New Parts for my 2260 and 2560 HPAs

[rsterne]

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]

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.... hey! ?....

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 

[fivestar45]

Hi, Bob If I read your post correctly you increased the stroke by .1", and extended the cocking pin slot the same to compensate, but if you are using a standard length bolt you might want to check that the back of the bolt is clear of your revised slot. I've had a couple of OEM Disco's that had a problem with this and in their case when shot the cocking pin hit the back end of the bolt, the bolt handle would jump up.
Just a thought, in your case I figured it might possibly undo some of your good work.
Anthony

[rsterne]

Thanks for that thought.... However, I'm using a custom breech and bolt, and it doesn't interfere with the cocking pin when fired.... While the new valve has the stem shortened 0.100", that's only 0.020" more than the old valve anyway.... I can't see shortening it any more, as when you push it to coil bind it's now flush with the end of the valve....

The plan is to make a new RVA (power adjuster) that is longer, and fit a QB hammer spring (which is also longer and slightly stiffer).... The previous hammer was drilled oversize with that in mind, and also tapped for a spring guide.... I've used that setup before on my Hayabusa, and it has plenty of hammer strike to test if that's the problem.... and it certainly gives all the indications that it is....

Bob

[lloyd-ss]

Hi, Bob If I read your post correctly you increased the stroke by .1", and extended the cocking pin slot the same to compensate, but if you are using a standard length bolt you might want to check that the back of the bolt is clear of your revised slot. I've had a couple of OEM Disco's that had a problem with this and in their case when shot the cocking pin hit the back end of the bolt, the bolt handle would jump up.
Just a thought, in your case I figured it might possibly undo some of your good work.
Anthony

Anthony,
That is a good observation to bring up.  The standard Disco setup has  a little extra clearance at each end of its travel. But doing modifications can certainly change that.

[lloyd-ss]

............... 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.................
Bob 

Bob, yes, I have run into that same same situation when opening up the valve throat and then reassembling with the same spring settings.  It effectively moves the valve lock pressure to a lower pressure.  All I have been able to do to fix it is increase the spring force.  Also, at the higher air pressures with the big valve throat, the velocities also seem to ramp up very quickly as the pressure drops from the valve lock pressure. Then after that rapid climb, the velocities level out.
Lloyd

[rsterne]

at the higher air pressures with the big valve throat, the velocities also seem to ramp up very quickly as the pressure drops from the valve lock pressure. Then after that rapid climb, the velocities level out.

Kind of like the "spring rate" felt by the valve is non-linear and related to the lift?....  That would make sense, as at low lift (ie partial lock) the pressure diffferential across the head would still be quite large, and the valve gets slammed shut.... When the pressure drops a bit, the valve lifts further, and the head is more "surrounded" by the HPA and hence the dwell increases.... This may have to do with the "lift to diameter ratio" of the valve.... My understanding is that once it reaches 1:4 (ie the lift is 1/4 the diameter) the flow doesn't change much after that (at constant pressure)....

What I'm trying to say is that initially, when the lift is smaller than 1/4 of the diameter, both the amount of flow and the duration (dwell) are affected.... Once the pressure drops a bit, and the valve lifts 1/4 of it's diameter, then the flow is more constant, but the duration continues to increase to compensate for the pressure drop.... giving our typical "self-regulating" bell-curve.... Just a theory....

Bob

[lloyd-ss]

Bob,
Yes, good description of what is happening. And that 1 to 4 ratio works out where the circumferential area around the valve equals the valve throat area. It seems that many valves have excessive overhang beyond what is actually needed to cover the sealing margin. That excessive diameter must affect the air flow and that could be a good thing or a bad thing.  ANother variable to play with, LOL.
If the valve throat is .236 dia, why does the valve head need to be .350 dia, other than to have a good seat for the spring coil. It certainly wouldn't help max power, but maybe it helps to flatten out the velocity curve?
Lloyd

[rsterne]

I agree, Lloyd.... but that Disco overmolded valve poppet is just soooooooooo tough it's hard not to use it.... When I get around to converting my Hayabusa to a big-bore I'll be drilling the throat out to 5/16", so that will use most of the available diameter.... *grin*.... Anyway, here's todays trials and tribulations....

