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

hello

thanks Howard

thanks MIC for the nice explanation, now it's clear for me.

did someone ever tried a timing like 86° 160°? because you all had only 150° bTDC beginning opening??

Another question:

if the inlet goes with the turning direction of the crankshaft, it's better to keep the crankcase open over the crankshaft or is it better to keep it closed?
has someone tried the difference??

thanks
manuel

PVO a écrit:

I've redone my sub ports since yesterday. The first two pictures show my old sub ports. The last two depict my new design. Hope this helps.

PVO, thank you for the pic, it seems that I was wrong, there is only an optical illusion.

Frits Overmars a écrit:

This may give you an idea of the way the auxiliary exhaust ducts exit from the cylinder:
[Vous devez être inscrit et connecté pour voir cette image]

Dear Mr. Overmars!

Good to see you! Thanks for the advice, I'll keep it in mind!

If you follow the comments, and you are here, please let me ask you about the FOS exhaust concept.
I have read in one of your articales from the 70's that in those times you calculated with 0,03 in the formula and only later began to use '(407 - exh. timing) / 7100 '. You wrote that it only gave us a general header diametre. I think this is obvious because you don't apply TA, flow area etc. through your calculation.
The diameter of the calculated Dx doesn't have to be the same as the usefull size of the exhaust port, does it? Because I'm affraid I can't see any connection between the area of Dx and the exhaust port's flow area neither in case of small timearea, nor in case of a large one.

Although I have observed only a few engines, but I have noticed that generally the exhaust port's flow area is equal to (Dx area)x 0,85-0,95.

Well I must say that I'm always in trouble when I have to choose the diameter and length of the exhaust passage to get the better power.

Or is there any general solution for this problem? Earlier I thought that the best choice is to apply the mean area of the exhaust port. But now I'm not sure about this at all. :)

B.

Forget about the mean area of the exhaust port, Forgi. Look at the blowdown area.
What I wrote in the seventies, is 35 years old! And what I showed in the exhaust concept that I posted some time ago, is an extreme simplification, only meant to help beginning tuners on their way.

If any of you want to see the Champ 440 race snowmobiles in action. google ( Fire on Ice Snowmobile Race ) youtube... This shows these premier class race sleds in action... This class should bring a smile to any two stroke lover and will prove two stroke racing is alive and well.. The knowledge we have learned on here will directly benefit our race effort...

Daniel A. a écrit:

Fully closed 93° after TDC, wow, I did not think that it would be so late.

Why is everyone using around ~160° timing on a port-controlled engine then?
Looking at the timing of a rotary valve over 180° timing should theoretically even work better!? But I think practically it won't.

Somebody knows why? I haven't tested such huge timings until now.

Regards

Daniel,you can use a lot more timing in a piston port engine.The old piston port TZ250's used 200-204 degrees duration and would start very easily.You just need to have the pilot jet/throttle slide cutaway right.

Howard Gifford a écrit:

Jan did you hard anodize the wear surfaces of the rotary valve cover and crankcase? It would seem that a hardened surface would wear less with carbon fiber since it is fairly abrasive but perhaps anodized aluminum and steel moving parts together are not a good match! We have experienced more wear on the motor than the cover. What is your experience or advise with this?

We tried carbon valves at Garelli in 1983, on the 50 and 125cc engines.
On track the rider did not notice any difference in vibrations.
On the dyno I saw a little less power with the carbon discs.
And there was a lot of wear on the crankcase and cover.
So we decided to continue with steel discs which never gave any problems.

At Aprilia carbon discs were used.
The cankcase and cover were coated, there was never any wear.
I am sorry but I do not remember the coating material.
I was not anodized!

if you're interested, I often use this company... [Vous devez être inscrit et connecté pour voir ce lien]

for me, surface treatments shown to decrease friction are:
[Vous devez être inscrit et connecté pour voir ce lien]

and:
[Vous devez être inscrit et connecté pour voir ce lien]

This last link, i used on the camme surface for decrease friction and decrease usury...

I think smaltiriva was the firm that treated Aprilia crankcases and inlet covers.

This is a coincidence... i work with Smaltiriva for my true job!!!

Frits Overmars a écrit:

Forget about the mean area of the exhaust port, Forgi. Look at the blowdown area.
What I wrote in the seventies, is 35 years old! And what I showed in the exhaust concept that I posted some time ago, is an extreme simplification, only meant to help beginning tuners on their way.

