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rsterne · Moderator

I'm in the process of tweaking my 2560 HPA.... nothing major, just seeing if I can get a squeek more power (like 46 FPE isn't enough!), and more shots by changing to an 18 CI tank instead of the 13 CI, still regulated to 1600 psi.... The smaller tank is moving over to my 2260 HPA which is getting a reversed tank block like the .25 cal has now.... I'm hoping that the much larger plenum (the entire 2260 tube) will give me the same power (36 FPE for the .22 cal) on 1600 psi as I was getting on 1800 with the small volume between the regulator and valve it has now.... Here are the new parts I've made....

I made an extended valve front end to move the O-rings in front of the roll-stamp on the 2260 tube.... It has a lot larger bore, and is 9/32" at the spring seat instead of the stock 1/4".... The valve spring has been enlarged at one end to 9/32 to increase the flow as well.... The rear half of the Disco valve is bored out to 5/8" ID (from 1/2") to unshroud the poppet.... The flow into the valve is so much larger than out, basically the valve volume is the whole 2260 tube, from a pressure drop point of view.... I thinned out the stem of the valve poppet from 0.157" to 0.109", shortened it 0.100" for increased hammer travel, drilled the throat of the valve to 1/4", milled the exhaust port to 3/16" on a 20* angle, and rounded the corners for better flow....

The hammer has been modified by grinding a flat on the bottom to allow for the extra travel without hitting the sear.... and the cocking slot in the tube lengthened 0.100" as well.... I made a spring guide from steel which sits in the hammer and adds 11 gr. to the weight, the total is now 68 gr. instead of the stock 60 gr.... The spring guide runs in the preload adjustment screw, and provides a cocking indicator as well.... Thanks to Ribbonstone for that idea....

The tank block is the reversed style I came up with a while back, and mounts to the threaded front of the 2260 tube using a drilled out CO2 nut riding on a hollow bolt.... The aluminum bushing carries an O-ring to provide a seal past the threaded area where the tube is full thickness.... O-rings (not shown) between the parts seal everything up.... The lower part of the tank block carries a standard regulator with the bottom of the tank back towards the trigger, and right up against the bottom of the 2260 tube.... The tank acts as the forestock.... In the bottom of the block is a 3-groove Picatinny rail for a bi-pod, sling-swivel, flashlight, or laser.... On top of the tank block is mounted the barrel band.... The vertical air passage is sealed on the bottom by a 10-32 low-profile SHCS carrying an O-ring.... If desired, I can drill and tap a 1/8" NPT hole on the side of the block for a gauge to show the pressure in the plenum....

This valve is also a prototype for a .25 cal pumper I plan to build in the near future.... The port areas exceed 3/16" diameter at all points.... and is the largest I have gone starting with a Disco valve.... It can be fitted with a gauge by leaving a thicker wall in the appropriate location.... and of course a pumper version will need a check valve in the front.... The pumper version will have a volume of about 5 cc....

Bob

rsterne · Moderator

OK so I finally got some time between Motel guests to get some time in the shop.... I pulled apart my 2560 and installed the new valve and hammer with the original 13 CI tank regulated at 1600 psi.... With the hammer preload at the maximum, the gun shot 911-914 fps with the 25.4 gr. JSB Kings.... This was about 30 fps LESS than it did with the old parts.... WTF ?....

I started swapping parts, and the hammer wasn't the problem.... with the old valve, the velocity peaked at the same value as before.... I put on my 18CI tank which was regulated at 1900 psi, and gained about 45 fps, shooting 990 fps at maximum hammer strike, still with the old valve.... I tried the new valve, and lost nearly 100 fps, now it was down to 895ish.... So what happened with the new "super" valve with the big ports?.... I was pretty disappointed as I couldn't believe that just the larger ports lost so much performance.... The only other possibility was that I shortened the valve stem an extra 0.020" compared to the previous valve, but I allowed for that in the length of the cocking slot etc.... I was perplexed, and decided to sleep on it....

That turned out to be a good decision, at least I think so, I haven't finished the experimenting yet.... I spent the day cutting and trimming the lawn while my ShoeBox was topping off my SCUBA tank and then it hit me.... What if the bigger valve throat was preventing the valve from opening fully?.... Well, not the throat, per se, but the fact that it is bigger means that the area the air pressure is pushing on is larger, so maybe the valve was not opening as much with the same hammer strike.... After dinner I installed the new valve, put a Disco fill fitting directly on the front of the 2260 tube, filled it to 2100 psi and fired a shot across the Chrony.... BINGO!, less than 800 fps.... and each shot I fired the velocity increased until I hit 1600 psi, where it peaked at 914 fps.... I kept shooting until the velocity dropped to 750 fps because I wanted to know how much air this new valve would flow at lower pressures, as I intend to use a similar one in my Disco pumper project.... Well, it turns out it only needs 850 psi to shoot 750 fps.... I refilled the gun and shot a string from 2000 psi down to 1200 with the following results (bear in mind this is only a 67cc tube = 4.1 CI)....