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 (at coil bind) 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) at 2000 psi.... 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]

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

[MustangMike]

when you guys talk about dwell time am i correct in thinking that is the time that valve is open after the hammer strikes it ?

also is the dwell time effected by the hpa coming over the valve seat and front of the valve seat on its way out. but because the i guess throat bottle necks the air before leaving the valve, the valve has to over come the same pressure as it did to be opened ?

want to make sure im understanding what im reading when you and lloyd make small talk.. lol

[rsterne]

Yes, "dwell time" (AKA duration) is the time the valve is open.... from the time the hammer strikes it until the air/spring pressure reseats it.... This is related to, but not the same as the "lift" which is how far the valve opens at the highest point in it's travel.... Generally, if you hit the stem harder, the valve will lift further, and the dwell will increase as well.... How they are related is very complex, and affected by the pressure difference across the valve seat, the diameter of the seat, the spring inside the valve (minor at high air pressures), the amount of lift relative to the diameter, the mass of all the moving parts (valve poppet, hammer, spring, guide if any), and how fast the hammer is moving at impact (which is affected by the spring rate, preload, hammer weight, and hammer travel, plus any drag in the system)....

The amount of flow through the valve is what governs the power of the shot.... more flow, more potential power.... For any given port size, the greater the air pressure, the greater the flow CAN be (ie more air mass is released), providing the hammer hit is strong enough to open the valve.... For any given valve/hammer setting, even that isn't a constant, it's affected by the weight and fit (drag) of the pellet.... The longer the pellet takes to get moving (or move a significant amount) the higher the pressure between the valve and the pellet will rise.... That (in theory) will increase the duration very slightly (less pressure difference across the valve seat trying to close the valve).... The higher pressure of the initial pulse (it's the most important part) will also increase the efficiency of the shot.... These factors can combine to increase the FPE of a heavy pellet compared to a lighter one.... The greater the power level of the gun, the more pronounced that difference tends to be....

All in all, it's a complex situation taking place in what is essentially a very simple device.... In a non-regulated PCP, it is how all those factors interrelate that gives us the "bell-curve" of velocities we strive for as the valve "self-regulates" the amount of air escaping to drive the pellet.... In a regulated PCP, once you have "enough" hammer strike to flow all the air that the gun can flow at that pressure (with that pellet) then adding more hammer strike only wastes air.... This is a very important lesson to learn that for the most part, goes ignored in the quest for power.... The way I determine how much is "enough" is to graph the velocity vs. the hammer spring preload.... Providing you have enough hammer power available, you will eventually see a plateau in the velocity.... Backing down the hammer strike to the start of the plateau.... or even better to just before the plateau starts.... gives the most power for that regulator pressure without wasting air.... If you continue to reduce hammer strike, the velocity will continue to decrease.... but you are better off the reduce the regulator pressure and start again....

Bob

[MustangMike]

Thanks for clearing that up bob, I do read this posts from you and lloyd and most times can figure out what you guys are talking about. Just need some clarification from time to time

[rsterne]

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

[1377x]

thats sure is a flat string!
nice job!

[lloyd-ss]

Bob,
Does the valve in the pictures int he very first post in this thread have a valve stem o-ring built into the valve body?
Thanks,
Lloyd-ss

[rsterne]

Yes, it has an internal O-ring to seal the valve stem.... Crosman are now omitting that in their current production valves.... I ground the notch in the top rear of the valve to miss the forward breech screw position in the breech I'm using....

Bob

[renowntwo]

Mr. Rsterne very nice work. Great results!!!

[AllanWJ]

Cool stuff Bob. There's something very sexy about those flat shot strings. 

I have an offtrack question. Is a Disco fill adapter threaded for a 22xx tube; i.e. are they plug and play? I could find out if I took my guns apart, but I thought I'd just ask.

Allan

[rsterne]

YUP, a Disco fill adapter threads right into a 22XX tube.... Don't forget about anchoring the valve and sealing it to the tube with O-rings, however.... I always use a Disco valve on my HPA conversions, mounted with three 8-32 (or 10-32) screws.... If you are drilling a 22XX tube, instead of drilling to 5/16" and using the pan-head Disco valve screws, drill the side holes 0.270" to match the bottom valve screw hole in the 22XX tube and use three 8-32 low profile socket head cap screws.... You can get them from McMaster Carr that are 180,000 psi tensile strength, and the head fits the bottom hole perfectly.... I think 1/4" is the shortest you can get, however, so you have to shorten them a bit....

Bob