Let's Talk Transfers

[paragonbuilder]

What's the difference in the timing on the 64cc vs the 79cc jug?

It's the same case, right?

I've got a 6400 here I can get some timing numbers if you want? I don't have a 7900 though...


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[Mastermind]

The 6400 jug doesn't have the long blowdown time that the 7900 had. I tried 7900 numbers in one after machine work, and it was a turd.

 

[Backyard Lumberjack]

<snip> . The swept volume of the engine is from the point of TRANSFER CLOSING to TDC. If you raise the transfers you are effectively shortening the STROKE.

I read this at one porter's site: Swept Volume - Volume of cylinder with piston at exhaust port opening to TDC. (4-stroke would be volume/cc's displaced by piston from BDC to TDC.)

this is my understanding, too. at transfer closing, the exh port is still open. on 044 bit better than 1/8th" closer to 3/16ths...

can you clarify your definition a bit further?

 

[drf256]

It makes complete sense that the 7900 jug has less transfer duration.

It has a proportianally smaller case for its displacement than the 6400. It's prolly got bigger jug tunnels. So it squirts the charge out under greater pressure.

What about the exhaust Randy? Is it more duration on the 6400?

 

[Mastermind]

I don't have notes on the 6400 Al. No one seems to want those ported.

They have three open transfer ports on each side, so case volume is larger than on the closed port configuration on the 7900.

 

[Backyard Lumberjack]

Really interesting thread guys thanks for all the valuable input

it is... think now, I might just odda set my cyl up with a degree wheel... and carry it all one step further... and read the numbers. see how the numbers match up to these theories and practical applications mentioned in this thread... i'd like to see 'my numbers' !

 

[Mastermind]

Here's a 6400 jug.

 

[drf256]

Wow.

What are the entry angles like on the three ports.

 

[Mastermind]

Pretty flat on the roof, and higher on intake side tapering down to exhaust.

 

[tree monkey]

I read this at one porter's site: Swept Volume - Volume of cylinder with piston at exhaust port opening to TDC. (4-stroke would be volume/cc's displaced by piston from BDC to TDC.)

this is my understanding, too. at transfer closing, the exh port is still open. on 044 bit better than 1/8th" closer to 3/16ths...

can you clarify your definition a bit further?

swept volume is bdc to tdc on any engine, no?
traped volume is ex port just closing to tdc on 2 cycle , and valve closing to tdc on 4 cycle, no?

 

[paragonbuilder]

So if I'm understanding the theme here correctly, for a long while the thinking was take basic 2-stroke theory and throw it away for chainsaws.
Now it looks like we are headed back in that general direction.
Seems like a lot of things work because they are adding time/area that was left on the table by the engineers, it's just bring achieved in many different ways.
If finger and bridge ports are giving you gains, then it may be possible to get the same gains by working the transfers a bit more. But in a 2 port jug it may be hard to get enough time area with out compromising ring support or compression.

Am I on the right track with this???


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[paragonbuilder]

Here's a 6400 jug.

View attachment 11234

Wow! Those tunnels are deep! Is that to add case volume. Seems they could be much shallower with the size of the port opening.
I didn't realize it had 6 ports.
I gotta pull mine apart and study it. It's s strong saw from the factory. They got something right on it!


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[Backyard Lumberjack]

swept volume is bdc to tdc on any engine, no? traped volume is ex port just closing to tdc on 2 cycle , and valve closing to tdc on 4 cycle, no?

sweep volume: you are right! according to:

https://en.wikipedia.org/wiki/Engine_displacement

thanks for the clarification(s)

 

[jmssaws]

Looks insane to me but I only work on stihls and a couple quad huskys

 

[tree monkey]

lol not much velocity in those transfers.

 

[paragonbuilder]

lol not much velocity in those transfers.

Right! But this is a very strong saw in stock form. Heavy, but strong. So this is an instance of volume trumps velocity?


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[Magic_Man]

Here's a 6400 jug.

View attachment 11234

Kind of remind me of my old poulan transfers.

 

[drf256]

swept volume is bdc to tdc on any engine, no?
traped volume is ex port just closing to tdc on 2 cycle , and valve closing to tdc on 4 cycle, no?

Thanks for the correction. Trapped not swept volume.

 

[paragonbuilder]

So with the 6 transfers, I'm guessing they are physically lower? More time/area, so no need to take them as high.
So by taking them up to 7900 numbers there was too much time area?
Maybe this is how they can keep compression so high. My 6400 has 205 psi and it's bone stock!


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[jmssaws]

I've got my 4200 apart and it's just like those. Reed valve motor though.