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Disco Transfer Port Size 
PostPosted: Thu Mar 15, 2012 4:13 pm Reply with quote
rsterne
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In case you were wondering how to detune a Disco for more shots, here is some information you may find useful.... I have been able to get three sizes of ports, the stock 0.140" (9/64"), one that measures 0.109" (7/64"), and a stepped one from a Canadian 1377 that measures a tiny 0.070".... I made some intermediate sizes by drilling out some of these, ending up with additional ports measuring 0.166" (as big as you can go on a steel Crosman port, and the same size as a poly port), 0.156" (5/32"), 0.125" (1/8"), 0.094" (3/32") and also one measuring 0.081".... I used a tethered Disco running on a regulator, and tested all the ports at 1600 psi, which is the top of the sweet spot in a stock Disco.... I then reduced the regulator pressure to 1200 psi, and then to 850 psi, and tested the commercially available ports.... A stock Disco hammer spring and rear plug were used for all tests.... I used 14.3 gr. JSB Express pellets, and shot 10 shots with each combination, recording the average velocity with the following results....



You will note that increasing the transfer port size without also increasing the size of the barrel port and/or the valve porting pays no benefit.... This makes sense, as the barrel port measures only 0.134", slightly smaller than a stock transfer port, and is obviouly limiting the flow.... Reducing the diameter, however, does reduce the velocity, although the 1/8" port hardly makes any difference at all.... When you fit the 0.109" port, the gun is noticably quieter, losing about 6% in velocity and 12% in energy.... From that point on, losses are pretty linear, with the smallest port resulting in a very much quieter report, a loss of about 25% in velocity, and about 45% in energy.... Very similar losses are observed at the lower pressures, starting from a lower level with the stock (0.140") port.... It is interesting to note that the gun is louder at 1200 psi than it is at 1600 psi with a stock port, as is observed with an unregulated Disco....

I hope that the above information will be helpful to anyone contemplating detuning a Disco to increase the shot count.... I did not try weaker hammer springs, they would, of course, increase the shot count at pressures below 1600 psi.... A spring from a 22XX can be made to work at about 1200 psi, and one from a 13XX at about 1400 psi.... or an RVA which allows the preload to be reduced would be beneficial as well.... Too much hammer strike results in wasted air and excess noise, so it must be balanced against the pressure used....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26
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PostPosted: Thu Mar 15, 2012 5:19 pm Reply with quote
radar
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Did you drill out the factory port and tap it for the cartridge ports you made? I assume it's just a drilled hole from the factory.

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PostPosted: Thu Mar 15, 2012 5:39 pm Reply with quote
rsterne
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The transfer port on a Disco is a small steel sleeve sealed by a rubber gasket.... It is pinched between the breech and the top of the valve.... The stock sleeve has an ID of 0.140"....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26
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PostPosted: Thu Mar 15, 2012 7:45 pm Reply with quote
radar
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I see. Thanks for the interesting work here, Bob.
Would it be better to adjust the hammer strike or slip in a new port if one wanted to shoot thier Disco for many shots at reduced power? I understand that the power adjuster doesn't require disassembly every time, but would reducing port size actually get more shots per fill, or more consistancy, or has that yet to be tested?

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PostPosted: Thu Mar 15, 2012 10:20 pm Reply with quote
rsterne
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If you back off on the hammer spring preload you have to reduce the fill pressure, but you also get to reduce the refill pressure.... Because the FPE per shot is less, you gain on the number of shots.... but you tend to lose a bit on the total pressure range within the shot string.... That means you don't gain as much as you might think on shot count....

When you restrict the transfer port, you tend to flatten the bell-curve.... Taken to the extreme, it becomes nearly dead flat over a very wide pressure range.... Because you didn't reduce the hammer strike, the gun still works at high pressure.... You gain shots because the FPE per shot is less, but you can also actually gain shots because of the flatness of the bell-curve.... The result can be significantly more shots per fill....

The best example I have seen is the Hatsan AT44.... The full power version gives 30 shots averaging 27 FPE (14.3 gr. at 890-950 fps) from 190 bar down to 115 bar.... They detune it for Canada by fitting a thicker valve stem which chokes off the airflow.... The result is 180 shots at 460-475 fps (7 FPE) from 200 bar down to 90 bar.... The total power per fill went from 810 FPE to 1260 FPE, over a 50% increase, with NO other changes to the gun....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26
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PostPosted: Fri Mar 16, 2012 8:22 am Reply with quote
radar
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Interesting.
I may like to make a small bore transfer port then, if I want to shoot the disco "all day" at reduced power and leave it that way for a while. I do have a homemade power adjuster, but I haven't worked out a spring binding issue with it yet, so I took it back off and then kind of put it aside for a leater project.

-Rob

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PostPosted: Fri Mar 16, 2012 10:17 am Reply with quote
rsterne
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Or if you are shooting at reasonable temperatures, just fill it with CO2.... Lots of shots in the 600s....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal:
.177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26
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PostPosted: Fri Mar 16, 2012 11:27 am Reply with quote
radar
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Location: Des Moines, Iowa, USA
rsterne wrote:
Or if you are shooting at reasonable temperatures, just fill it with CO2.... Lots of shots in the 600s....

Bob


Oh sure, give me the logical solution. Laughing
I actually kind of forgot that I could do that. Thanks!

-Rob

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Disco Transfer Port Size 
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