machined with a protrousion that helps to secure it by a press fit into the wrist pins ID. Plus if any part of the plug ever rubs the cylinder wall or becomes un lodged, damage to motor parts is highly unlikely.

It will probably melt. All non-metallic plugs we tried did!

I guess PEEK was among the tested materials? If not, that would be very high on my list of possible candidates with a Tg of ~145 degrees C, and a melting point above 340 degrees C.

Jan was referring to the tests carried out at Aprilia. I continued developing my piston pin plugs after Aprilia closed its two-stroke racing department. The temperature at which a material still retains its mechanical properties, lies a lot lower than its melting point. PEEK was tested, and it was unsatisfactory. And I am not going to tell you which material was finally found adequate. I can tell you that it is horribly expensive!

TECASINT

UNI name is "PI"


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It is no secret what to use. DuPont makes a product called Vespel SP. I have used it and it works quite well at temperatures up to 500 degrees F and yes sir it is super expensive. SP-22 is the material of choice for pin plugs in that it has a thermal expansion rate that is identical to that of cast aluminum.
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Howard Gifford a écrit:

It is no secret what to use....

Thanks for letting the cat out, Howard. Now I can forget about that new Porsche I was going to order .

(To put your mind at ease: I doubt if those plugs will ever return their developing investment).

At the price DuPont charges for even a small sample piece I doubt you will sell much of it at a profit. If anyone wants any plugs made I can make a limited amount but at around $120.00 a plug I doubt anyone will be beating on my door with orders.
By the way I have a mint condition 1990 944 Turbo for sale Frits if your'e interested?

Howard Gifford a écrit:

By the way I have a mint condition 1990 944 Turbo for sale Frits if your'e interested?

Can I pay in piston plugs or do you prefer money?

Frits. Tell you what! If you can help me make 140HP with my motor at 10300 rpm I will give you the Porsche!! So far we are just over 130.

How much is a train ticket from Holland to Ottawa? (Only partly kidding; I would love to visit Canada, but not in January).

any of you know the diameter of rad shafts RSA?

Do you think for a motor 47x47, 5 with a shaft diameter of 62mm and width of 31mm, how could it be?

Frits Overmars a écrit:

Daniel A. a écrit:

Okay, it was clear for me, that f.e. higher revs require steeper angles in general, but in case of your exhaust concept the angle won't be "really" big anyway, because it's a 2-stage diffusor.
Exhaust pipes with a 3-stage diffusor most time have a relatively big angle at the third stage and I am wondering, if in this case there is a specific critical angle so that, if exceeded, the flow will get REALLY bad, what could result in a clearly noticeable powerloss.

You did not do your homework, Daniel. The steepest diffuser angle in the Aprilia pipe on the previous page is 17,2° and its maximum lenght from piston to tail restrictor is 803 mm. We know that the exhaust timing is 202° and that maximum power is produced at 13.000 rpm, so a bit of reverse engineering will tell us that the average speed of sound in that pipe was 587 m/s.
Enter those same values in my simple concept-for-beginning-tuners (which, by the way, does not have a 2-stage, but a 1-stage diffuser; the header does not count as a diffuser) and you will find that this single diffuser has an opening angle of .... 17,2° .

Anyway, try to get rid of the idea that a diffuser has to have a certain angle. In gas dynamics there is no such thing as an optimal difusser with a fixed angle; it will look more like the horn of a musical instrument.
But that might create the misimpression that an exhaust pipe is an acoustic device. It most definitely is not;
you might want to read my 'Helmholtz Blues' elsewhere in this forum to discover why acoustic rules do not apply.

aprilia have 202° timing for Exhaust?

Main exhaust, yes.

@ Frits:

Okay, I understand. But obviously the angle with your exhaust concept is also depending on cubic capacity. If I use the values you wrote down of the RSA for a 250cm³ engine I get a total angle of 24,1°, if I use it for 50cm³ I get only 10,9°. Almost every 50cm³ exhaust I know has got steeper angles and this exhausts seem to work very well. This might be like this, because your concept is "simple", as you already said yourself. But I don't really know, that's why I ask

And this is why I wonder, if the biggest angle of an exhaust is REALLY depending on cubig capacity or if there are some general "gas dynamic rules". Let's say I wanted to enlarge the whole RSA engine up to 250cm³. Should I use a MUCH steeper header and keep the 17,2° at the steepest part or should I use just a little steeper header and use a steeper diffusor, for example 24,1°? (paying attention to the pipe volume in every case)

At the same time: Am I on the right way when assuming that reed valve engines "require" steeper angles than rotary valves (simplified)?

Best regards and thanks for your efforts!

