Gas velocity - port hogging relationship

Gas velocity - port hogging relationship

Joined: May 22nd, 2007, 3:28 pm

June 30th, 2012, 2:23 am #1

I don't have much experience with airgun mods, also haven't been on the forums for quite a while.

I got to wondering about opening up ports. Seems that where you have a large volume chamber and a small orifice for air to pass through on its way to being exhausted, the restriction would work under the venturi effect. This would cause the gas to speed up exponentially at point of restriction, effectively causing a ram induction force into exhaust, increasing volume more quickly and speeding gas flow.

If that transfer port or whatever is opened up then there would be a loss of venturi effect and loss of gas velocity. Has any research or data crunching been done regarding this, to show where an intended improvement actually causes a net loss in pressure or gas velocity behind pellet? I can see advantage to smoothing contours or sharp edges however it seems there's a danger of backpedaling when hogging ports. Apologies if this has already been over thunk and discussed ad infinitum...
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Joined: April 28th, 2010, 12:23 am

June 30th, 2012, 2:27 am #2

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Joined: March 1st, 2002, 12:22 am

June 30th, 2012, 3:47 am #3

I don't have much experience with airgun mods, also haven't been on the forums for quite a while.

I got to wondering about opening up ports. Seems that where you have a large volume chamber and a small orifice for air to pass through on its way to being exhausted, the restriction would work under the venturi effect. This would cause the gas to speed up exponentially at point of restriction, effectively causing a ram induction force into exhaust, increasing volume more quickly and speeding gas flow.

If that transfer port or whatever is opened up then there would be a loss of venturi effect and loss of gas velocity. Has any research or data crunching been done regarding this, to show where an intended improvement actually causes a net loss in pressure or gas velocity behind pellet? I can see advantage to smoothing contours or sharp edges however it seems there's a danger of backpedaling when hogging ports. Apologies if this has already been over thunk and discussed ad infinitum...
seems for a given port size you can wrangle a range of performance... either a slow pellet speed with a number of shots, or some power for much fewer shots.

Also seems that the more open the more curve you shot speed is gonna show.

But the upshot is you can work with that using differnet springs and fills...... so its in there, but its a matter getting the various factors to work together.... much time over the chrony


dr_subsonic's pneumatic research lab

the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic
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Joined: May 22nd, 2007, 3:28 pm

June 30th, 2012, 4:36 am #4

Okay, thanks! Just sort of a passing thought, couldn't believe it hadn't been addressed and yeah, it had been!
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Joined: June 25th, 2002, 1:34 pm

June 30th, 2012, 11:13 am #5

I don't have much experience with airgun mods, also haven't been on the forums for quite a while.

I got to wondering about opening up ports. Seems that where you have a large volume chamber and a small orifice for air to pass through on its way to being exhausted, the restriction would work under the venturi effect. This would cause the gas to speed up exponentially at point of restriction, effectively causing a ram induction force into exhaust, increasing volume more quickly and speeding gas flow.

If that transfer port or whatever is opened up then there would be a loss of venturi effect and loss of gas velocity. Has any research or data crunching been done regarding this, to show where an intended improvement actually causes a net loss in pressure or gas velocity behind pellet? I can see advantage to smoothing contours or sharp edges however it seems there's a danger of backpedaling when hogging ports. Apologies if this has already been over thunk and discussed ad infinitum...
the volume of air and the pressure of the air passing through the port. I have found that five thousandths of an inch increments in diameter is a good step size for checking transfer port performance. hth
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Joined: November 17th, 2006, 3:51 am

June 30th, 2012, 2:32 pm #6

I don't have much experience with airgun mods, also haven't been on the forums for quite a while.

I got to wondering about opening up ports. Seems that where you have a large volume chamber and a small orifice for air to pass through on its way to being exhausted, the restriction would work under the venturi effect. This would cause the gas to speed up exponentially at point of restriction, effectively causing a ram induction force into exhaust, increasing volume more quickly and speeding gas flow.

If that transfer port or whatever is opened up then there would be a loss of venturi effect and loss of gas velocity. Has any research or data crunching been done regarding this, to show where an intended improvement actually causes a net loss in pressure or gas velocity behind pellet? I can see advantage to smoothing contours or sharp edges however it seems there's a danger of backpedaling when hogging ports. Apologies if this has already been over thunk and discussed ad infinitum...
Change the valve in some way, and the results will change, but if just looking at transfer port, can get an idea. Might be toosimplistic, but it seems to point in the right dirction for me.

So lets say you are playing ONLY with the transfer port.

Set your sweet spot velocity limit to whatever you choose, just keep the same strict limit for all transfer port sized tested. BEST would be a percentage limit...I tend to 3% of max.

Then it becomes the number of shots X the average energy.

Lets say we got the following 3% sweet spot with various sizes of transfer ports:

.07 = 35 shots @ 8 foot pounds = 280
.075= 30 shots @ 10 foot pounds = 300
.08= 25 shots at 13 foot pounds = 325
.085=20 shots at 16 foot pounds = 320
.09= 15 shots at 19 foot pounds = 285

That would lead me to believe the that at that pressure range, with that valve set up, a transfer port giving something in the 13-16 foot pound range is probably at its optimum. Change some other factor, like the operating pressure or the valve cycle, and would need to retest.
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Joined: May 22nd, 2007, 3:28 pm

July 1st, 2012, 5:37 pm #7

Thanks, guys,

I was thinking that the pellet starts at zero velocity then accelerates up the barrel till exit. So there's a time lapse of millisconds while pellet travels bore. Gas behind pellet escapes though valve at a particular velocity and the narrowing of any ports or whatever act as a venuri, to speed up that air.

