[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked)

Mic a écrit:

Frits I tried your FOS pipe calculator using RSA125 values and ended up with this...

Frits Overmars a écrit:

I repeat: these exhaust calculators were put on the internet by several people who based their programs on my 'simple exhaust concept'. I take no responsibily for these programs and any questions about them should be addressed to their makers, not to me.

Mic a écrit:

The total length looks rather small compared to the tubo 102 which had a total length of 803mm..... with speed of sound set to 587m/s the length fits with tubo 102 but are there some guidelines for estimating the correct value?

Frits Overmars a écrit:

The FOS exhaust concept is only meant to help beginning tuners on their way. A lot of important factors, like compression ratio, ignition timing, type of fuel, carburettor diameter, crankcase volume and angle.areas, are not taken into account. Instead of all those factors that I left out, I included one variable, the speed of sound.
Starting with 550 m/s will get you in the right ballpark, after which you can vary this value according to your findings.

Anyone else with questions to which you can find the answers on the previous pages? I am not writing french, am I ?


I think I need a holiday . 

I know that the multi stage difusor wasn't in your exhaust concept.
Sorry i should have made that clear.

I understand your other explanations.
Thanks for the information.

I think
" some knowledge about engines and physics" is a big understatement.  

Sander

Manuel Rainer a écrit:

frits when you calculating the angle.area you take the effective area of ​​the channel and not the area on the circumference. is there a regular relation in which the exhaust area should stand to inlet area and in which relation to the blowdown?

I do not quite understand what you mean by 'the area on the circumference', Manuel.

A port, 20 mm high and 20 mm wide, has the same area as a port 10 mm high and 40 mm wide, but the first port has a lot more angle.area. So instead of looking for relations between areas, you'd better concentrate on relations between angle.areas.

There is a simple fixed relation between blowdown angle.area and inlet angle.area: both need to rise linearly with rpm.
There is no regular relation between blowdown angle.area and exhaust angle.area. The blowdown angle.area is all-important while the angle.area of the total exhaust port hardly matters.

Measuring the port's circular area vs. its chordal

ambike is David McNeal in DFW, Texas.

It is a pleasure to be here.

Please add my Thank You to Mr. Jan Thiel, to Mr. Frits Overmars, and to all others for their time, information, & participation in such an excellent thread.

ambike a écrit:

ambike is David McNeal in DFW, Texas. It is a pleasure to be here. Please add my Thank You to Mr. Jan Thiel, to Mr. Frits Overmars, and to all others for their time, information, & participation in such an excellent thread.

Hi David, nice to have you here. Now I understand what Manual meant: radial outflow versus parallel outflow.
Radial outflow offers more area, provided that the exhaust duct permits it. I certainly do take this into account when calculating angle.areas but I did not mention it here because I wanted to keep the angle.area explanation simple.
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hi frits let me ask something. i hope it a quick answer so you dont need to type much  ,

anyways i was reading some history about the 250cc engine mostly used in dirtbikes. prior to year 2000 all manufacturers (ktm,honda,kawasaki,suzuki,yamaha) were using various bore/stroke combinations. almost like they couldnt make up their mind.  in 2000 the manufacturers all adopted the 66.4 x 72 combo. did a light bulb shine on their head suddenly and they realized something good with this exact ratio ?  further more, is there some advantage to the mostly large undersqaure ratio ? ( they could of easily just all went to 68 x 68 ).  and 'if' theres some good advantage then why didnt 125cc use large undersqaure also ?

brokedown a écrit:

... prior to year 2000 all manufacturers (ktm,honda,kawasaki,suzuki,yamaha) were using various bore/stroke combinations. almost like they couldnt make up their mind. in 2000 the manufacturers all adopted the 66.4 x 72 combo. did a light bulb shine on their head suddenly and they realized something good with this exact ratio ?

It would seem so, wouldn't it?

Citation :

is there some advantage to the mostly large undersquare ratio ?

Keep reading, Brokedown, and you will find answers to a lot of your questions, for instance here:
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Citation :

'if' theres some good advantage then why didnt 125cc use large undersquare also ?

