GTA
Airguns by Make and Model => Crosman Airguns => Topic started by: WyoMan on June 20, 2017, 08:05:41 PM
-
I’ve been working on this platform for almost a year now. It’s been on again, off again (mostly off) but I shot it again last night...
(http://i.imgur.com/oufI0gX.jpg)
I cranked the RVA all the way in, made a tweak or two, and got it to dump 10 pumps pretty consistently. Then shot the top shot string...
(http://i.imgur.com/DmP7lnn.jpg)
It averaged 575 fps (11.7 fpe) with 10 recharge pumps per shot. I had to give it 11 pumps twice and 9 pumps once to refill the valve to the same pressure (approx. 1000psi).
The pump effort isn’t that bad at that pressure. The horsepower is mainly coming from the extra valve volume. At 3.3 cc, it’s nearly double the stock volume.
(http://i.imgur.com/2o2WC26.jpg)
(http://i.imgur.com/gXr87Tk.jpg)
But I’m going back to the 4 recharge setting which is much better for 10m paper – quiet, no recoil, and tighter ES than the 3 recharge setting. I still haven’t shot this thing much. It has shown some amazing accuracy so I need to see what it can really do... thanks -
Wyo
-
Clever, using the tube (by removing valve sides) to add volume. ;D
I seem to be missing something - do you know how much air is being retained? Or put another way, could another shot be taken? Or is that outside of the design or desires?
-
I didn't explain things very well, Craig... and this...
It averaged 575 fps (11.7 fpe) with 10 recharge pumps per shot. I had to give it 11 pumps twice and 9 pumps once to refill the valve to the same pressure (approx. 1000psi).
Should have been followed with "instead of the 10 pumps used for the other seven shots in the string".
The RVA and short, stiff spring (slingshot hammer) release a metered portion of the accumulated valve volume. Since the total accumulated volume is 17 pumps, you just count the number of recharge pumps it takes to get back up to pressure. You have to fine tune the RVA so it doesn't use some fraction of a pump. The pressure can only be recharged with whole pumps.
I use an indicator for the valve pressure... really simple but it works. With a F/T valve and piston, most folks can feel the piston make contact with the valve... for the initial 4 or 5 pumps or so. I set the piston length to make contact with the valve for the first 16 pumps. When there's no contact, I stop the recharge pumping. It has held consistent for some time now... hope that helped with your question
And thanks, btw, but the skeletonized valve was invented over a decade ago... I'm not sure if it's been done with a 13xx valve. thanks -
Wyo
-
I am clueless about pumpers so don't laugh too hard but could a valve, tube and end cap be configured to use the entire tube volume to hold pressure? Would take a ton of pumping initially... but I suppose that's p-rod territory anyway.
-
A good amount of atmospheric pressure air is needed to be compressed into the high pressure holding area (the valve), so as you imply, that is the hand pump of a pcp and its pressure tube.
-
Hi Mark,
You would only be able to use the portion of the tube between the hammer travel and the piston travel. Some clever folks, like Bob S., have made additional valve volume outside of the tube. Valve volume (I've come to realize) is the real key to the retained air MSP. More volume has no downside because you only have to fill from empty once (in theory, but you'll probably do it a lot ;))
Wyo
-
Should have been followed with "instead of the 10 pumps used for the other seven shots in the string".
... hope that helped with your question
Yes, and the fact I did not see the "17 pumps" in the graph. :-[
A great piece of work!
-
Thanks again, Craig
Wyo
-
Dang. That's a pretty gun, Wyo. ;D ;D 8)
-
Thanks, Scott... I had a chance to squeeze in a few rounds...
10-shot groups from 10m:
(http://i.imgur.com/60N37eA.jpg)
Cherry-picked ;D
(http://i.imgur.com/D5ZPRDJ.jpg)
A few more details...
(http://i.imgur.com/oct93BA.jpg)
The RVA and short, stiff spring are really pretty simple. Combined with a slippery fluted hammer and the 1701 T-group, it delivers a consistent hammer strike with no second opening of the valve.
I'm using .25" Delrin tube stock here for the transfer port. The ports (valve, TP, and barrel) are at .165"...
(http://i.imgur.com/8tefYGi.jpg)
I'm really thinking about converting my 1325 to this platform. In its present state, it has a mortar round trajectory unless you pump it 8 or 9 times. This .22 conversion went from 10 fpe at 13 pumps, to 11.7 fpe at 10 pumps. Apples to apples it's about a 40% increase... just by modifying the internals to operate like a PCP.
Wyo
-
Just like all your airguns that I've seen, not just beautiful, but power to spare!
Do you have any built on the mk-177 tube and valve? If you're not familiar, it shares the same stroke as 13xx guns, but has a 3/4" tube instead of 5/8". The valve is bigger and can be hogged out for even more volume. A few guys here have used these with good results.
-
Hi 2K1TJ... I've never used a different tube for the 13xxs (I have ten, I think)... but have seen plenty of others use bigger tubes including the 1400 and 2100 tubes. The earliest I've seen was documented in 2004:
http://www.network54.com/Forum/275684/thread/1081019718/Introducing+the+1377XLT+Hi-Comp.+.+.+.+.+Several+pictures+inside...+--%29 (http://www.network54.com/Forum/275684/thread/1081019718/Introducing+the+1377XLT+Hi-Comp.+.+.+.+.+Several+pictures+inside...+--%29)
I think it's a great idea... you can fit a pistol grip frame, attach a steel receiver, get rid of the clamshell, etc. But, not my cup of tea (they're really rifles, not pistols). As you mentioned, the additional swept (pump) volume coupled with more mechanical advantage (longer lever) will have its due in making power. Thanks -
Wyo
-
I let the pistol sit for a week and then shot it again. Doesn't seem to be losing any pressure. The last tune was set to dump four of the seventeen total pumps, so I pumped it up four times and took a shot... then 9 more with four pumps per shot:
(http://i.imgur.com/4jcK9BN.jpg)
It's very quiet and tame (like a P17) at this tune - shooting jsb 15.89 at just over 400 fps. This is my most accurate .22 cal pistol.
The barrel is very good - an LW from FX that I cut to fit the 13xx platform:
(http://i.imgur.com/0uwVVx2.jpg)
(http://i.imgur.com/ZtFl7i8.jpg)
So far, the air-retained setup has been all positive... I'm a believer now ;D... Just started the conversion on my 1325...
Wyo
-
Wow. :o Beautiful gun and incredible results.
I'm going to have to study up on this retained air thing....
-
Thanks, bhh. The retained-air setup has been around for a while... mostly with rifles (e.g. custom - Bob's Millennium Pumper; commercial - FX Independence). But it has also been done with the 13xx platform, over a decade ago. It's been called DV1322 (debounced valve) or 1322ACP (air conserving pumper) among other things.
Most use a gauge to read valve pressure... but that's not easy to do with a pistol platform. I simply indicate pressure when the piston / valve contact disappears - then it's up to pressure, stop pumping, simple.
You will generally need five modifications to make it work:
1. A large volume valve – to maintain high breech pressure and efficiency
2. A high pressure pump – reinforced pivot, secured valve, low headspace
3. A hammer energy adjuster – for tuning
4. A hammer energy arrester – to retain air efficiently
5. A pressure gauge / indicator – to recharge the valve to the same pressure
Simple, after it took me three attempts to get it right :D
Wyo
-
Thanks. That's a great, clear explanation. I have a 1377 and a PM66 that I had plans to do something with next winter. I think I'll give the retained air thing a shot.
Thanks!