I've been thinking about the possibility of an extended valve to fit in a 2250 tube for some time, and the work I did over the last couple of days was to establish a baseline for this experiment.... I was wondering what would happen if I increased the valve volume, and since a 2250 tube is 1.6" longer than a 2240 tube, I simply made a 1.6" long extension to fit in the middle of the valve and lengthened the piercing pin by that amount.... Here are the parts....
The extension has a small pocket milled in the bottom to miss the front trigger mounting screw.... Above that, back to the valve seat, the extension is drilled 3/8" ID, and in front of that it is drilled 33/64" and the front part is tapped 9/16"-18NF.... The spring seat is just like in the original valve front, except the air passage is 17/64", and the male threads are shortened to 0.30", and the same is done on the valve front (originally they were 0.50" long).... These changes basically triple the volume inside the valve to about 5 cc.... The piercing pin was also lengthened 1.6" with a pointed piece of 1/8" piano wire, joined to the old pin (which was ground flat on the end) with a sleeve of 1/8" ID brass hobby tubing 1" long and glued in place with shaft retainer.... Here is the assembled valve....
The original spring was used, and the original poppet (except for the longer piercing pin).... plus 2 of the original size O-rings.... The valve just drops into the 2250 tube and is secured by the single 6-40 screw on the bottom as per the original setup.... You simply drop in a CO2 cartridge and use the original 2240 CO2 cap to hold it in place.... Cock the gun and fire it to pierce the cartridge, exactly as per the original....
Before disassembling the gun to fit the new valve in a 2250 tube, I installed a new RVA.... I got a Crosman power adjuster from the Challenger, which uses a 24 TPI screw instead of the 16 TPI one I had.... In addition, it has longer travel, and I lengthened it even more by drilling out 3/16" of the threads on the inside to allow the hammer spring to move back further.... It now has 18 turns of travel (3/4") from coil bind to the minimum setting.... Since the adjuster was new, I did a few tests to correlate the previous information to the positions of the new adjuster, then stripped the parts out of the 2240 tube and reassembled them into the 2250 tube, but using the extended valve.... I dropped in a new cartridge and started testing by checking to see not only if and how much the velocity changed, but how that change related to the new RVA.... Here is the results of that initial testing....
The black line is the 2240 with the original valve, and you can see that 12 turns out is where the velocity starts to decline, above that it plateaus.... With the new, extended valve (blue line), the plateau is moved 3 turns to the left (more preload), and the exciting part is that the velocity increased 55 fps.... That is an 11% increase in velocity and a 23% increase in FPE - JUST from the new valve, with no other changes.... In .25 cal, the difference is even greater.... The plateau also shifted 3 turns to the left, and the velocity increased 65 fps (15%), increasing the FPE by 31%.... That's a BIG increase in power from just increasing the valve volume.... I was hoping to see an increase, but I'm overjoyed to see it so dramatic.... Now it was time to shoot a string with the .25 cal and see what I got.... I loaded a new cartridge, shot three shots just to confirm the velocity results at 3, 5, and 7 turns out on the preload, set the adjuster to 9 turns out, and shot one shot per minute until the velocity dropped to 300 fps....
You can see the first three shots, mirroring the velocities in the above graph, and then the string at 9 turns out beginning on shot #4.... One thing became obvious during this, and the first cartridge I shot using the new valve, and that was how stable the velocity was.... Admittedly, part of the level shot string is from shooting every 60 seconds instead of every 30, but there is no question that the larger valve not only increased the performance a lot, but decreased the shot-to-shot variation as well.... Including all 28 shots, the average velocity was 466 fps (12.35 FPE), extracting 28.1 FPE from each gram of CO2 (nearly as good as the best results yesterday in .22 cal) and the efficiency worked out to 0.84 FPE/CI.... If you looked at only shots 4 - 23 (ie 20 shots), the average was 476 fps (12.8 FPE) with only a 19 fpe ES (4%).... and the efficiency would have been higher as well....
I'm extremely pleased with these results.... I've never seen anyone make a valve extension to stretch a 22XX valve to fit into a 2250 tube before, and when you consider the big improvement in performance WITH OTHERWISE STOCK VALVE AND PORTING, this may have the potential to unleash more performance on CO2 when combined with other modifications.... The idea I had, which seems to have been proven, was that by using a larger valve, the pressure drop inside the valve during the shot will be less, increasing the AVERAGE pressure during the shot cycle.... The fact that more preload was required to reach the velocity plateau is proof that indeed that happened.... The piercing pin nearly blocks the inlet to the valve completely, so all we have to work with is the volume inside the valve.... The only other alternative is to go to a bulk fill arrangement and increase the diameter of the valve inlet, but that brings another set of problems.... the major one being that if you raise the muzzle on a bulk fill gun, the valve fills with liquid CO2, resulting in inconsistent velocities and a horrendous increase in CO2 consumption as the cloud of liquid blows out the barrel.... If you stored this gun with the muzzle up, I suppose this could happen, but the clearance between the piercing pin and the hole in the valve front is so small that it would take a while.... I think this is an excellent way to get (nearly?) bulk-fill performance and still be able to use the 12 gr. cartridges....
The fact that the .25 cal responded even more to the extended valve than the .22 cal makes perfect sense.... The bore volume (for the same barrel length) is 33 % greater, so it NEEDS a bigger valve anyway.... In fact, I hope to be able to use this greatly increased valve size to experiment with even larger calibers, building a 3050 and maybe even a 3550.... I wonder how large I can go on caliber and still get a pistol that shoots over 400 fps on CO2 ?.... Sean and I have the barrels available, in .300 cal and .357, designed for the new JSB pellets.... The 25" rifle barrels are just begging to be cut in half and tried on a CO2 pistol with the new extended valve....
Bob