Formula for piston size, port size, caliber?

[dkwflight]

Hi
My experiment now has me wondering.
Is there a fomula for the piston size and port size for a specific caliber.

Or is all guesses and experience?

[rsterne]

More than likely experience of who did the original design with possibly some experimenting thrown in.... Then in the case of clones probably just copying the original.... In many cases, the port size doesn't change even when they change the caliber, which may or may not be correct.... Then throw in different pellet weights and that's yet another variable....

Most experienced shooters don't fool around with the transfer port as the chance of losing performance, and even damaging the gun, is greater than the likelihood of any significant gain.... as you already found out.... Having said that, if I were going to play with it, I'd drill out and tap the transfer port for screw-in inserts that could be easily changed.... Even that will only get you close, because changing the taper on both sides can change things as well....

The Cardew brothers published one set of experiments where they found the optimum size was 1/8" but that was for a gun that only shot 428 fps in .22 cal.... A 1/64" change either side of that only lost ~3-4 fps.... but a 1/32" change lost 5-8 fps, and a 3/64" larger port lost 14 fps and a 1/16" larger port (ie 3/16") resulted in the piston hitting the end of the chamber.... 3/64" smaller lost 40 fps, but that port was only 5/64" which is tiny.... The point was that there was indeed an optimum but that a 15% change either side made very little difference.... Greater changes made BIG differences in either velocity and/or longevity....

Piston size (actually the swept volume) determines the maximum potential performance of the gun.... Some numbers I calculated just the other day indicated that typical Springers can produce about 1 FPE for each 3cc of swept volume.... or about 5.5 FPE per cubic inch.... Several designs seem to have about the same numbers, so I can only assume that is a good balance between smoothness and power....

Bob

[K_sqrd]

Here is a link that may be of interest to you...

https://www.pyramydair.com/blog/2010/02/changeable-air-transfer-ports/

[rsterne]

Interesting that roughly the same dimensions worked on a 19 FPE .22 cal R1 that Cardew's used on a much less powerful gun (~ 6FPE) many years previously!.... 1/8" to 9/64" (3-3.5 mm) worked well in both cases....

Bob

[dkwflight]

Hi

Interesting the relationship.
I have nee thinking that this an area that nneds more time and thought.
Thanks for the link to the article

[rsterne]

BTW, just measured the transfer port on my .22 cal B-26.... it's 3mm (0.118")....

Bob

[RedFeather]

Curiously, has anyone seen a trans-section of the transfer port?  I'll see if I can dig up the picture.  Not at all what you guys might think.

OK, took a while but I found it. 



Please note the transfer port on this 34.  While you can bore it out at the pellet side all you want, you will find that it does not effect 90% of the port.  The Cardews found that many guns had flat transfer ports on the piston side and a slight chamfering increased velocity.  However, some manufacturers already did this.  Not so sure about today but I'll bet many are.  What you end up doing by increasing the hole on the port side is introduce a little vortexing.  Air's a fluid and the shape of the port can increase or decrease efficiency in ways that seem contradictory.  For example, certain shapes tend to create turbulence that impedes air flow even though you would think they ought to promote it.  Others that look "wrong" have the opposite effect.

Something that the Cardews also discovered is that a long transfer port becomes less efficient since it provides more space for the combustion to take place in and, hence, a lower resulting pressure.  Opening the port too much on either side adds to that volume.

But, they are your guns so have fun.

Oh, and that apparently inefficient offset port up there really isn't. 

[dkwflight]

HI

so far my limited examples of spring guns use straight holes.
I have been thinking that the hole act like a valve under the high speed increase in pressure.
It is possible there is a hypersonic flow for a short time.
The hole might be improved by polishing and rounding the piston side to promote smooth flow.

The pressure time values could be studied with the proper instrumentation. Beyond me at this time.

[rsterne]

I've seen "drawings" of various designs, just a drilled hole, both ends tapered (countersunk/beveled), only the breech end tapered, straight and on the centreline of the chamber, straight and offset above the center of the chamber but in line with the barrel (my B-26 is that way), angled from the centerline of the chamber upwards to the bore.... I would think the design varies gun to gun, no?....

Bob

[rsterne]

I hadn't seen one drilled from both ends before like the one in that photo.... The angled one I saw was straight but angled (in the drawing anyway).... My B-26 is parallel to the bore, straight, no kink, and offset upwards from the center of the chamber....

Bob

[dimitri]

-------------|
                 |___________
A1              ___A2______
                 |   V2
-------------|
V1

Calculation is simple: A1v1=A2v2

[Pete]

-------------|
                 |___________
A1              ___A2______
                 |   V2
-------------|
V1

Calculation is simple: A1v1=A2v2

OK... give us a working example of all yer A & Vs

A = area?
V = volume ?
V = Velocity ?

[dimitri]

a=area; v=velocity

[dkwflight]

Hi

I have been thinking about these guns.

What I am thinking now is the small port delays pressure build up and when the piston flies to the port the pressure flow to the base of the pellet is relatively slow and low pressure until the wave front builds to a point when it bursts through the port to the base of the pellet like an explosion.
in an oversize port the air flows too easily through the port allowing a slow build up of pressure at the base of the pellet, the pellet starts too early and does not build the speed the original constricted port does.

[rsterne]

Cardew's claim that up to 60% of the power of springers that are over 500 fps is created by the combustion of some of the thin film of lubricant on the wall of the chamber.... If the pressure rise is too slow, the temperature needed to create that combustion would not occur.... Springers are not quite as simple as people think....

The formula give by dimitri would only be valid if there were no compression heating, let alone combustion, taking place.... and also no frictional losses....

Bob

[RedFeather]

I may disremember, but I thought that the Cardew nitrogen (inert gas) experiment only showed a decrease of about 150-200 fps when no combustion was allowed.  Not 60%.

Is the B26 a break barrel?  Are you sure the port isn't angled upwards from the center of the compression tube?  The reason for the angle is to keep the barrel more or less in line with the tube.  If it were centered, your barrel's bore would be quite low, necessitating a very high front sight (not to say looking a bit odd.)  Now, guns with a retracting breech like the 48 are directly in line and, thereby, a bit more efficient. 

[rsterne]

I believe it was "up to 60% of the energy".... which would be up to about 26% of the velocity.... which would agree with your recollections (156 fps on a 600 fps gun - 756 including combustion)....

Yes, the B-26 is a breakbarrel, a copy of the HW95.... and yes I'm sure that the port is parallel with the barrel, not angled downward to the center of the compression tube.... The port is above center in the compression tube.... which is one of the variations I have seen a sketch for....

Bob

[john]

Googling "cardew airgun" yields

http://airgunner.org/?p=50

[Mark 611]

Well fellas have any of you done port work before?

[Mark 611]

a Bob you are correct the port is in line with the barrel!