Author Topic: Air volume increase-enlarging port  (Read 1017 times)

rifle 50

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Air volume increase-enlarging port
« on: February 16, 2017, 11:16:01 PM »
Bob, or anyone capable of the calculation?

You seem to love these calculations BOB? If a valve port diameter is increased from .335" to .404", what is the possible increase of air passing through at PSI 3500-4500 psi? Given in % increase.........Thanks,

Carl


  • Fresno, CA

rsterne

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Re: Air volume increase-enlarging port
« Reply #1 on: February 17, 2017, 08:37:46 AM »
Assuming it's just a port with no obstruction, such as a valve stem, the potential increase in airflow is proportional to the increase in the area, which is the ratio of the diameters, squared.... So in your example, (0.404 / 0.335) ^2 = 1.206 x 1.206 = 1.454, or a 45% increase in flow at whatever the pressure you are working at.... If there is a valve stem in the way, for example inside the throat of a valve, then that area must be deducted from the throat area before making the percentage calculation....

Although there is no hard and fast rule governing the FPE increase from such a port increase, my experience has been that up until the point where the port diameter equals the caliber, and for a PCP shooting at the maximum possible, the FPE increase is proportional to the port diameter (not the area).... Again, this assumes that the other factors (pressure, barrel length, caliber) remain constant.... Therefore, using your example, you could expect a maximum potential increase in FPE of about 20%, which for a given bullet weight should mean a velocity increase of roughly 10%....

HTHs....

Bob
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rifle 50

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Re: Air volume increase-enlarging port
« Reply #2 on: February 17, 2017, 10:17:42 AM »
Thank You  BOB.  I did ask on GTA the same question, but your answer here gives me the information to proceed. If you copied and pasted over there is fine but I do have and answer and it satisfies my curiosity. A stem is involved, but FOR ME the potential increases are worth trying the mod........Have to get some machining lined up and particularly the enlarged SEAL!!!

Many Thanks.............Carl
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rsterne

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Re: Air volume increase-enlarging port
« Reply #3 on: February 17, 2017, 12:49:04 PM »
If you give me the diameter of the stem, I can do the same comparison.... Additionally, the size of all the ports between the valve throat and the bullet is necessary to insure there are no other restrictions in the system.... The smallest area between the HPA reservoir (including the inlet side, before the valve) and the bullet is what will prevail.... I like to see the valve throat area about 10% larger than the exhaust side, and the inlet side at least the clear diameter of the throat (so even larger again in area)....

Bob
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rifle 50

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Re: Air volume increase-enlarging port
« Reply #4 on: February 17, 2017, 05:05:13 PM »
If you give me the diameter of the stem, I can do the same comparison.... Additionally, the size of all the ports between the valve throat and the bullet is necessary to insure there are no other restrictions in the system.... The smallest area between the HPA reservoir (including the inlet side, before the valve) and the bullet is what will prevail.... I like to see the valve throat area about 10% larger than the exhaust side, and the inlet side at least the clear diameter of the throat (so even larger again in area)....

Bob

I forgot in my THINKING or LACK OF, to figure depth of O ring groove....After doing that, I WON'T be opening up throat quite as much, as have to LEAVE SOME METAL for support ::) Will open about .020" less than  I FIRST figured.......Already have the person machining and making a larger SEAL agreeing to do it and it has been SHIPPED PRIORITY MAIL this afternoon.......It will be done, and CHRONO will tell me how we did.........Work on 2 factory valves in same vein going SLOW, showed 30% increase in FPE......

So thanks for your HELP Bob.........I DON'T have the stem diam..........It has been shipped.......Other 2 valves I have here are factory,  and stem is smaller than Jeremy's (S. Africa)........Modifiying it now beyond what has been  done on the factory throats, as it doesn't produce numbers as good as the altered factory valves as YET  ;D

Carl

Carl
  • Fresno, CA