New Parts for my 2260 and 2560 HPAs

[AllanWJ]

Thanks Bob. I'm still playing with HiPacs and stock valves but I'm ordering a Disco valve for another conversion and slew of other parts - learning as I go. I'll throw a fill adapter on the order and have something to play with. Gotcha on the retaining screws.

Allan

[lloyd-ss]

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

Bob,
I've been reading and re-reading this post.  Is another way of describing the thought behind the "light bulb" moment, that the faster you can close the valve, the more efficient the shot is likely to be? And in general, that would mean backing off from any max velocity shots.  But as we have often seen, you don't necessarily need to loose too much velocity.  I agree that that is a good rule of thumb.

Regarding your regulator re-start lag.  I have experienced that in other kinda similar situations and have my theory. As the air pressure on the input side of the regulator continues to drop with each shot, the o-ring(s) tend to get pushed to one side of their gland. Then when the tank is recharged, the o-ring is moved to the opposite side of the gland. That cause that first-shot anomaly.  Then, after the first shot, the o-ring gets shifted back to its normal position.  I have tightened up the groove width to reduce the shifting of the o-ring, and have also been a bit more consistent with a good silicone lube.  If you want to chase that gremlin  , that might help.
Lloyd-ss

[rsterne]

Thanks, lloyd, I'll do some testing and see if it is indeed only the first shot after a refill, or the first shot after the gun sits for a while.... If it's the latter, it could be that the delrin poppet is taking a set over time and, since the seat on a Disco valve is slightly rounded, the diameter is increasing as the metal seat sinks into the Delrin head.... That would increase the effort to lift the poppet off the seat on the first shot.... A squared off seat on the metal part of the valve might avoid that diameter change.... If it's the first shot after refill only, I'll likely not worry about it....

I wouldn't say it so much how "fast" the valve closes as how "early" it closes relative to pellet travel down the barrel.... The earlier it closes, the more efficient....

Bob

[rsterne]

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

[Rescue35]

Looking good!

[lloyd-ss]

Very nice Bob.  So the objective is to get the unit to function properly at a pressure that is very low for the amount of power you will be producing.  I think I have that right.  And that generally means getting a big fat burst of air out very quickly, and then shutting the valve very quickly. 
Forgetting efficiency for the moment and going strictly on shot count, I wonder if this 1600 psi low pressure setup will give more total 50fpe shots than a higher pressure might.  This is a good experiment.
Lloyd

[rsterne]

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....

Lloyd, the jury is still out whether or not you can get more shots by using a higher setpoint and a lighter hammer strike for a given power level.... The plus is that the higher pressure for a set FPE gives a higher FPE/CI.... The downside is that it you go too light on the hammer strike relative to the pressure, then after you come off the regulator the velocity climbs.... plus with a higher setpoint you have less pressure range available between the fill pressure and the setpoint, which mean fewer shots.... It's a balancing act, and so far I have had the best success by doing just that.... balancing the hammer strike and setpoint pressure to allow the gun to shoot a few shots below the setpoint.... I seem to get the flattest shot string that way because although the gun is regulated, it's also "self-regulating" as well....

Bob

[lloyd-ss]

Bob,
Fantastic results, and a nicely documented history and explanation of your methodology. The steps you go thru to optimize the pressure and spring preload demonstrate how you have a true understanding of the real world cause and effect of the numerous parameters in the system. This is a valuable reference for anyone who does serious tuning of PCPs. It is appreciated!

I look forward to seeing the stings from this high power setup.  The hammer is around 75 gms, with a QB78 spring? Could you add the exact specs of both? Your adjuster is 16 pitch?
Thanks again!
Lloyd-ss

[rsterne]

Will do... Striker and guide is currently 73 gr. but I will be shortening the guide slightly as I need to redo the RVA (currently too long).... Since I have plenty of hammer strike I can afford to reduce the weight slightly.... The QB hammer spring is 0.33" OD x 2.33" long, 0.047" wire, 20 coils.... and the adjuster is 16 TPI.... I don't plan on doing any strings running the gun at high power.... It not worthy sooooooooo much air and the reservoir is only 67cc including the valve.... so I'm fighting an uphill battle as it is.... I'm really pretty excited that I got over 90 FPE from that small a reservoir, but the pressure drop was HUGE (over 1000 psi) so in fact the average pressure during that 92 FPE shot was something like 2400 psi.... The gun recoils like a .22 LR, and LOUD, even when not supersonic....

What I did learn from this is that there are gains to be made in increasing the porting beyond 3/4 the bore size.... With a bottom port, you have to start going to an elongated barrel port to maintain the area, but this gun has the ports about 88% of the caliber, and it outperforms anything I have done before.... Realistically, it is overkill for most applications, but it does allow more FPE at lower pressures.... When you think about it, it would be a lot easier to use 75% ports and just bump the pressure to 1800 psi, which in the last version gave the same velocity as this gun at 1600 (965 max).... This version was strictly to learn more about porting, but I already have an application in mind.... Plans are underway for my version of the "Millenium Pumper".... I plan to build on what I learned with my .25 cal Disco Carbine Pumper, but utilize a small internal reservoir, a 23.8" LW barrel and MRod trigger, nestled in one of Norm's Blaster stocks.... You can bet that I will be using a valve similar to this one....

Bob

[rsterne]

Well, Lloyd, I have some preliminary results from testing the 2560 on 1600 psi.... The relationship between velocity and air usage is in another thread so I won't repeat it here.... http://www.gatewaytoairguns.org/GTA/index.php/topic,38918.0.html

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.... 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 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.... You 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 possible but I wasn't sure.... After these results, I decided to use your 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, your 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.... Today was spent tearing the gun down, resetting the regulator, and retesting.... You'll have to wait for tomorrow for the complete results.... as my ShoeBox is topping up my SCUBA tank.... Let's just say your spreadsheet guided me in the right direction.... 

Bob

[rsterne]

Here are the results for my 2560 HPA regulated to 1800 psi instead of 1600.... 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