855
875
896
904
904
899
879
854

Average = 883 (44 FPE).... Total FPE = 352.... Air used = 800/14.5 = 55 bar.... CI used = 55 x 4.1 = 226 CI.... Efficiency = 1.56 FPE/CI.... WOW!!!.... That's the highest efficiency I've ever seen in a .25 cal PCP....

Now I have to make a hammer setup that will allow me to use more than 1600 psi, or to at least allow more air to pass through the valve at 1600 psi.... That will be the next project....

Bob

rsterne · Moderator

I built a longer RVA (power adjuster) today so that I can use the hammer spring from a QB 78/9.... I added a spring guide to a hammer I had used in the gun previously which I had threaded to accept it.... The total weight of hammer and guide is 73 gr.... This arrangement has the ability to be adjusted for a much heavier hammer strike than the previous arrangement, in fact to duplicate yesterdays results I had to back it off about 8 turns (1/2") from coil bind.... Using 2000 psi, the velocity was 990 fps from maximum preload all the way to 5 turns out.... At 6 turns out, it was only down 7 fps.... At 1800 psi I could use 7 turns out, and at 1600 psi 8 turns out before the velocity started to drop.... This means I no longer have to worry about getting enough hammer energy.... That is the good news....

The bad new is that even with the new valve with the significantly larger ports, I'm still stalled out at 990 fps with the JSB Kings (55 FPE).... While that's not too shabby for a 22XX based gun, it's the same as I got yesterday with the old valve and a weaker hammer strike.... It's possible that the rest of the porting in the gun (transfer and barrel ports) which were already at 3/16" are the limiting factors.... however, my Hayabusa uses a 3/16" barrel port and I've hit 1060 fps with JSB Kings at 2000 psi with that rifle....

When I set the gun up for the same velocities I shot in yesterday's string (8 shots starting at 2000 psi, peaking at 905) I got a virtually identical string.... However, the gun used more air to do it, and on the last shot or two, you could hear some hammer bounce.... It used 1000 psi instead of 800 to produce the same results.... so the hammer bounce is costing me about 20% on efficiency, and I'm pretty certain that is only in the last half of the string, when the pressure is under 1600 psi.... I have seen hammer bounce before (rarely) and it seems worse when there is more preload on the valve and a lower spring rate than when the hammer spring is stiffer with less preload....

Anyway, I have learned something about valve throat size.... As you go larger, it requires more hammer energy to open the valve at the same pressure.... Without that, the peak of the velocity curve will move to a lower pressure.... This makes perfect sense, it's just the first time I've seen it first hand.... It also explains why my Hayabusa required a stronger hammer strike at 2000 psi than anything I had tried previously.... it had a 1/4" throat just like this valve.... The next test will be to put back in the old valve (with a 0.234" throat) and test it with the new hammer with guide and QB spring....

Bob

rsterne · Moderator

Tried some heavier pellets today with the hammer spring preload 3 turns out (more doesn't help).... These were shot at 2000 psi....

25.3 gr. JSB Kings - 990 fps = 55 FPE
30.9 gr. H&N Baracudas - 940 fps = 60.6 FPE
35.3 gr. EunJin Domes - 900 fps = 63.5 FPE
41.7 gr. EunJin Points - 855 fps = 67.7 FPE

I guess I can't complain too much, this is 10% more than I've ever had before with a 22XX based PCP.... and within 10% of my Hayabusa....

Bob

rsterne · Moderator

After a rather long enforced break due to busy season in the Motel, I got a chance to get back to this project over the past couple of days.... I sold off some of my earlier 2260 HPA parts, which forced me to commit to the "tank under" conversion of that gun, which saw the destruction of about 100 ground squirrels and marmots this spring.... I used the tank mounting block, extended valve, and hammer spring guide from earlier in this thread, and reassembled the gun using a 13 CI tank regulated to 1600 psi and carrying a 1.8K burst disc.... The hope was that the increased plenum volume of the full length 2260 tube between the regulator and valve would allow me to achieve the same power levels on less pressure than I was using previously (1900 psi), and I'm please to say that was the case....

The initial tests were done without the spring guide (which increases the hammer weight by 11 gr.) with the following results....

As is typical with a regulated gun, the velocity plateaus if you have more hammer strike than needed for the regulator output.... In this case, there is no loss in velocity with reducing the preload one turn from coil bind, and only a 20 fps loss at 2 turns out.... That setting gives me 943 fps with 21.0 gr. H&N Baracudas (41.5 FPE), and 1000 fps (40.2 FPE) with the 18.1 gr. JSB Exact Heavies.... One more turn out drops the 'Cudas to 890 fps (36.9 FPE) and the JSBs to 954 fps (36.6 FPE), which is right on the velocity that proved so deadly with those pellets in this gun in the spring.... One more turn out, and the power level drops to 30 FPE....

