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Thread: Transfer port length, what's the best?

  1. #1
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    Transfer port length, what's the best?

    Is it the shorter the better, is there an optimum? Diameter not a consideration at this point but let's sat 3,2mm for the moment. Reason for the question is that I am drawing up my own break barrel Air arms seeing as they are dragging their feet!!
    Plinkerer and Tinkerer

  2. #2
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    tinbum is offline Killer Vampire Lesbians on scooters
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    It is dependant on the diameter as it's the volume that matters. Search the forum for "SCR" or static compression ratio. It's all relative.
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    Hi Tinbum, yes I know the scr has to be right but is shortest the best, diameter can be changed to achieve the desired scr. Current design has a 3mm long port but things would be easier if I went to 6/7mm but I don't want to compromise if its better at 3mm.
    Plinkerer and Tinkerer

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    I have experimented , in my tx , with a 21mm diameter comp tube . Down to 4mm long , i have found gains down to 6mm, shorter than that not so much, as i say that's with 21mm dimeter comp tube , will be different at other tube diameters .

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    Quote Originally Posted by maximus View Post
    Hi Tinbum, yes I know the scr has to be right but is shortest the best, diameter can be changed to achieve the desired scr. Current design has a 3mm long port but things would be easier if I went to 6/7mm but I don't want to compromise if its better at 3mm.
    According to more modern thinking, the ultra short port may not be the Holy Grail. In my view 6mm is short enough and you still have the benefit of more "meat" at the cylinder end.
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    This is the sort of info I was after, I have read that longer ports produce a softer shot cycle but good too long and efficiency is compromised.
    Plinkerer and Tinkerer

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    first question - what kind of shot cycle do you want ? smaller/longer-softer - but obviously that can go too far and lose efficiency quite quickly.

    ballpark I'd be looking around the range of 3.5mm x 6mm to 3.0 x 10mm
    Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.

  8. #8
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    I think I will go for 7/8mm. May even look at interchangeable transfer ports for experimenting.
    I started drawing it up last year and only picked it up again yesterday. So far have done the breech Jaws and cylinder, breech block and barrel interface. I even bought a second hand TX so I could measure up for swapping parts over. Looking to use the piston, trigger assy and stock.
    Plinkerer and Tinkerer

  9. #9
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    Quote Originally Posted by maximus View Post
    This is the sort of info I was after, I have read that longer ports produce a softer shot cycle but good too long and efficiency is compromised.
    That is illustrated well in the Gamo Paratrooper repeater, massive spring power, but low ft.lb. energy out the barrel.


    Baz
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