GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: K.O. on April 01, 2014, 01:44:31 AM
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I decided I had to many things to do to be able to do the valve mods one at a time and test to see the effect of each so today I thinned the
exhaust poppet shaft, went to a weaker spring(cock before pumping now),coned the exhaust poppet and took off a couple threads to increase
valve volume. well I should have waited on the valve volume it takes me 27 pumps to get what I think I could get at about 20. Live and learn.
The good news is that with CPHP 14.3g at 27 pumps I am getting 689 fps and it is seeming to only have about a 10 fps variance throughout.
I will take up some space in the valve until I get another nose but I think I just may have a chance of getting 700 fps at 20 pumps or so once
I do. Experimenting with volume later this year when I have more time I bet it will do it in 15-20 pumps when I get things just right.
It would be nice to get 725 fps or so by 25 pumps but ??? I have a feeling it will be as accurate at 30 yards as 20 now but did not have time
to test , I will soon though. For now I am pretty happy with the 15 fpe it is getting. I will fiddle with it as time allows but it does show it was
worth doing IMO.
Just for those who do not know what I am updating http://www.gatewaytoairguns.org/GTA/index.php?topic=64968.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=64968.0)
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well I took up some volume but then it topped out at 665 fps at 24 pumps.
I stuffed the large end of the piston and got back to 682 at 25 pumps.
It seems the limit is the cup system and with how it is flow ported. the max pressure is where I am so in a while I will take the spacer back
out. I did not have the right Dremel bit when I ported this valve and I think I can make more gains with a bit more valve work. I took the
other one further and rounded the side edges a bit I think that will help 12 pumps on up. Also I have the pump cup shimmed to where it
barely makes contact with the piston. Under high pressure it just might be getting compressed enough to cause just a touch of head space so
I will try shimming another .01 or so. I also may order a couple Disco and 2100 springs and see if that helps.
I shoot most of the time 12 pumps and right now that's ~595 fps. I cant help thinking hey that's what it used to get as a .177 :D
I still think I might make 700 fps but think it will be between 20-25 pumps...
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The more I think about it the more I think it is the porting job I did on this valve that is holding it back.
I think I got it just about right on my 14.5' barreled 1322 to get the 640-650 fps at 22 pumps. If I put this 18.75" barrel on it I would/should
get another 40ish fps (680-690 fps) from the extra 4.25" at 22 pumps. I still have not stuffed the piston on that one... It got me 20 fps on
this one so that gets it to 710 fps...
I expect close to the same from this build. Two rifles with a similar state of tune will perform close to each other not to much different.
What the math says is this one should be hitting 710 fps at 18 pumps instead of 22. that is what the 4.2" pump stroke
should get me..
At 14.3 for atmospheric pressure the 13xx's 3.5" stroke at 22 pumps and 60% efficiency says you get about 1484 psi in the valve
At 14.3 for atmospheric pressure the 664's 4.2" stroke at 18 pumps and 60% efficiency says you get about 1457 psi in the valve
assuming they have the same volume .13 ci in this case.
this one has the above differences from last post also. oh the nose of the valve on the 1322 has not been sleeved and this one has.
so logic says buy another good porting bit to replace the one that got broken and shim just a bit tighter and 710 at 18 pumps is not
unreasonable.
Oh I found this on the other Crosman forum and it lets me know I am doing ok... He is got 695 fps at 15 pumps with a flat top set up...
http://www.network54.com/Forum/275684/message/1396371412/1322ish+rifle (http://www.network54.com/Forum/275684/message/1396371412/1322ish+rifle)