Part Three: The Transfer Ports

[Mastermind]

Think I'll STFU now...feel like I'm asking too many questions

No. Ask more questions.

 

[Adirondackstihl]

Ask more questions.

Why you not rent pigs?

 

[Mastermind]

Why you not rent pigs?

Because I don't have any.

 

[Stump Shot]

It was mentioned in thread 1 that exhaust port timing had a relationship to the amount of cc's displaced. Does the amount blow down have this same corelation? I've heard the number of twenty degrees thrown around in the past. Is that a generic number like .020" squish band clearance? Which I believe, really is wrong for every size engine as well.
Isn't it also time for thread number four to come along?

 

[Stump Shot]

Part four did come along. http://opeforum.com/threads/part-four-compression.7318/#post-387683

 

[srcarr52]

Hmmm...
Air Fairies..

View attachment 68691

It's an industry nickname for people who do Computational Fluid Dynamics work. I'm sure they also have misnomer for my realm of work.

 

[Dub11]

@Dub11 has that saw now.

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Yup it ran fantastic too and thats why I gifted it to my ol buddy that was looking for a jred

 

[t4driller]

What about stratos and how they work? I'd like to here some ideas/ theories on them.

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[Hedgerow]

It's an industry nickname for people who do Computational Fluid Dynamics work. I'm sure they also have misnomer for my realm of work.

Turd polisher?
Metal Jockey?

 

[Chainsaw Jim]

The way I'm looking at it, the velocity through the transfers is more so coming from a negative pressure vacuum and with that source being so close the turbulence characteristics are not quite the same as general air flow. I think this is why the carb and intake play such a huge role in transfer size and shape.

When you blow a pressurized stream of air it builds its own momentum and will continue until laws of motion play their role to slow it down, which in a two stroke would blow the entire fresh charge right out the exhaust regardless of soundwaves. When you vacuum that same volume of air the momentum doesn't occur and the air flow stops the instant the pressure equalizes. And allows the soundwaves to work.

 

[Fruecrue]

I'd be very interested in seeing smoke flow through transfers. I've seen water used to demonstrate but not smoke.

 

[wcorey]

The way I'm looking at it, the velocity through the transfers is more so coming from a negative pressure vacuum and with that source being so close the turbulence characteristics are not quite the same as general air flow. I think this is why the carb and intake play such a huge role in transfer size

Unless I'm missing you're point entirely... Sounds like you're thinking more in 4 stroke terms where the intake charge is pulled in through the port by the negative pressure from the combustion chamber side of the decending piston.
The 2 stroke intake charge is pushed through the transfers with positive pressure from the crankcase side of the decending piston.

Edit; Maybe you're talking about the stratos?

 

[Deets066]

The way I'm looking at it, the velocity through the transfers is more so coming from a negative pressure vacuum and with that source being so close the turbulence characteristics are not quite the same as general air flow. I think this is why the carb and intake play such a huge role in transfer size and shape.

When you blow a pressurized stream of air it builds its own momentum and will continue until laws of motion play their role to slow it down, which in a two stroke would blow the entire fresh charge right out the exhaust regardless of soundwaves. When you vacuum that same volume of air the momentum doesn't occur and the air flow stops the instant the pressure equalizes. And allows the soundwaves to work.

So it's not the piston coming down and compressing the mix in the case to give the velocity? It's vacuum?

 

[paragonbuilder]

So it's not the piston coming down and compressing the mix in the case to give the velocity? It's vacuum?

If that was the case, I think case capacity would be much less of a factor, and the bigger the uppers the better. But that doesn't seem to be true... at least not always...


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[srcarr52]

Turd polisher?
Metal Jockey?

Probably like tire humper, or digital cowboy.

 

[Chainsaw Jim]

So it's not the piston coming down and compressing the mix in the case to give the velocity? It's vacuum?

At high rpm. I think the down stroke compression plays more into making a break between each charge to prevent flood limiting.

 

[RIDE-RED 350r]

I don't think vacuum plays a role in transfer. With the exhaust port open, how would vacuum build above the piston to pull the transfer charge into the chamber? Without adequate blowdown, exhaust gases will back feed into the transfers, that can't happen if there is a vacuum generated above the piston as it is driven downward by combustion...... Just thinking out loud

 

[Hedgerow]

So it's not the piston coming down and compressing the mix in the case to give the velocity? It's vacuum?

Hence the term "scavenging".
Better scavenging makes for a better pump.
Degrees of CC compression needs a head start moving stuffs. Otherwise it's gotta do all the work.

 

[Chainsaw Jim]

With the exhaust port open, how would vacuum build above the piston to pull the transfer charge into the chamber?

How would it pull without it? The escaping gases burn the oxygen and created a very strong vacuum. Something has to fill the void.

 

[Mastermind]

It was mentioned in thread 1 that exhaust port timing had a relationship to the amount of cc's displaced. Does the amount blow down have this same corelation? I've heard the number of twenty degrees thrown around in the past. Is that a generic number like .020" squish band clearance? Which I believe, really is wrong for every size engine as well.
Isn't it also time for thread number four to come along?

The numbers I posted were just a little something to consider. Maybe a good starting point on a work saw.

Blowdown is a tricky thing. Take a 7900......it likes about 30° of BD.......and some two port Stihls run great with 16°.