Exhaust port width and GS piston

[KTy]

Alloha ,

I'm back again on my Polini 133 project , well it's more a KTy 134 now...

I have a 58mm GS piston and I'm doing a port map of my actual cylinder (already modified)...

I didn't work too much on my exhaust port width, and now what I measure is odd too me.

On *the paper* of my port map, I measured a maximum exhaust port width of 37mm. So, the actual width of the port is: 58*sinus(37/58) = 34.5mm , that is 59.5% of the diameter of the piston. Could this be correct ?

I'm planning on increasing the exhaust port to 69-70%, that would be a 40.5mm port width (44.9mm width on the plannar port map), 6mm more compare to my existing port ! :plemplem:

So if I haven't made any calculation mistakes, here goes my question: On a Polini 133, can you enlarge that much the exhaust port without making a "hole" in the exhaust transfer ?

I don't want to see the studs through my exhaust port !! :plemplem:

Thanks for your help ;)

KTy -

[Paola]

yo.

there is enough flesh around the exhaust Port all the way out, but you have to be very careful.

another thing to concern is the amount of sealing surface for the header.

this is why I suggest to put a 1 cm piece of iron between the Cylinder and the header flange.

you than got a longer exhaust port to smooth the flow between a rather "squared" exhaust window and the round header pipe.

the polini exhaust port is too short anyway and by this method you can achieve a round outlet of the desired diameter without any sealing problems.

regards

[Brosi]

I measured 36mm sehnenmaß (can´t find the english word, but what you called "actual width of the port"). But the port might have been enlarged already, as it´s second-hand . I enlarged it to 40mm radian measure (on port map) wich is 70 % of the cylinder diameter. I also widened the exhaust side of the port to 32 mm diameter and had no such problems as "holes"...

[oli-san]

On *the paper* of my port map, I measured a maximum exhaust port width of 37mm. So, the actual width of the port is: 58*sinus(37/58) = 34.5mm , that is 59.5% of the diameter of the piston. Could this be correct ?

Yes.

I'm planning on increasing the exhaust port to 69-70%, that would be a 40.5mm port width (44.9mm width on the plannar port map), 6mm more compare to my existing port ! :plemplem:

This correct.

So if I haven't made any calculation mistakes, here goes my question: On a Polini 133, can you enlarge that much the exhaust port without making a "hole" in the exhaust transfer ?

I don't want to see the studs through my exhaust port !! :plemplem:

You have to be careful, when you continue with the exhaust diameter to the flange and raise the diameter to 1,1 x 40,5 mm. The wall is very thin and my polini has two holes where the studs end.

my guess is, that many people raise the exhaust port to 70% and adjust the flange diameter accordingly. but the projected path of the exhaust gases is not considered enough (or more posts would have come up here).

[KTy]

Ok, thanks

Any advices and experiences on different shapes for the exhaust port ?

I read that a rectangular port is more noisy but delivery very sharp wave pulse that could make the exhaust works better. Oval shape is common, but could reduces exhaust efficiency. Finally, V shape or trapezoid is often found on modern cylinder...

I was thinking that because I have to enlarge of 6mm (!!) the width, I could enlarge the exhaust port width to 70% just for the blowdown area... Just an idea.... :wasntme:

[Brosi]

Topic about ehaust port shapes with nice drawing by gerhard

And another one

[rasputin]

sehnenmaß

"chordal width".

r

[KTy]

Topic about ehaust port shapes with nice drawing by gerhard

And another one

Danke

:grins:

[100 spezial]

look avatar..

there arent holes

Bearbeitet 7. März 2006 von 100 spezial

[KTy]

@100 spezial : Very nice :love:

For those of you interested in some numbers and pictures,

PortMap HighRes: http://ktylife.free.fr/vespa/KTy133/200507...133.PortMap.JPG

Excel sheet fed with some number more or less exact: http://ktylife.free.fr/vespa/KTy133/KTy134.xls

[huegenbegger]

nice Excel sheet

How did you correct the steep upward angle of the boostport?

I made the same work yesterday and didn´t find any hint how to.

So I measured(in fact I calculated) the height of the Boost port rectangular to achieve the real port hight. I took the relation of measured to real hight and corrected the width (omg my english )

example Porthight: 10, real porthight by calculating with the upward angle: 5.

So I got (calc height)/(mapped portheight) -> factor 0.5

I multiplied the width of the port by the factor to correct the area.

MacDizzy did it by software but gave no hint about the formula he took, the result seems a bit odd to me.

I don´t think it´s meant to take the Boostport in account that much.

Afterwards I calculated the same values with bimotion, there´s not that big difference in the results of timearea.

Have You already any idea which range to take as targets? What about the blowdown area?

In your portmap I found the aux transferport having a bigger value ATDC, maybe You should correct the mean port area line of this port.

have fun decrypting

If anyone´s in need of Bimotion, I will send you, it´s not available any more and not allowed to be linked, but to give away person to person.

b

Cell N41 Bmep has to be "=N42/1.36/D30/D8*600000" isn´t it?

Bearbeitet 8. März 2006 von huegenbegger

[KTy]

nice Excel sheet

How did you correct the steep upward angle of the boostport?