The purpose of using the full length 2260 tube and reversing the tank to sit underneath it, was to greatly increase the volume of the "plenum" between the regulator output and the valve seat.... The extended front end on the valve allows more air to enter the valve than can escape during the shot, so the pressure at the valve seat drops less because of that, raising the AVERAGE pressure during the shot.... My calculations show that the air usage (33 CI at 1 bar) when the gun is tuned for 37 FPE drops the pressure to 1480 psi when the valve closes at the end of the shot, making the average 1540 psi during the shot.... That means the valve is releasing 4.9 cc of HPA to do the work.... This rifle is a .22 cal with a 24" barrel, which places the bore volume at 14.5 cc, so that air can expand by nearly a factor of three in the barrel, which should lead to reasonable efficiency.... Here is a graph of what is happening in this case....

You can see that the efficiency tracks virtually parallel to the ratio of the bore volume divided by the volume of HPA used for the shot.... This is an important piece of information I learned from Steve in NC on the Green Forum, and I thank him for that "lightbulb moment".... After doing the baseline work, I added the 11 gr. hammer spring guide, and found that I could now back off the hammer spring preload an additional 3/4 turn and still get the same velocities.... At this point I also changed to a new tin of JSB Heavies, and it turns out they were 0.1 gr. heavier, so I fiddled with the preload to bring the velocity to 950-960 fps and shot a complete string, filling to 3000 psi....

As you can see, the results are an extremely flat string, with an ES of only 12 fps over shots 2-45.... I'm still under the hex of the "low first shot after a fill" gremlin, but at 938 fps, it was only 2% below the average velocity of 957 fps (37 FPE).... The bottom line is that I had 44 shots within a 12 fps range, and 47 shots with 2.5% (24 fps).... The pressure after the last shot was 1280 psi, meaning that I can shoot 300 psi below the regulator setpoint without a noticable drop in velocity.... That shows a VERY good balance between pressure and hammer strike, which is confirmed by the efficiency of 1.12 FPE/CI.... All that remains is to do some long range testing to find the highest velocity at which the JSB Heavies will hold good groups.... and to test a few other pellets to see if any others are up to scratch....

Bob

rsterne · Moderator

After many months I finally got back to working on my 2560 HPA.... This round of mods is going to see an 18 CI tank instead of a 13 CI, with the regulator set as low as I can and yet achieve 50 FPE with the JSB Kings.... Ideally I would like to keep it to 1600 psi and use a 1.8K burst disc, but the way the valve is mounted in this gun can stand 3000 psi easily, so I can certainly run a higher regulator setpoint and a 3K burst disc if I need to.... I decided that this time around I would hog the Disco valve out even more, and enlarge all the ports to 7/32" (0.219") or the equivalent area.... Here are the results on the valve, with a stock one beside it for comparison....

In the first photo, the stock valve is on the left.... The port is 9/64" diameter and vertical.... Note that you can't see the valve throat at all.... The modded valve is the one on the right.... The port is 9/32" diameter and angled 20 degrees.... You can clearly see the valve throat....

In the second photo, the stock valve is on the right, and the modded one is on the left.... The drills inserted into the ports show the huge difference in the diameter and the direction of the ports.... The new valve has nearly 2.5 times the port area of the stock one.... and it is 36% bigger than it was during the last set of tests.... The valve throat was drilled previously to 1/4" (stock is 7/32"), and the stem of the poppet has been slimmed from 5/32" to just under 7/64".... That means the throat area is slightly larger than the rest of the valve ports, and over double the area of a stock valve.... The transfer port is 7/32" ID, and the barrel port tapers from a 7/32" circle where it meets the transfer port to a 1/4" long x 3/16" wide oval on the inside.... That maintains the area, while keeping the width of the transfer port to 75% of the bore size to avoid pellet loading issues....

I also had to drill the top hole in the main tube to 3/8", and also the breech.... The new transfer port is made from 3/8" brass stock, turned down to 1/4" OD on the ends and sealed with O-rings directly to the valve and barrel.... I'm not too far from being able to assemble the gun for initial testing, tethered to a regulated air supply so that I can change the pressure without pulling the gun apart....

Bob

Alstone · Moderator & Site Supporter

Thats a cracking shot string Bob, it should make for some very accruate shooting. Thumb Up!

