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 Difference in port timings

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rbrown8828



Nombre de messages : 5
Localisation : US
Date d'inscription : 01/06/2019

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MessageSujet: Difference in port timings   Difference in port timings Icon_minitimeSam 1 Juin 2019 - 18:48

I am getting different port timings for the exhaust duration based on the exhaust opening at 89* ATDC
So.... 180 is 89 = 91 x2 = 182 degrees exhaust duration.

However, when using an online calculator by measuring from TDC of barrel to top of exhaust port (32.4358 mm) and the distance of TDC of barrel to piston crown (.3556 mm) I get 173.83 with a 54 mm stroke and 102 mm rod length.

Can anyone explain why these are different and which one is more accurate?
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JanBros



Nombre de messages : 250
Localisation : Belgique
Date d'inscription : 05/12/2011

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeSam 1 Juin 2019 - 21:45

both are correct according to my excel's, so the most probable answer is : the one you measured most acuratly is the most correct.
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Frits Overmars

Frits Overmars

Nombre de messages : 2246
Age : 71
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeSam 1 Juin 2019 - 22:29

rbrown8828 a écrit:
I am getting different port timings for the exhaust duration based on the exhaust opening at 89* ATDC. So  180 - 89 = 91 x2 = 182 degrees exhaust duration.
That is correct. But where does this 89° value come from? Did you hear it from the milkman, read it in the engine specs or measure it with a degree disc?
Citation :
However, when using an online calculator by measuring from TDC of barrel to top of exhaust port (32.4358 mm) and the distance of TDC of barrel to piston crown (.3556 mm) I get 173.83 with a 54 mm stroke and 102 mm rod length. Can anyone explain why these are different and which one is more accurate?
That is correct as well, assuming TDC lies 0,3556 mm below the cylinder top plane (this does not automatically follow from your description) and assuming there is zero piston pin offset. I assume you measured these values yourself, so I suppose these would be more trustworthy than the degree-value.
I just wonder: how on earth did you manage to establish so many decimals? Not even a conversion from imperial to metric units could give such precise results. I wish my measuring tools were this accurate...

PS: how about a little introduction? What are you working on at the moment?
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rbrown8828



Nombre de messages : 5
Localisation : US
Date d'inscription : 01/06/2019

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 0:58

Ok I think I see the problem now that you mention it. The 89* value comes from a spreadsheet that a friend gave to me. His values may be measured differently depending on the deck height values that were used. I am unsure at this time on how he came to those measurements, ill have to ask.

My measurements are from gauges and a micrometer :
TDC of barrel to exhaust roof = 32.44mm
Top edge of piston crown to TDC of barrel is .356mm

32.44-.356 = 32.08mm
which should put the crank angle at 93.08 deg ATDC
With a duration of 173.84 so that makes sense now.

Its from a direct drive karting engine 125cc and I was worried that the 119.5 transfer duration might be a little too short. The project has been gathering engine data to design an expansion chamber. Im trying to figure out a way to get a pipe to start work around 8,000 and peak around 12,000 rpm. However, I need the ability to over rev to 17,000. I know thats asking alot from a pipe but those are the typical rpm ranges on the local tracks.

My exhaust port opening at the flange is 35mm and the ID of the stock header is 43mm which I thought may be a little small. Maybe the manufactures did this to widen the power band. Not sure if trying a 38mm ID diameter would be worth trying.

Its been really hard to find any information on direct drive expansion chambers. Seems like most use perforated baffle cones that are enclosed inside another chamber before they hit the stinger. Its hard to know what will work best on a design like this.

Transfer angles point towards the back of the cylinder away from the exhaust opening. And there is only one on each side.
the exhaust is squarish oval with 2 auxiliary ports on the sides.



Any help on a design that would get me something close to those ranges from you guys would be greatly appreciated.

Here is the port layout. Its a pdf from another motor so the measurements are a little different but the layout is almost identical.

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Frits Overmars

Frits Overmars

Nombre de messages : 2246
Age : 71
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 2:07

rbrown8828 a écrit:
Its from a direct drive karting engine 125cc and I was worried that the 119.5 transfer duration might be a little too short. The project has been gathering engine data to design an expansion chamber. Im trying to figure out a way to get a pipe to start work around 8,000 and peak around 12,000 rpm. However, I need the ability to over rev to 17,000. I know thats asking alot from a pipe but those are the typical rpm ranges on the local tracks.....Its been really hard to find any information on direct drive expansion chambers. Seems like most use perforated baffle cones that are enclosed inside another chamber before they hit the stinger. Its hard to know what will work best on a design like this.
.... On the other hand it's quite easy to know what won't work. Been there, done that, and wept.
I once (in the 1970s) designed a proper pipe instead of those 'perforated horror pipes', for a 100cc direct-drive engine.
It produced so much more power that its driver could overtake at least two competitors on the straightaway lap after lap.
Minor problem: he lost three places coming out of every corner lap after lap because the rather effective new pipe not only gave more top end power; it also produced a huge torque dip at around 8000 rpm.
So those 'perforated horror pipes' weren't so bad after all. They are called Vevey pipes and you'll find their dimensions below.
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rbrown8828



Nombre de messages : 5
Localisation : US
Date d'inscription : 01/06/2019

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 2:51

So my question is should I base my pipe design on something with a good power curve close to what I need and then start adding holes to the baffle cone till i find the sweet spot? I would assume its far more complicated considering the volume in the back end of the pipe could effect everything.