If the ports are opened too much then at some point the gas velocity escaping behind pellet will eventually be lowered, maybe under the optimum speed for pushing that pellet mass, that it might actually slow down the pellet speed through bore instead of speeding it up as expected.

I guess what I was suggesting is that there may be an upper limit on how big a guy wants ports and that it's not always the case that more or bigger is better. Of course a lot else is going on with air chamber size & volume of air, relative beginning pressure before discharge through exhaust valve, total volume of discharged air before valve closes, valve closing speed or elapsed time, speed of gas through valve & any restrictions there causing venturi effect, port angles & smoothing sharp edges etc. plus more.

Large subject, mostly beyond my skills or brains. I've played with fluid & gas flow dynamics in fuel systems and seen much of this so that's where the thoughts here originated, I think I'll leave them there. Thanks for your replies & comments!
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Joined: March 1st, 2002, 12:22 am

July 1st, 2012, 7:05 pm #8

I don't have much experience with airgun mods, also haven't been on the forums for quite a while.

I got to wondering about opening up ports. Seems that where you have a large volume chamber and a small orifice for air to pass through on its way to being exhausted, the restriction would work under the venturi effect. This would cause the gas to speed up exponentially at point of restriction, effectively causing a ram induction force into exhaust, increasing volume more quickly and speeding gas flow.

If that transfer port or whatever is opened up then there would be a loss of venturi effect and loss of gas velocity. Has any research or data crunching been done regarding this, to show where an intended improvement actually causes a net loss in pressure or gas velocity behind pellet? I can see advantage to smoothing contours or sharp edges however it seems there's a danger of backpedaling when hogging ports. Apologies if this has already been over thunk and discussed ad infinitum...
Oh man.... its a wide open field.... Look at guys that modify intake manifolds... only need a few thou for huge gains. Then there is the "boundary layer" school. We had a really long discussion here and on the yellow a while bck about whether the air path should be smooth or have some texture...

We have to remember its a shooting system. Change one thing, and it affects the rest of the system. I think our collective problem is that there are so many variables that can impact post mod performance.

Think its gonna be a matter of segrating one section of the system, identifying where it can be "improved" and test it. Then move to the next subsystem, and make the improvements to it, and see what happens. Repeat thru your whole gun...

Good thing is there are a few of us working on it, and we can confirm or refute each others efforts.


dr_subsonic's pneumatic research lab

the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic
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Joined: May 22nd, 2007, 3:28 pm

July 1st, 2012, 11:48 pm #9

Exactly what I was thinking but couldn't express myself. Just like short ram/long ram, textured surface/smooth surface etc. you have limits. Trick is to find farthest limit in a sub-system then address another aspect of system.

Part that struck me is that within air conservation and pellet speed you might come up with a net loss if air passages were too large. I was wrong in original thought because I pictured an air column and dynamics of gas motion but with the open/close valve and pellet in barrel it's less dynamic, a closed system. Motion is actually similar to a hydraulic jack where small piston moves large piston-ram: that air is smaller piston and pellet is larger piston-ram.

There's always going to be air loss equal to volume of gas between closed valve and length of barrel behind exiting pellet, can't be helped. Was thinking in terms of accelerating pellet inside barrel to max velocity long before end of barrel, thereby conserving what would have been wasted gas while achieving hghest possible pellet speed.

Seeing it sort of like my old M44 where bullet runs out of barrel length and exits long before total gas charge is used up due to powder's slow burn rate. Was thinking to keep air velocity highest possible throughout travel of pellet in bore, as if having a quicker powder burn rate, forcing higher velocity sooner in pellet's travel through bore. Have to think it again.
Last edited by Airfierce on July 2nd, 2012, 4:48 am, edited 1 time in total.
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Joined: April 12th, 2002, 5:26 am

July 2nd, 2012, 5:54 pm #10

you are working with. I think that approach is key to good efficiency(good power at low gas consumption)I look at the valve as a metering device, analogous to a fuel injector...it's easy to see gas gun valve dwell as the length of the fuel pulse/injector timing.

One of the models that's been discussed here a lot over the years is the "pellet horizon"(note..just checked, it's not pellet horizon, it must be something else..edit..it's "sonic horizon"..thanks Curt)the point in pellet travel at which any further gas release can not further accelerate the pellet. That horizon seems to be somewhere less than halfway down the bore..though it's a timing issue, and related to power and bore size, as I understand it.

This model is why I'm such a fan of fast, light strikers, valve bodies wide open to the reservoir, and open porting...kick the pellet in the skirt with a high pressure pulse, and get the valve shut as fast as possible..then don't let it open again.

My feeling is that( at least in the upper power range, above 20 ft/lbs or so) port diameter will be limited by damage it does to the pellet during loading long before it get's grossly oversize and affects efficiency...IF you meter with the valve/striker system, not the port.

I feel strongly that using the traditional "gas engine intake port model" as a guide for gas gun performance mods will lead you down the wrong path..it's wrong in too many details...hydraulic systems experience may be a much better teacher.
Last edited by classicalgas on July 4th, 2012, 6:20 pm, edited 1 time in total.
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