[/quote]

We tried to close the auxilaries to see where power improved.
It improved at 95000 rpm and below.
So we thought it was useless to put a valve on it.
We lost more than 12HP by closing the auxilaries.
Raising then would cause a lot of torque loss.
And max. power did not improve, only overrev.[/quote]

Thanks Jan.
What a delicate balance between torque and blow down

Frits Overmars a écrit:

Jan was referring to the tests carried out at Aprilia. I continued developing my piston pin plugs after Aprilia closed its two-stroke racing department. The temperature at which a material still retains its mechanical properties, lies a lot lower than its melting point. PEEK was tested, and it was unsatisfactory. And I am not going to tell you which material was finally found adequate. I can tell you that it is horribly expensive!

Of course, that's why I mentioned the glass transition temperature, Tg.

Polyimide looks much better though, thanks Howard!

Daniel A. a écrit:

Main exhaust, yes.

@ Frits:

Okay, I understand. But obviously the angle with your exhaust concept is also depending on cubic capacity. If I use the values you wrote down of the RSA for a 250cm³ engine I get a total angle of 24,1°, if I use it for 50cm³ I get only 10,9°. Almost every 50cm³ exhaust I know has got steeper angles and this exhausts seem to work very well. This might be like this, because your concept is "simple", as you already said yourself. But I don't really know, that's why I ask

And this is why I wonder, if the biggest angle of an exhaust is REALLY depending on cubig capacity or if there are some general "gas dynamic rules". Let's say I wanted to enlarge the whole RSA engine up to 250cm³. Should I use a MUCH steeper header and keep the 17,2° at the steepest part or should I use just a little steeper header and use a steeper diffusor, for example 24,1°? (paying attention to the pipe volume in every case)

At the same time: Am I on the right way when assuming that reed valve engines "require" steeper angles than rotary valves (simplified)?

Best regards and thanks for your efforts!

Booster? Primary? Secondary? Coanda?

t0nix a écrit:

any of you know the diameter of rad shafts RSA?

Do you think for a motor 47x47, 5 with a shaft diameter of 62mm and width of 31mm, how could it be?

What do you mean by 'rad shafts'?

Howard Gifford a écrit:

It is no secret what to use. DuPont makes a product called Vespel SP. I have used it and it works quite well at temperatures up to 500 degrees F and yes sir it is super expensive. SP-22 is the material of choice for pin plugs in that it has a thermal expansion rate that is identical to that of cast aluminum.
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Howard,

Are your plugs reliable?
Those made in Holland were not 100%.
I think they are not used anymore.
And most important: have you ever tested the difference they make on HP?
And where in te powercurve?
In Holland this was not done, so I am still curious!
And at 120$ apiece, so 240$ a piston, the plugs are too expensive
The welded piston pins made by Pankl in Austria were cheaper!
And 100% reliable.

t0nix, I also don't really understand you. But if you want to know the RSA timings, simply read the thread.

Daniel A. a écrit:

...obviously the angle with your exhaust concept is also depending on cubic capacity. If I use the values you wrote down of the RSA for a 250cm³ engine I get a total angle of 24,1°, if I use it for 50cm³ I get only 10,9°. Almost every 50cm³ exhaust I know has got steeper angles and this exhausts seem to work very well. This might be like this, because your concept is "simple", as you already said yourself.

Did you ever consider that a 250 cc single cylinder engine cannot possibly produce its maximum power at 13.000 rpm, like the RSA125 does? If you wish to compare engines of different sizes, you cannot use the same rpm for all of them; you must use the same piston speed in all cases.
The RSA125 has a bore and stroke of 54 x 54,5 mm. Its piston speed at 13.000 rpm is 13.000 x 54,5 / 30 = 23,6 meter per second.
A 250 cc engine with a bore and stroke of 68 x 68,5 mm will have this same piston speed at 54,5 / 68,5 * 13.000 rpm = 10.343 rpm. And a 50 cc engine will have that piston speed at 54,5 / 40 * 13.000 = 17.700 rpm.
Now do your homework again and you'll see that the diffuser angle is exactly the same in all three cases: 17,2°
That angle depends on exhaust timing, speed of sound and piston speed; not on cubic capacity;
my exhaust concept may be simple, but it is not inadequate.

Citation :

Am I on the right way when assuming that reed valve engines "require" steeper angles than rotary valves ?

No.

I never wanted to claim that your concept was inadequate, I only try to understand the context! Sorry if it sounded criticising.

