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PostPosted: Sat Nov 12, 2016 8:29 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8977
Location: Coalmont BC
Today I tethered the Disco Double to a 1900 psi regulator instead of 2900.... With the much lower operating pressure there was lots of adjustment available on the SSG gap before the valve "fell off the cliff".... It is interesting that you can change the gap by 1/16" when you are on the plateau and the velocity only changes a few fps, but when you get to the cliff, a change in gap of just 0.020" (1/3rd turn) drops the velocity from 870 fps down to 310 fps.... I set the gap quite close to the cliff at 1900 psi, and then shot a string with the 78 gr. JSBs down to 1000 psi, which took 14 shots, starting at 872 fps and ending at 705 fps.... The Pressure drop per shot was significantly less than the tests I did with the SSG set up for 2900 psi, which shows it pays to be closer to the cliff to get better efficiency....

I then reduced the preload on the SSG from 7 lbs. down to 5 lbs. and reduced the pressure to only 1500 psi and retuned the gap on the SSG for a bit above the cliff at the lower pressure.... I then shot a string from 1500 psi down to 1000, which was 9 shots, starting at 805 fps and ending at 705 fps.... The air usage per shot on this string almost exactly matched the previous one.... I then plotted and normalized the results, which are presented in the table below....

Image

If you compare the pressure drop per shot in this table to the previous one, shot with the SSG adjusted for 2900 psi, you will notice that it is using less air, so the efficiency is even higher than before.... I am extremely pleased with these results, so rather than remove the valve and start playing with it to see if I can get a bell-curve, I now plan on installing my .25 cal barrel, tethering the gun at 1900 psi, and seeing what I can get for results.... I have high hopes for good power combined with good efficiency, and will test it with 25 gr. Kings, 34 gr. Heavies, and possibly with bullets as well....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Nov 18, 2016 11:59 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
I had to spend a couple of days pruning our fruit trees for the winter, but the outside winterizing projects are now done, so I was back in the shop this afternoon.... I machined the barrel port on the .25 cal, 25" long barrel to match the new 3/8" OD x 1/4" ID transfer port, using an oblong barrel port of 3/16" x 5/16" at the boreline, so the porting is full bore area straight through.... I was expecting great things when I tethered the gun to my 1900 psi regulator, and I wasn't disappointed.... The first shots with JSB Kings were nearly 1100 fps.... I was able to use the same preload and gap setting on the SSG as I used in the .357 cal at the same pressure, and since that velocity was too high, I switched over to the 34.2 gr JSB Heavy Mk 2s.... The velocity was still nearly 1000 fps, with the gun doing 75 FPE at 1900 psi on PELLETS !.... The best I had ever seen before at that pressure with the old valve was 67 FPE, so the larger ports certainly did their job.... The ideal pressure with the Heavies is 1700 psi, which produces a velocity of 960 fps at an efficiency of 1.04 FPE/CI.... This was a lower efficiency than I was hoping for, but better than I have seen at such a low pressure before.... Here is the table with the normalized results for the 34.2 gr. JSBs....

Image

I then switched over to a 50.6 gr. BBT, and ran two strings, one from 3000 psi down to 1900 (using a smaller SSG gap), and the other from 1900 down to just below 1500 psi.... At 3000 psi, I got 1018 fps (116 FPE), which is a bit more than I ever got with my own valve at the same pressure.... The big difference is that it developed that power on a LOT less air than my previous best setup.... Tethered at 1900 psi, I got 885 fps (88 FPE) at an efficiency of 1.30 FPE/CI.... By comparision, the unregulated version would do an 11 shot string at the same average FPE, but the efficiency was only 0.80 FPE/CI.... Backed off so that the peak velocity matched the Cothran valve, I could get a 14 shot string within a 4% ES, at 0.95 FPE/CI.... Mind you, I did not have an SSG fitted to the unregulated version.... Here is the table of normalized results with the 50.6 gr. BBTs....

Image

I am quite pleased with the results with the Cothran valve in a regulated .25 cal PCP.... It can shoot 34 gr. JSB Heavies as fast as you would want to on just 1600-1700 psi, at pretty decent efficiency.... You could expect to get about 18 shots with a 13 CI (215 cc) tank, or 30 shots with a 22 CI (360 cc) tank.... If you used a 500 cc bottle (30.5 CI) it should be possible to get over 40 shots at about 950 fps.... I think that any of those combinations would be a dynamite Varmint rifle.... The plenum on my Disco Double is the entire upper tube (135 cc), and you really only need half that with the 34 gr. pellets.... A 2260 tube would be the perfect volume, and 1700 psi should do the job.... The added benefit is that the gun would be very easy to cock....

If you wanted a bullet shooter for the 51 gr. BBTs, you would need about 2000 psi to drive them at 900 fps (91 FPE), or 2400 psi to hit 950 fps (102 FPE).... Using a 3000 psi tank wouldn't give you a lot of shots at the higher pressure, but not too bad with a 2000 psi setpoint.... Of course if you had a 4500 psi tank, then you could get a ton of shots.... With a 500 cc bottle, you should be approaching 50 shots at 100 FPE.... The Cothran valve flows enough air to handle even heavier bullets, and a .257 cal shooting the Lyman 257420, which weighs 73 gr. should get into the mid 900s without too much problem.... If we assume that is attainable at 3000 psi, with an efficiency of 1.0 FPE/CI (which I think is nearly a given).... then a 500 cc 4500 psi bottle, regulated at 3000 psi, should get about 20 shots at 145 FPE.... While this is only theory until proven, I think it is a reasonable assumption, based on the testing I have done.... In his testing of the MRod Cothran Powerhouse Valve, which has the same port sizes, Lloyd got 950 fps with a 73 gr. bullet in .257 cal at 2800 psi (in a 30" barrel), with excellent efficiency.... There is no reason the Disco version should not do the same.... A 28" barrel at 3000 psi should have the same power as a 30" barrel at 2800....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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