It is quiet interesting Frits! So are you telling me that the exhaust port's time area doesn't mind? :-) Only the blowdown what is important? Well I always intend to make the whole exhaust port to have a convenient time area between 0,0170-0,0180 and blowdown between 0,0013-0,0014 (road race).

So all of this means that from the exhaust port's flow area we can't deduce to the area of the passage? It is interesting but now I'm a little bit confused because I thought I knew the secret.:-)

CRECY a écrit:

Daniel A. a écrit:

Fully closed 93° after TDC, wow, I did not think that it would be so late.

Why is everyone using around ~160° timing on a port-controlled engine then?
Looking at the timing of a rotary valve over 180° timing should theoretically even work better!? But I think practically it won't.

Somebody knows why? I haven't tested such huge timings until now.

Regards

Daniel,you can use a lot more timing in a piston port engine.The old piston port TZ250's used 200-204 degrees duration and would start very easily.You just need to have the pilot jet/throttle slide cutaway right.

Ok, but this won't give more power, because it will close a lot too late! Regarding the rotary valves timing (close = 93° ATDC) ~186° should be the best timing for a port controlled engine. But I doubt that this will result in more power in practice.

Nevertheless, maybe I will try it sometime.

CRECY a écrit:

Daniel A. a écrit:

Fully closed 93° after TDC, wow, I did not think that it would be so late.

Why is everyone using around ~160° timing on a port-controlled engine then?
Looking at the timing of a rotary valve over 180° timing should theoretically even work better!? But I think practically it won't.

Somebody knows why? I haven't tested such huge timings until now.

Regards

Daniel,you can use a lot more timing in a piston port engine.The old piston port TZ250's used 200-204 degrees duration and would start very easily.You just need to have the pilot jet/throttle slide cutaway right.

200, - 204 sound like exhaust durations..

I have a question. When the stinguer restrictor goes smaller what usually happens to the carburation? I goes leaner or ricther?

Frits, I think that the formula you posted to build exhausts give to small restrictions?

For the RSA for example with your formula sqrt(54)*3.06 = 22,49, and is 23,3, on the Honda sqrt(50?)*3.06 = 21,63 and is 23

Thanks

Romeuh, the target power does not refer to wheel or trans.shaft power, but 'crank' power or better target BMEP power.
Here's a shortcut to the math in case somebody runs out of ink: [Vous devez être inscrit et connecté pour voir ce lien]

Would this be a similar surface treatment?
[Vous devez être inscrit et connecté pour voir ce lien]

I notice that the cylinder design PVO has posted has the floor of the aux exhaust ports angled downward toward the main exhaust. I would think that will uncover the aux exhaust to the piston cutaway when the piston is at tdc... I have always tried to prevent this ... Is this a non issue ? It does not seem to adversely effect our engines in testing but I would like to know for sure..

diskvalve a écrit:

I notice that the cylinder design PVO has posted has the floor of the aux exhaust ports angled downward toward the main exhaust. I would think that will uncover the aux exhaust to the piston cutaway when the piston is at tdc... I have always tried to prevent this ... Is this a non issue ? It does not seem to adversely effect our engines in testing but I would like to know for sure..

The piston always need to be long enough not to make an opening between crankcase and exhaust at TDC. This the case for the RSA and I'm sure PVO has choosen a piston which won't do what you descripe.

Aprilia even choosed a piston pin with capped ends in order not to fill the hole inside the pin with exhaust gasses when passing over the aux exhaust ports which then would escape into the crankcase.

Very good question diskvalve. I believe the extra length in the cutout area on the cast piston is to keep this closed off at TDC. I don't know about the forged piston design though. It looks like it is cut somewhat deeper dosen't it? See top of page 20 for cast piston pic.

Forgi a écrit:

Frits Overmars a écrit:

Forget about the mean area of the exhaust port, Forgi. Look at the blowdown area.

So are you telling me that the exhaust port's time area doesn't mind? :-) Only the blowdown what is important?

Yes!

Frits what is the result if you have excessive blowdown i.e. too big auxillary exhaust ports?

diskvalve a écrit:

I notice that the cylinder design PVO has posted has the floor of the aux exhaust ports angled downward toward the main exhaust. I would think that will uncover the aux exhaust to the piston cutaway when the piston is at tdc... I have always tried to prevent this ... Is this a non issue ? It does not seem to adversely effect our engines in testing but I would like to know for sure..