Thanks for the explanation, but there is still a point in this context I do not understand: Noone lets his 50cm³ engine rev 17700 (with a "normal" scavenging), because there won't be enough port time area for such high revs. Most of these engines have their maximum power around ~13000, too. So let's assume: Same exhaust timing (202°), same speed of sound. BUT: Lower piston speed. The result is 10,9°. An angle that is clearly lower than the ones I've seen until now. (they move between 15 and 20°, while the engines have their maximum power always between 11500 and 13500 --> "relatively" low piston speed, nevertheless a relatively high angle). Maybe the exhausts I've seen simply are crap and ones with lower angles would work better?

Thanks again and sorry if my questions are annoying.

Daniel A. a écrit:

Maybe the exhausts I've seen simply are crap

Compared to the Aprilia RSA125, the 50 cc engines you have seen are crap. I know what I'm saying; the 50 cc European champions of the last four years are good friends of mine and I know their engines inside and out. They too realize that they still have a long way to go.
A 50 cc engine, developed to the same level as the Aprilia RSA125, should produce 29 HP at 17.700 rpm, as Jan Thiel has repeatedly explained. No current 50 cc engine comes near these values, because their development has been below par ever since the 50 cc class lost its GP-status.

For 13000 rpm you do not need 202° exhaust timing. 192° is enough because the maximum attainable rpm is limited by the insufficient transfer angle.area, like you say. And because the power level is lower, so are the exhaust gas temperature and the speed of sound. Entering an exhaust timing of 194° and a speed of sound of 500 m/s in my exhaust concept will give a diffuser angle of about 14°.
If you wish to continue this conversation (I don't mind; I like a good sparring-partner), please present some power curves of the engines you are mentioning .

Jan Thiel a écrit:

Howard Gifford a écrit:

It is no secret what to use. DuPont makes a product called Vespel SP. I have used it and it works quite well at temperatures up to 500 degrees F and yes sir it is super expensive. SP-22 is the material of choice for pin plugs in that it has a thermal expansion rate that is identical to that of cast aluminum.
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Howard,

Are your plugs reliable?
Those made in Holland were not 100%.
I think they are not used anymore.
And most important: have you ever tested the difference they make on HP?
And where in te powercurve?
In Holland this was not done, so I am still curious!
And at 120$ apiece, so 240$ a piston, the plugs are too expensive
The welded piston pins made by Pankl in Austria were cheaper!
And 100% reliable.

hello

has some one tried to close the pin wit an aluminium rod?

I was thinking to press the aluminum rod into the pin and may set with loctite 648. after then work one it with the lathe.
can that work?

Manuel[left]

Manuel Rainer a écrit:

Jan Thiel a écrit:

Howard Gifford a écrit:

It is no secret what to use. DuPont makes a product called Vespel SP. I have used it and it works quite well at temperatures up to 500 degrees F and yes sir it is super expensive. SP-22 is the material of choice for pin plugs in that it has a thermal expansion rate that is identical to that of cast aluminum.
[Vous devez être inscrit et connecté pour voir ce lien]

Howard,

Are your plugs reliable?
Those made in Holland were not 100%.
I think they are not used anymore.
And most important: have you ever tested the difference they make on HP?
And where in te powercurve?
In Holland this was not done, so I am still curious!
And at 120$ apiece, so 240$ a piston, the plugs are too expensive
The welded piston pins made by Pankl in Austria were cheaper!
And 100% reliable.

hello

has some one tried to close the pin wit an aluminium rod?

I was thinking to press the aluminum rod into the pin and may set with loctite 648. after then work one it with the lathe.
can that work?

Manuel[left]

Yes, we tried that, and it worked.
First the pin was honed on the inside, and a piece of aluminium was pressed in.
The pin expanded a bit through the pressing-in.
So we fitted smaller needles.
Then it worked quite well, but it was a bit heavy.
We also tried magnesium, but that worked loose.
Then welding the pin was tried, that was lighter.
But sometimes there were reliability problems, the welded-on caps came loose.
This caused massive engine failure.
Finally we had our pins made by Pankl in Austria.
These were 100% reliable.

Jan I will let you know how they stand up in a race situation. They seem quite good on the dyno but we do not do sustained runs on the dyno so I will find out shortly . I havent seen any big gains on the dyno either but with our auxillary exhaust communicatinbg with the transfewrs at tdc you would think that they can only help.
I tried machining two pieces out of aluminum that were bolted together by an allen screw. They had a locator pin and were curved on the belt sander to match the profile of the piston. They worked well also but they were heavier than I liked. The Vespel plugs weigh 4 grams each so weight is not a concern. I just wish I could give them a proper test on the track before I determine weather they are worth the bother.
Yes the cost is silly for the material and they are difficult to hold properly to machine so Im hoping they will become unnecessary. Especially since Frits promised to come to Canada to wring out some more «hp out of my motor.
BTW Frits the porsche actually belongs to my wife and she says it`s not for sale.