Seems as if the perforated holes are in place to weaken the return wave and help with over rev, somewhat similar to what lengthening the belly would achieve. Have you had any experience with the vevey pipes? Or know of anywhere I can further educate myself on them?

I was thinking the size of the holes and how many could affect things differently even if the open area % was the same. For example one baffle cone would have a lot of 1 mm holes vs a different baffle cone with not as many 4mm holes but both having an equal open area percentage. In theory the small diameter holes would give a stronger return and maintain more pressure on header side of the pipe. Seems like maybe there would be some room there to gain some power somewhere without losing too much over rev. Maybe im way off here just thinking out loud.
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Frits Overmars

Frits Overmars

Nombre de messages : 2246
Age : 71
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 3:36

rbrown8828 a écrit:
So my question is should I base my pipe design on something with a good power curve close to what I need and then start adding holes to the baffle cone till i find the sweet spot? I would assume its far more complicated considering the volume in the back end of the pipe could effect everything.
Seems as if the perforated holes are in place to weaken the return wave and help with over rev,  somewhat similar to what lengthening the belly would achieve. Have you had any experience with the vevey pipes? Or know of anywhere I can further educate myself on them?

I was thinking the size of the holes and how many could affect things differently even if the open area %  was the same. For example one baffle cone would have a lot of 1 mm holes vs a different baffle cone with not as many 4mm holes but both having an equal open area percentage. In theory the small diameter holes would give a stronger return and maintain more pressure on header side of the pipe. Seems like maybe there would be some room there to gain some power somewhere without losing too much over rev. Maybe im way off here just thinking out loud.
It's not just the holes in the end cone; you could approach that with a shallower hole-less cone. The header diameter plays an even more important role. It's far too wide for pipe efficiency, but then that efficiency would turn against you below the power band. It's the same with the cylinder and the pipe. Just about everything on these direct-drive engines is 'wrong', but they are the end result of an awful lot of development work.  Improving on them won't be easy.

By the way: how direct is your direct-drive? The old 100cc engines had a sprocket fixed on the crankshaft and you cannot get more direct than that. But the 125cc engines I know had a centrifugal clutch between crankshaft and sprocket. Those clutches were tested at scrutineering: they had to start grabbing at no higher than 5000 rpm. But when lubricating the chain on the start line, one could accidentally spray the clutch instead of the chain which would result in an amazing start....
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rbrown8828



Nombre de messages : 5
Localisation : US
Date d'inscription : 01/06/2019

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 4:05

Yeah the clutches seem to grab almost immediately. So when you say shallower hole-less cone you mean shorten it up as far as length. Effectively increasing the angle using the same size diameter on the small end? And if shorter where would a good starting point be? 20 mm at a time? Also leave all the other lengths the same? Seems like that would kill the over rev.

If I can find something that is pretty wide on the powerband with a little over rev then I could start dropping teeth on the sprocket until it causes problems in the corners. That would allow me to stay closer to pipe power on the long runs. I just dont know if thats going to be realistic with an expansion chamber capable of that wide of a band.

I plan to build several pipes to see if i can get anything to produce better laps times. I went another route and shortened the belly from 110 to 60 mm. Lengthened the diffuser 20 mm and added a little bit of header length. I think i ended up adding 10 mm to the header. I moved the baffle cone inward the 50 mm which is what made the belly shorter. I left the end chamber and stinger location where it was originally. So now it has a little more volume back there. Probably not the right direction to go but that is my first pipe for testing which will be tomorrow.
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Frits Overmars

Frits Overmars

Nombre de messages : 2246
Age : 71
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 4:20

A shallower cone would be longer, not shorter. But I'm afraid you missed the point I was trying to make: everybody and their mother has worked on these engines and pipes for decades, so chances of improving them are almost non-existent.
But go ahead, try your ideas and you will gain experience. My advice for now: focus on the header dimensions.
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rbrown8828



Nombre de messages : 5
Localisation : US
Date d'inscription : 01/06/2019

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MessageSujet: Re: Difference in port timings   Difference in port timings Icon_minitimeDim 2 Juin 2019 - 5:20

I understand. Thanks for taking the time and giving me a direction. Its probably a lost cause but like you said maybe ill get some experience and learn something along the way. Cheers!
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