help me get a grip on air flow in a vavle.

[csdilligaf]

  I want to understand the relationship of my transfer port diameter vs valve head diameter.  I have been getting plenty of power on my latest air gun build.  More than I need so I have reduced my transfer port size.  Now I am thinking I can also reduce my vavle head size thus reducing the force needed to open it.  So my question is, can I take the area of the transfer port and make the valve head minus the valve stem area equal and it will flow the maximum that the transfer port is capable?  Another way to say it is the void area around the valve stem is equal to the transfer port area.   Does tha make sense?  For instance, my transfer port size is .140" and with 1000 psi I am getting 26 fpe with an 18.1 grain 22 cal.  For the Heavy Varmint class I only need 20 fpe.  I may want to go down too .125 transfer port and make the valve smaller.  My valve stem is .073" dia so what would you guys calculate my vavle head size to be optimum?  I plan on sticking with .22 cal but going down to the 14.3 or 13.4 grainer.

[Motorhead]

Running Regulated or Not ?

[rsterne]

The basic answer to your question is "yes" you can get good efficiency by having the throat area (hole area minus stem area) equal to the transfer and barrel port sizes.... For a few years that was exactly what I did, but when going for absolute maximum power I have found that adding about 10% to the transfer port area for the throat area has a small but measurable gain.... PROVIDED that you have enough hammer strike to open the larger seat....

Using your numbers, for the equivalent area of a 1/8" transfer port, and using a 0.073" stem diameter, you would need a throat size as follows:

TP = 0.125 ^2 x PI / 4 = 0.0123 sq.in.
Stem = 0.073 ^2 x PI /4 = 0.0042 sq.in.
Throat = 0.0123 + 0.0042 = 0.0165 sq.in to 0.0182 sq.in.
Throat diam. = (4 x area / PI) ^0.5 = 0.145" to 0.152" .... personally, I'd use a 5/32" throat, which with the seat margin will end up being about a 3/16" seat or more....

There is one caveat on this.... and that is the directness of the flow path between the seat and the exhaust port.... As an example, on the Disco, where you can angle the exhaust port, you can see quite easily through the throat.... On an MRod, however, where the valve is a larger OD and the port is so close to the seat that you can't get any angle on it, with the same size ports you can't see through the throat at all, or barely.... The MRod benefits from having a larger seat relative to what a Disco needs so that the flow doesn't have to get around that hard corner.... It's actually easier to get power from a Disco valve than an MRod valve at the same pressure....

I'd try a 5/32" throat to go with your 1/8" transfer port and see what happens.... The only reason that is even remotely large enough is because of your very slim stem....

Bob

[csdilligaf]

Yes, it is regulated to 1000 psi.  I have always gone for power so having to reduce it to make the rules for a certian class is something new to me.  Interestingly though the 20 fpe HV class normally has better scores than the 35 fpe Open class.  One of the guns I built for the Open Class(for Paul Bendix back east) did recently shoot the first 750 score outdoors though which was great.  He's a rimfire guy as well and a superb shot.

[Motorhead]

1k working pressure in .22 cal would in my opinion would be on the low side for a 20 ft lb tune.
In a match gun set up generally you have two schools of thought ....
* Operating on lower pressure Small valve and small passageways with an ample hammer strike.
Velocity gets mostly regulated by Poppet/Throat/transfer Passage size, dwell in longish but velocity is consistent at lower to mid power levels.
* Operating at closer to 1.5 / 1.8k pressure in .22 cal you can get away with larger porting on throat & passages, but given the same size poppet as above, valve is now harder to open, but still manageable.
Now you can use a lighter faster striking hammer that SNAPS the valve open very fast, yet because of less weight can reverse dirrection allowing a faster valve closure. * Much reduced dwell.

Difference between the two one could view like a simple pea shooter or blow dart gun ...
First is a long breath of lower amplitude ... sorta wooosh.
Second is more akin to puffing up you cheeks with air, then clapping them ... more a Pop effect.


Yup breathing dynamics of PCP's really have a broad range that can be made to work, some more of less stable in velocity and/or air usage.
There is MUCH already said on this very diverse subject within past GTA threads .... Time to do some reading 

[Bill G]

Scott and Bob have given solid advice.  IMO your reg set point should be 1600 psi.  This will give several shots off reg that are still consistent thus maximizing your efficiency.  Scott makes some light weight hammers that may be of use to you. Based on a 26 gram hammer and a 6# spring at 6 turns preload, assuming 2.5" spring, you should have no problems attaining your goal.  if your reservoir is  at least 250cc, you may be getting close to 90 shots @20 fpe. Letting your valve breath and act quickly will make the rifle very adjustable and predictable.  I think that Scott likes to let his valves move air quickly under regulation, I take this approach myself.  I like to have a barrel port of .165 for .22cal.  Couple that with Scott's light hammer and the valve snaps and doses air very nicely. Scott has some very nice write ups on the matter.  LW hammer, light weight hammer, valve dwell are some good searches that will reveal.  Bob has awesome visuals with charts and graphs plus his input.  Your very close.

Bill