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PostPosted: Sun Jul 01, 2012 5:01 am 
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Location: Montreal, Quebec.
I am planning on making another valve for my 9mm PCP gun and plan to keep the exhaust port at 0.250" I.D. but my questions is I want to make the valve port equally restrictive as the exhaust port (not more, not less!).. What would be the optimal size to drill the valve port to?? :?


My most logical guess is to calculate the surface area of the valve stem (not to be confuse with valve stem sealing check) and also calculate the surface area of the exhaust port and add the two together to get the size I need to make the valve port..

I will need to take my valve apart and measure my valve stem since I reduced a bit of O.D. on the Discovery valve stem where the sealing check is!

edit: My Disco valve stem should be about 0.125" (modded) so if I take the area of that and add it up with the area of the exhaust port 0.250" it will give me the total area of what the valve port need to be..

After all calculations I got 0.280", that is the optimal porting size for the valve port if I keep the exhaust port at 0.250"

My current 0.325" valve port is too big for my 0.250" exhaust port! :roll:

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PostPosted: Sun Jul 01, 2012 6:52 am 
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you should just do it straight thru at .25 , or .28 or .3'' or .32, with a big bore just make your valve stem open longer at .25 ..... but a .3 port straight thru and long stem travel will obviously get more power .

since your valve is to big at .325 why not make it .5 and force a delrin sleeve (.51) in and drill it /surface it for .25 .


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PostPosted: Sun Jul 01, 2012 12:06 pm 
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jezX wrote:
since your valve is to big at .325 why not make it .5 and force a delrin sleeve (.51) in and drill it /surface it for .25 .


Actually I was planning on doing that today, along with one for the valve stem to ride on because it's a little rough! I'll see what I will do once I get the gun apart and have a good look at the valve again.. What are the dimension on your valves that you use for your 9mm PCP guns? Also you use delrin valve stems, what size are they too?

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PostPosted: Sun Jul 01, 2012 12:41 pm 
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I understand that the Korean 9mms are usually modded to have a 6mm (0.236") port size (or equivalent) right through to improve them over stock.... The limiting factor will be about 75% of the bore size or the pellet will hang up on loading.... That means about 0.265" is the maximum barrel port size practical with a bottom ported barrel.... I personally would use 1/4".... maybe 17/64" maximum.... I would size the valve throat/stem to flow the equivalent of a 17/64" (0.265") hole.... maybe 19/64" with a 1/8" stem....

Bob

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PostPosted: Sun Jul 01, 2012 8:02 pm 
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Well I didn't waste anything this morning, I made delrin insert for the valve and can now use a metal valve stem seal check..

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The original Disco valve stem was FUBAR! Judging by the picture it couldn't handle the pressure at 2500 psi and just caved in, I might have not had this problem is the valve port was smaller..

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Anyway I am using the Disco valve stem without the sealing check, it's straight metal that will seal against the delrin portion on the valve.. My question is will the delrin hold up to the pressure or is the valve stem just going to dig in? The is not much area around the valve stem head seating on the delrin possible 0.328" sitting against 0.297".. :?

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PostPosted: Mon Mar 02, 2015 6:22 pm 
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rsterne wrote:
I understand that the Korean 9mms are usually modded to have a 6mm (0.236") port size (or equivalent) right through to improve them over stock.... The limiting factor will be about 75% of the bore size or the pellet will hang up on loading.... That means about 0.265" is the maximum barrel port size practical with a bottom ported barrel.... I personally would use 1/4".... maybe 17/64" maximum.... I would size the valve throat/stem to flow the equivalent of a 17/64" (0.265") hole.... maybe 19/64" with a 1/8" stem....

Bob


Bob, can you clairify for me what you mean by "The limiting factor will be about 75% of the bore size or the pellet will hang up on loading" ?

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PostPosted: Mon Mar 02, 2015 6:40 pm 
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Hi Cadeau what I think BoB meant was the fact that if the barrel port is larger than 75% of the pellet diameter it can easily create feeding/loading problems.

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PostPosted: Mon Mar 02, 2015 7:12 pm 
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Correct.... For a 9mm, that would be 6.8 mm or 17/64" (0.266").... If you need more area than that, you can make the barrel port oblong.... ie stay at that width and make the port longer.... then blend it into a round, larger diameter transfer and exhaust port.... and an even larger valve throat (to allow for the stem area).... The limit is, of course to have all the areas equal to the bore area.... In fact, you can obtain a slight gain in efficiency (but usually not power) by having the total throat area (throat ID minus stem OD) about 10% larger than the ports to allow for turbulence in that area....

If you are shooting only bullets, not pellets, which are more tolerant of a large barrel port, you can go as large as 9/32" (0.281") on a 9mm/.357 without loading problems, providing it is properly deburred, of course.... That size port, running at 3000 psi, is capable of over 250 FPE.... I used a 21/64" throat, with a 1/8" stem (equivalent to a 0.305" port).... To get more than that, you need to persue bore-size porting, which should put 300 FPE within reach, providing you have a long enough barrel (or more pressure)....

Bob

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Proud Member of the 2000+fps Club!


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