Author Topic: Transfer port question  (Read 182 times)

Mrshosted

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Transfer port question
« on: March 06, 2020, 10:14:16 PM »
This is a picture of the tranfer port of my .30 with a bore scope looking down in to the valve.  The tranfer port is smaller the the port in the barrel.  Do you think this has a negative impact on the efficiency of the gun? Would be a good idea to inlarge the lower area of the transfer port to match the port into the barrel.  Is it bad to have the area lower transfer port larger then port on the barrel.  Have often wonder becouse so many gun are starting to have multiple barrels optional.  I'm not trying to build the highest power.  I'm more interested in high shot count with lowest standard deviation. Thanks Mont                                                                       


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rsterne

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Re: Transfer port question
« Reply #1 on: March 07, 2020, 08:56:30 AM »
I just love fuzzy, out of focus pictures where I don't know what I'm looking at....  ;D

You say that is looking down into the valve, so I assume the vertical bit in the middle is the valve stem?.... Is the barely visible edge where the arrows are pointing to the barrel port?.... What part is the transfer port, and what part is the valve exhaust port?....

Anyway, I start with the barrel port, and make the transfer port and the valve exhaust port the same diameter.... I then calculate the area of the throat (the area of the hole under the poppet minus the area of the valve stem)…. and make sure that area is a bit larger than the area of the port leading to the barrel (exhaust, transfer and barrel ports)…. If I am understanding your photo and description correctly, I would be enlarging the transfer and exhaust ports, and blending the exhaust port into the valve throat.... which I might also have to enlarge (or thin out the valve stem, or both)….

Using larger ports for the same FPE created tends to increase the efficiency, and in a regulated PCP also the shot count.... This is because for the same FPE you can drop the setpoint (regulated) pressure, and reduce the hammer strike (dwell)…. However, in an unregulated PCP, it's like putting a hot cam in an engine.... It increases the maximum available power, but narrows up the "power band", which in this case would be the pressure range between the fill pressure and refill pressure.... You can back off on the hammer strike to get back to the original FPE, but that will reduce the fill pressure you use.... Either way, larger ports in an unregulated PCP tend to make for a lower shot count.... Before the days of regulated PCPs, Field Target shooters, who needed lots of shots at medium power, used to strangle up on the transfer port and then lean on the hammer spring to get the power back.... This resulted in a flat shot string and more shots per fill....

So, the answer to your question depends on if your gun is regulated or not....

Bob
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Mrshosted

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Re: Transfer port question
« Reply #2 on: March 09, 2020, 07:53:55 AM »
I'm sorry about the edges of the picture being fussy.  The bore camera has a small mirror to angle down at 45 degrees.  The fussy area is the back of the barrel. The center of the pictures is the exhaust port.  With the depth of the port it photographs wired. I add the arrows with hope you'd understand what I was concerned with.  It is a regulated gun. The arrows are pointing at the smaller area of the breach.  The outer area is not as clear,  the brass area of barrel port.  The valve port is not visible only the stem crossing.
I'm understanding it would be in my best interest to gain efficiency and shot count.  To port match the exhaust port to the barrel port. Then blending them into the valve area. I'll also check the size of the valve.
Do you recommend a percentage of valve size to exhaust port. You mention a bit larger.
Bob, Thank you very much. I really appreciate the help with helping me understand. 
Sincerely Mont
  • Riverton Utah

rsterne

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Re: Transfer port question
« Reply #3 on: March 09, 2020, 09:22:48 AM »
To calculate the area of the valve throat, you must figure out the area of the hole beneath the poppet, and then subtract the area of the stem that partially obstructs it.... For example, use a 1/4" throat and a 1/8" stem....

Throat area = 0.25 x 0.25 x PI/4 = 0.0491 sq.in.
Stem area = 0.125 x 0.125 x PI/4 = 0.0123 sq.in.
Difference = 0.0491 - 0.0123 = 0.0368 sq.in.

You can figure out what the diameter of a matching hole would be as follows....

Equivalent diameter = Sqr.Root(0.0368 / (PI/4)) = Sqr.Rt (0.0469) = 0.217"

If the exhaust, transfer and barrel ports were all 0.217", you would have the same area for flow all the way through the valve.... However, the flow in the valve throat has to travel around the stem, so really it needs to be a bit bigger.... I like to see the throat about 10% larger that the rest of the ports, so in reality that 1/4" throat with the 1/8" stem would only be large enough to feed ports of (0.9 x 0.217) = 0.195" or so.... You can make the throat area larger by either drilling out the hole, or slimming the valve stem in the throat area.... Drilling out the throat increases the area being sealed by the poppet, and so may require a harder hammer strike to open the valve.... Slimming the stem will not do that....

Bob
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Mrshosted

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Re: Transfer port question
« Reply #4 on: March 09, 2020, 10:28:29 PM »
When adding hammer strike do find any advantage with adding weight to the hammer over increased spring pressure?  Ive seen where a weight could be easily added inside the spring just behind the hammer, possibly even lowing the spring pressure behind the hammer.  Or do you prefer to increasing the hammer spring adjustment?
Mont
  • Riverton Utah

rsterne

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Re: Transfer port question
« Reply #5 on: March 10, 2020, 08:51:28 AM »
Increasing the spring force and/or the hammer travel increases the hammer energy, and therefore the valve lift.... and it also increases the hammer momentum, and therefore the dwell.... Increasing the hammer weight does not change the energy, but it does increase the momentum.... Therefore changing the hammer weight changes the ratio of lift to dwell.... A lighter hammer results in the valve opening and closing faster, but not being open for as long a period of time.... This is better for efficiency, but may not deliver the dwell you need to produce the FPE you want.... The only time I would increase the hammer weight is if you can't reach the velocity plateau without excessive cocking force.... For the same FPE, a heavier hammer will require less hammer spring preload (or hammer travel)….

Bob
  • Coalmont, BC