Transfer port size !

[tokoloshe]

I have recently been reading all i can find about transfer ports (more interested in PCP though) and have some thoughts:

the idea of a venturi is an interesting one (I get involved with them in my work) but here I dont think you could get them to work as its the increase in velocity through the narrowing of the venturi that produces a pressure drop at that point and thus creates suction which is used to draw more air, here you have an initial increase in pressure with little or no flow so air would pass put of the venturi, which means you would need a pretty efficient none return valve on the suction leg?

One thought that did occur to me is why can you not have a small cylindrical secion on the end of the piston that is sligthly smaller than the port so that it fills the port as the piston reaches the end of its travel which means no where for the air remaining in the cylinder to go so cushioning the piston may be it could seal maybe it could be sized to create some air passage?

I have also been reading some conflicting info on polishing the ports, I assume this is done to "ease" the path of the air? there is some thinking that this is a bad thing as it leads to laminar flow whereas turbulent flow is preferable? what about a rifled transfer port to impart spin to the air?

Been working on this idea for the last month with some interesting results. Still work in progress as I have still to get through various combinations of length/diameter and shape of 'rods' that enter the port. Using a nose extension with 4mm hole in center to accept threaded rods. My TP is 3.9mm dia and initially started using M4 nylon bolts reduced to 2mm for fear of a nasty thwack and one knacked compression tube. Am confidently using M4 brass bolts now and have had them full length of transfer port at 3.4mm in dia.
One reason I wanted to do this was to try and work out where the piston stops and hence cylinder gap. Another reason was to see if I could eliminate bounce on ultra light pistons. Results thus far indicate, on my TX, the piston stops somewhere around 1.8 to 3 mm but need to do further tests. Also want to try with different strokes and piston weights - so many variables! One 'rod' I tried changed my aim points quite substantially at the longer ranges, accuracy bombed after about 30 yds so shape is another headache!
Anyway, it's all fun and keeps me busy when the telly is crap or kids/wife doing my head in!

[bigtoe01]

what you need is an inertia driven transfer port tube thingy that on full extension actually comes thru into the barrel the depth of the transfer port.

That way you get maximum compression from the piston, as it stops dead the extension tube shoots forward, being hollow to the depth of the transfer port it captures all the air it is holding and further compresses this air as it shoots forward into the transfer port. The trick would be a slight taper going to parallel to it self centres, o ring round its base so it seals into the transfer port or have an O ring set into the transfer port at the barrel end or just have a breech seal that seals onto the outside of the extension tube.

If you were clever with the design of this you could make it to it was a surge damper also ..Like a harper valve on a springer LOL

[Lol Moore]

Been working on this idea for the last month with some interesting results. Still work in progress as I have still to get through various combinations of length/diameter and shape of 'rods' that enter the port. Using a nose extension with 4mm hole in center to accept threaded rods. My TP is 3.9mm dia and initially started using M4 nylon bolts reduced to 2mm for fear of a nasty thwack and one knacked compression tube. Am confidently using M4 brass bolts now and have had them full length of transfer port at 3.4mm in dia.
One reason I wanted to do this was to try and work out where the piston stops and hence cylinder gap. Another reason was to see if I could eliminate bounce on ultra light pistons. Results thus far indicate, on my TX, the piston stops somewhere around 1.8 to 3 mm but need to do further tests. Also want to try with different strokes and piston weights - so many variables! One 'rod' I tried changed my aim points quite substantially at the longer ranges, accuracy bombed after about 30 yds so shape is another headache!
Anyway, it's all fun and keeps me busy when the telly is crap or kids/wife doing my head in!

Another concept (though more complicated) in conjuction with the inserted barrel idea of Tony L (Can a break barrel ever be as efficient as a sliding breech springer) is to have a piston with a recess in the end which passes over the barrel which is inserted through the cylinder end wall, as the piston aproches the end of the travel it seals on the barrel outer thus creating a toroidal area of trapped air between the barrel outer and the cylinder walls/end to act as a buffer

It may be possible to set so as to stop bounce back and slow the piston relatively gradually, it could also mean that as the pellet is travelling up the barrel the air behind it is trapped though this may be a disadvantage as there may not be enough energy to keep it accelerating?

all my thinking is currently conceptual as i really dont want to do any maths I am not being paid to do

[bigtoe01]

Another concept (though more complicated) in conjuction with the inserted barrel idea of Tony L (Can a break barrel ever be as efficient as a sliding breech springer) is to have a piston with a recess in the end which passes over the barrel which is inserted through the cylinder end wall, as the piston aproches the end of the travel it seals on the barrel outer thus creating a toroidal area of trapped air between the barrel outer and the cylinder walls/end to act as a buffer

It may be possible to set so as to stop bounce back and slow the piston relatively gradually, it could also mean that as the pellet is travelling up the barrel the air behind it is trapped though this may be a disadvantage as there may not be enough energy to keep it accelerating?

all my thinking is currently conceptual as i really dont want to do any maths I am not being paid to do

would it not be easier to have a step in the piston body, much like you see on a Theoben gas ram piston, however instead of the inertia weight getting in the way of the spring (On a springer) have the weight on the outside of the piston, with O ring seals to the piston body and the inner face of the compression cylinder..so as inertia drives the weight forward it is compressing air at the same time...so acting as an air brake?