You want the aux ports as big as possible to have more blowdown.
You want the most blowdown with a limited exhaust timing in order not to lose power at low revs.
But there are limits.
You have to keep away from the transfer ports to prevent fresh charge going into the aux. ports.
That is why their bottom is inclined.
The same happens if you make the aux. ports too wide.
You loose fresh charge.
This also cools the exhaust gases, so you loose revs.
Symmetry is also important.
We tried a piston with a closed pistonpin hole on one side.
With this piston power was very bad!
Worse than with a 'normal' piston!
Closed piston pins did not improve power at all revs.
Only at some points in the power curve.
Mostly at high revs.
At some points in the power curve you loose something, but not much.

With aux. ports you can use a lower exhaust port, so that you have more power at low revs.

Forgi a écrit:

Frits Overmars a écrit:

Forget about the mean area of the exhaust port, Forgi. Look at the blowdown area.
What I wrote in the seventies, is 35 years old! And what I showed in the exhaust concept that I posted some time ago, is an extreme simplification, only meant to help beginning tuners on their way.

It is quiet interesting Frits! So are you telling me that the exhaust port's time area doesn't mind? :-) Only the blowdown what is important? Well I always intend to make the whole exhaust port to have a convenient time area between 0,0170-0,0180 and blowdown between 0,0013-0,0014 (road race).

So all of this means that from the exhaust port's flow area we can't deduce to the area of the passage? It is interesting but now I'm a little bit confused because I thought I knew the secret.:-)

Forgi,

What really counts is blowdown.
But discharging of the burned gases may not be complete until BDC.
In this case you get 'short circuiting' of the fresh charge from the transfer ports into the exhaust.
By using auxiliar exhaust ports you can improve blowdown quite a lot.
So that you might be able to raise the exhaust's 'floor'.
This helps improve blowdown flow, as seen on a flowbench.
In 2007 we started to raise the underside of the exhaust port.
The first results were very promising!
But at the end of 2007 I retired, so I could not finish what I started.
The idea was to raise the exhaust underside as much as possible until power dropped.
And then, with a smaller exhaust port underside it might have been possible to widen the A-ports more
without losing the fresh charge into the exhaust.
The ideal situation would, of course, be that all the burned gases are discharged from the cylinder before
the transfer ports start opening.
So it is difficult to have too much blowdown.
But in trying to achieve enough blowdown you can arrive at a too high exhaust port.
Which first causes power loss at low revs, and if exaggerated still more also a loss of max. power.
Because of shortening the power stroke.

diskvalve a écrit:

CRECY a écrit:

Daniel A. a écrit:

Fully closed 93° after TDC, wow, I did not think that it would be so late.

Why is everyone using around ~160° timing on a port-controlled engine then?
Looking at the timing of a rotary valve over 180° timing should theoretically even work better!? But I think practically it won't.

Somebody knows why? I haven't tested such huge timings until now.

Regards

Daniel,you can use a lot more timing in a piston port engine.The old piston port TZ250's used 200-204 degrees duration and would start very easily.You just need to have the pilot jet/throttle slide cutaway right.

200, - 204 sound like exhaust durations..

Since I am new to this forum I will introduce myself.My name is Garry Treadwell.I am 61.I have been modifying two stroke engines and designing expansion chambers since 1970.
Mike,Yamaha used 200-204 degree intake duration on their piston port GP engines from the TD3 to the TZ250L.They were very easy to start and they had to be because there were no clutch starts back then,push start only!As a comparison Honda's first piston port 125 MX engine had 181 degree intake duration.
I too am a big fan of snowmobiles.They are the horsepower kings of the two stroke world.Have a look at this website [Vous devez être inscrit et connecté pour voir ce lien] There are dynotests on here of some very powerful snowmobile engines including Tom McConkey's 1725cc "Quadzilla" @530hp and Justin Durand's incredible 800cc turbo VMax @560hp!!
How fast is "Quadzilla"? This fast [Vous devez être inscrit et connecté pour voir ce lien]

Jan Thiel a écrit:

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!

Interesting. What would be your solution?
Instead of the bottom-screwed-cylinder four spacer bolts up to the cylinder head? (Of course wide enough to not disturb the coolant or the scavenge).