Al

rsterne · Moderator

I assembled my 2560 today, but with just a fill fitting in the end of the tube, no regulated tank.... This was just to find out if I can set the regulator to 1600 psi and achieve the 50 FPE I was hoping for with 25.4 gr. JSB Kings.... I set the hammer spring preload at maximum, filled the tube to 2000 psi, and shot the first shot across the Chrony.... 1032 fps (60 FPE).... So far so good, more power than I have ever seen in a .25 cal 22XX based gun before.... I refilled the tube, reduced the hammer spring preload a turn and tried again.... 1032.... well, at least I have some hammer adjustment left, that's good.... I repeated the procedure for 10 settings, from maximum to 9 turns out, with the following results....

You will notice that for the last half of the settings I also tested the gun at 1600 psi.... I was pleased to find out that the gun will shoot at least 964 fps (52.4 FPE) at that pressure, so my first regulated trials will be at 1600 psi.... Out of curiousity, I then set the hammer spring back to maximum and filled the gun to 2900 psi.... The valve screws are high tensile screws so I know they still have a huge safety margin at that pressure.... I tried several different pellets, topping up between shots.... I then tried the more common pellets at 2400 psi as well.... Here is what this gun can do when the pressure is cranked up....

You will notice that at 2900 psi the Kings are right on the verge of going supersonic, and the lighter ones have a nice loud CRACK to them.... The last thing I tried at 2900 psi was two Kings loaded together, for a total weight of 50.8 gr.... The resulting 92 FPE put quite a smile on my face.... it's the most power I've ever got out on one of my .25 cal guns.... At 2400 psi, the gun shot nearly 70 FPE with Baracudas, and almost 80 FPE with EunJin Points.... so it packs quite a punch even at moderate pressures....

Bob

rsterne · Moderator

I finally got around to assembling and testing my 2560.... It uses a 17 CI tank regulated to 1600 psi.... While I was able to get the power level I wanted for this gun (50 FPE with JSB Kings, ie 950 fps) I couldn't get the efficiency required to get the shot count I wanted (unless I dropped the velocity a bit).... Here is the shot string on 1600 psi, with the hammer spring backed off to where the velocity is just at the beginning of the velocity plateau....

That was with a 3000 psi fill with the 17 CI tank, and after the last shot the pressure was 1350 psi.... so the gun is using nearly 70 psi per shot, for an efficiency of only 0.63 FPE/CI.... Lloyd-ss asked me the other day if I thought I might get better efficiency by using a higher pressure and running a lighter hammer strike, and I said it was likely, but I wasn't sure.... After these results, I decided to use Lloyd's Internal Ballistics spreadsheet to do some modeling.... I put all the numbers in for the setup above, and then tried various pressures up to 2000 psi, reducing the dwell to get the velocity back down to 950 fps.... As you increase the pressure, the efficiency goes up, but the amount of air in the tank you can use goes down, of course.... Well, the spreadsheet predicted that for my gun (with no other changes) that I should be able to get the most shots using 1800 psi at 950 fps.... I spent a day tearing the gun down, resetting the regulator to 1800 psi, and retesting.... Lloyd's spreadsheet certainly guided me in the right direction....

Without any change to hammer spring preload from the 950 fps @ 1600 psi setting, the velocity jumped to 977 fps....I backed the preload down one turn, and the first thing I noticed was how much quieter the gun was.... That meant I should see a jump in efficiency, and was I in for a treat.... The efficiency jumped up from 0.63 FPE/CI to 0.96 FPE/CI at the same 950 fps.... The shot count increased by 52%, from 23 shots to 35 before the first shot under 940 fps.... I was able to shoot down to 1500 psi (300 psi below the setpoint) before any drop in velocity.... With the gun regulated at 1600 psi, I was able to shoot down to 1350 psi, so I'm getting that huge increase in the number of shots on less air.... I'm now only using 43 psi per shot instead of nearly 70.... Here is how the gun responds to the hammer spring preload....

You will notice that I'm now running the gun on the "knee" of the curve (at 950 fps).... Yesterday, when I was running the gun at 1600 psi, I had to run it just where the plateau starts in order to get the same velocity.... By bumping up the setpoint just 200 psi (which moved the plateau up about 27 fps), I was able to avoid the velocity plateau where the velocity tops out but the air use skyrockets.... I learned something VERY valuable here.... The best balance between high power and efficiency occurs BEFORE you reach the plateau.... I can now get over 4 full 8-shot MRod clips on a fill at nearly 51 FPE.... That is exactly the same (over 4 full 10-shot clips) as I get with my 2260 HPA, which delivers 44 shots at 37 FPE.... Both guns shoot at 950 fps, which should give me virtually identical trajectories, which was one of my goals.... I plan to fine tune them to that end.... Here is the new shot string.... Compare it with the one above....

Lloyd's spreadsheet proved itself invalulable, because it gave me the ability to choose the optimum setpoint pressure to get the most shots from my setup.... I'm convinced it remains the best tool I have for modeling what is happening inside a PCP....

Bob

toadmyster223 · Senior Member

Your ingenuity never ceases to amaze me. Great work