I made the same work yesterday and didn´t find any hint how to.

So I measured(in fact I calculated) the height of the Boost port rectangular to achieve the real port hight. I took the relation of measured to real hight and corrected the width (omg my english )

example Porthight: 10, real porthight by calculating with the upward angle: 5.

So I got (calc height)/(mapped portheight) -> factor 0.5

I multiplied the width of the port by the factor to correct the area.

MacDizzy did it by software but gave no hint about the formula he took, the result seems a bit odd to me.

I don´t think it´s meant to take the Boostport in account that much. Afterwards I calculated the same values with bimotion, there´s not that big difference in the results of timearea.

First point is, I didn't correct the boost port area . Reading MacDizzy I was planning to do it, then I forget ! But, my understanding of this problem of steep angle is that it shouldn't change the timing nor the heigth of the boost port. A corrected factor should only affect the area, as few mixture can flow through this port... But that's my understanding ! I have also milled a lot this port even though it's still not much...

Have You already any idea which range to take as targets? What about the blowdown area?

That's where I'm stuck right now... I don't really know what to target in term of T.A values and BMEP... And anyway, my timing are already "extreme" (199/133/35), so the only thing I can try to optimize is the transfer width and of course, the exhaust width. As for the Blowdown Area, ... what is the question ? :plemplem:

In your portmap I found the aux transferport having a bigger value ATDC, maybe You should correct the mean port area line of this port.

Maybe I don't calculate well the mean line . I thought it was only a function of the transfer opening timing... As both primary and secondary transfer have the same height (there is a drawing mistake but I correct it), I thought they would have the same mean line... :wasntme: ?

If anyone´s in need of Bimotion, I will send you, it´s not available any more and not allowed to be linked, but to give away person to person.

I have the old DOS based version... I have to investigate more how to use it...

Cell N41 Bmep has to be "=N42/1.36/D30/D8*600000" isn´t it?

Hum.... I haven't made this sheet, so I assumed all formulas were ok !

Maybe you're right even though doing =N42/1,36/D30/D8*600000 gives strange results...

Hum.... I haven't made this sheet, so I assumed all formulas were ok !

Maybe you're right even though doing =N42/1,36/D30/D8*600000 gives strange results...

According to MacDizzy http://www.macdizzy.com/formulas.htm it is the correct formula...

[huegenbegger]

First point is, I didn't correct the boost port area . Reading MacDizzy I was planning to do it, then I forget ! But, my understanding of this problem of steep angle is that it shouldn't change the timing nor the heigth of the boost port. A corrected factor should only affect the area, as few mixture can flow through this port... But that's my understanding !

I think so too, thats why I took the hightfactor to correct the widht

I have also milled a lot this port even though it's still not much...

That's where I'm stuck right now... I don't really know what to target in term of T.A values and BMEP... And anyway, my timing are already "extreme" (199/133/35), so the only thing I can try to optimize is the transfer width and of course, the exhaust width. As for the Blowdown Area, ... what is the question ? :plemplem:

question ist the same as on the other TA targets, I have definitely no Idea (except of the chart in the escel sheet)

Maybe I don't calculate well the mean line . I thought it was only a function of the transfer opening timing... As both primary and secondary transfer have the same height (there is a drawing mistake but I correct it), I thought they would have the same mean line... :wasntme: ?

sorry, I didn´t get, that there´s a drawing mistake, then you´re right, it should be the same mean line

I have the old DOS based version... I have to investigate more how to use it...

Hum.... I haven't made this sheet, so I assumed all formulas were ok !

Maybe you're right even though doing =N42/1,36/D30/D8*600000 gives strange results...

According to MacDizzy http://www.macdizzy.com/formulas.htm it is the correct formula...

hmm the bmep for your engine: 135ccm at 8500rpm and 26hps should be 10 bar, the sheet told something about 147 that might be cool, but not that realistic.

I have the old DOS Version of Bimotion too, its not that bad. Bit tricky to use but helpful once you got it.

there´s a function "targets" in main menu. Enter this and your data, there it will show you youre bmep (you can choose which entry to make)

greetz, b

Bearbeitet 8. März 2006 von huegenbegger

[Brosi]

The BMEP is in psi. 1bar=14,5psi....

[oli-san]

135ccm at 8500rpm and 26hps should be 10 bar

Which 135ccm engine has 26 BHP at 8500rpm?

[KTy]

Which 135ccm engine has 26 BHP at 8500rpm?

Mine

Seriously, these data are pure speculation

question ist the same as on the other TA targets, I have definitely no Idea (except of the chart in the escel sheet)

So, does anyone have an idea on TA target ? :)

With the previous version of this engine, I already had *no problem* to pull all the gears with a 2.34 + zirri 4th + zirri silent.

I will rebuild it with a 2.56 primary with short 4th in order to have a really fun scooter...

[huegenbegger]

The BMEP is in psi. 1bar=14,5psi....

that explains a lot, I´m not used to psi, so I corrected the formular to bar

So, does anyone have an idea on TA target ? :)

Bimotion gives hints about TA targets, but I don´t know anything about the quality of these hints.

Bearbeitet 8. März 2006 von huegenbegger