Good question. We just never got round to investigating it to the last detail. Too many other things to try....

thnxs frits. the disadvantaqe of over sqaure is pretty clear to me. the undersqaure is not as clear  but ill keep reading to find the answer. yet to this day on other sites people will take a slightly over sqaure engine and shorten the stroke even more. please dont ask me why

something else im working on. 72x72 case reed engine. starting pipe diameter is 40mm. unfortunatly the exh flange in part of the cylinder casting. so i better think how to install new flange perhaps closer to 50mm  

brokedown a écrit:

......  in 2000 the manufacturers all adopted the 66.4 x 72 combo.

Honda started the trend late 80's if im not mistaken,the rest folowed as usual...

Frits Overmars a écrit:

ambike a écrit:

ambike is David McNeal in DFW, Texas. It is a pleasure to be here. Please add my Thank You to Mr. Jan Thiel, to Mr. Frits Overmars, and to all others for their time, information, & participation in such an excellent thread.

Hi David, nice to have you here. Now I understand what Manual meant: radial outflow versus parallel outflow.
Radial outflow offers more area, provided that the exhaust duct permits it. I certainly do take this into account when calculating angle.areas but I did not mention it here because I wanted to keep the angle.area explanation simple.
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Hi

Thanks david to explain it better what i mean.

thank Frits so you have to look how is the shape of a channel and than take radial outflow or parallel outflow.
in your image with this form of channel you had to take radial outflow. but iff you look at this picture you then must take parallel outflow but what wide? the red marked, or the green that stands 90° to the channel side?

thanks manuel
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Green. When in doubt, try to imagine pushing a sphere through it.

XpTpSMTT a écrit:

brokedown a écrit:

......  in 2000 the manufacturers all adopted the 66.4 x 72 combo.

Honda started the trend late 80's if im not mistaken,the rest folowed as usual...

yes honda was first around '86. eventually years later all other manufacturers stopped at 66.4x72. perhaps its a magic number

hey guys i have something since theres been pipe discussion in the last page or so. have you seen sometimes the first section of pipe after the cylinder will change colors usually to blue i think. im guessing its from the exh gas heating the metal to a certain temp but i dont know the exact temp. but anyways have you seen the stinger pipe section change to this blue color also ? would that indicate maybe its diameter is to small or its ok ?

brokedown a écrit:

XpTpSMTT a écrit:

brokedown a écrit:

......  in 2000 the manufacturers all adopted the 66.4 x 72 combo.

Honda started the trend late 80's if im not mistaken,the rest folowed as usual...

yes honda was first around '86.  eventually years  later all other manufacturers stopped at 66.4x72. perhaps its a magic number

MX 250 engines need power at low rpm and smooth power delivery.The best way to do this without using power valves is to keep the exhaust port timing low.The best way to increase power while maintaining these characteristics is to increase the stroke which gives more time area for the same port timings.I think the Maico was the first undersquare 250 MX engine at 67x70.Suzuki followed in 1977/8 with the same bore and stroke as Maico but then switched back to a 64mm stroke!The 1981 Honda CR250 was 66x72.I don't think there is anything "magical" about these dimensions as they probably are the best compromise.If the stroke is too long piston speed can become too high for acceptable piston life and also it becomes more difficult to get a high enough crankcase compression ratio which is probably why Honda used a relatively short 125mm rod in the CR.

CRECY a écrit:

.I don't think there is anything "magical" about these dimensions as they probably are the best compromise

i could believe that. seems they were changing something nearly every year. whether it be chassis, airbox, suspension, ignition, head, reedvalve, etc etc etc. always looking for a better compromise i guess  

hi manuel and frits. in the pic you can see also the yellow line. is it still best to use the measurement of the green line ?


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brokedown a écrit:

in the pic you can see also the yellow line. is it still best to use the measurement of the green line ?

Yes. The length of your yellow line is not restricted at its upper end.
Beyond the green line the inflowing charge can expand.

hi frits i hope you dont get sick of alot of questions  

but i have a strange situation and maybe you have seen it before. in the pic you can see the yellow line which represents a short circuit between crankcase and pipe , slightly before/ during/ slightly after TDC. this cylinder is unmodified right now. but i have modified this type of cylinder in the past and always just enlarged the aux port in the upper portion so the shortcircuit hole didnt get any bigger. but im wondering now if that was the best decision. unfortunatly there is no alternative pistons available with longer side skirting.

since this is a situation of best compromises what do you think is the better option ?

a) only enlarge the port in the upper portion so the shortcircuit doesnt get larger than it already is ?
b) enlarge the port in all directions because the increased amount of blowdown is more important than the negative affect of a larger shortcircuit hole ?
c) weld the shortcuirt hole closed and only enlarge the port in the upper section ?



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brokedown a écrit:

.. in the pic you can see the yellow line which represents a short circuit between crankcase and pipe , slightly before/ during/ slightly after TDC.

Old KTM, is it?

Citation :

since this is a situation of best compromises what do you think is the better option ?
a) only enlarge the port in the upper portion so the shortcircuit doesnt get larger than it already is ?

That can't hurt, provided you don't enlarge it so much that you create a leak path via the piston pin bore.

Citation :

b) enlarge the port in all directions because the increased amount of blowdown is more important than the negative affect of a larger shortcircuit hole ?

That is impossible to predict without knowing everything about engine and pipe.

Citation :

c) weld the shortcuirt hole closed and only enlarge the port in the upper section ?

Plaster it closed and take it to a dyno before you start welding.

ya old ktm. im not sure if the engineers didnt notice or perhaps just didnt care. or maybe they were fully aware and knew it might be alittle power loss but didnt consider it a large issue in the big picture of everything. a couple hp loss in a 50hp 500cc recreation mx bike probly wasnt a big concearn i suppose.  but its a shame because theres alot of valuable blowdown in that lower portion. i thought of raising the cylinder a couple mm, which would nearly close that hole and allow the lower portion to be alittle bigger but since the transfers enter the cylinder at 7* upward, raising the cylinder would screw up my plan to put more upward angle on the transfers. so raising cylinder 2.5mm would put transfers at 130, pretty much close the hole and allow alittle more blowdown in the lower portion but i wouldnt have any good way to raise the transfer axial angle above 7*, unless with epoxy inside the duct at the upper part of the outer wall. but i dont think thats a good idea since the ducts dont appear to be of good design on the inside wall. frits im sure over the years you also seen a few bad design engines ?

brokedown a écrit:

frits im sure over the years you also seen a few bad design engines ?

Yes I have, but I must add that some designs that we frown upon now, didn't seem so bad at the time. We learn every day.
This is why I hate to be asked to improve on an old design. I don't fancy dealing with problems that have since been solved, like drum brakes, contact breaker points, fixed ignition timing, air cooling, sleeved cylinders, short-stroke two-strokes, short con rods, pistons with multiple rings, carburetters etc.

Hello, I would like to know your opinion on the form of the crank webs.
I'm trying to design a crankshaft that could improve the flow. Unfortunately we are working on carters that weren't designed for performance, so we try to do as best we can. Here are some pictures of what I mean:

This is more or less the usual situation; the crank webs partially block the flow from the reed valve:
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What I would like to try:
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At least from a simplistic point of view, seems that the flow would improve by some amount. So my question is, do such "chamfered" crank webs have any negative effect on the gas dynamics that happen in the carter? The carter volume shouldn't be a problem, until now, every increase in carter volume had positive effects on performance.
Any thoughts appreciated.

roost a écrit:

.... do such "chamfered" crank webs have any negative effect on the gas dynamics that happen in the carter? The carter volume shouldn't be a problem, until now, every increase in carter volume had positive effects on performance.

There may be a positive effect on inflow and there will certainly be positive effects on big end lubrication and cooling, and on mixture homogenisation. And you already discovered the positive effect of increasing the crankcase volume.

frits & roots - maybe a backdraw of this design is the weakened press fit for the big end pin?