There has been a lot of discussion lately about how changing the transfer port diameter affects the energy of a PCP.... I decided to assemble a test bed based on a Discovery, only trouble is I didn't have a Disco main tube handy, but I did have one from a 2260, which is basically half the volume (65 cc instead of 135 cc).... I drilled the sides of the tube to 0.270" to fit the heads of 8-32 low profile SHCSs, which I had in a 180K tensile.... That will make the valve installation good for 3000 psi with a 3:1 safety margin in case I want to use the tube at that pressure for other things.... The valve I used is a brand new, stock Disco valve, the latest model with no O-ring on the stem and the hollow spring seat on the poppet.... The only modification I did was to notch the top and bottom of the back to miss the forward location of the 4-48 breech screw (I used a 22XX breech) and to allow any air that bypasses the stem to escape through the hole in the bottom of the tube behind the lower valve screw.... Trapping air between the hammer and valve can lead to hammer bounce, venting that area eliminates that possiblilty.... I didn't have a Disco gauge block, but I did have one from a PRod, so I used that.... I don't think it will affect the results in any significant way at stock power levels.... I used a stock .22 cal barrel, with a stock 0.134" barrel port.... and I used a stock Disco hammer spring and rear plug, without adjuster, so the spring preload is stock.... Here are the parts....



I had a selection of transfer port diameters running from 0.070" to 0.156".... There were a couple of the smaller sizes I was missing, but I'll run those tests later and add the results.... I filled the gun to 2000 psi for each shot string, and recorded all the shots until the velocity fell 4% below the peak for each string.... I then dropped any shots from the front of the string that were less than 4% below the peak.... It turns out that with this particular combination of parts, for a stock transfer port (0.140") the usable string started at 1800 psi and ran to a refill pressure of 1000.... This is about 100 psi lower (both fill and refill) for the 4% ES portion of the string than the last Disco I tested.... I'm not sure if that means the hammer spring I used is a fraction weak, or if the larger hole in the PRod gauge port allows a bit more airflow into the valve, increasing the average pressure during the shot, and hence moving the bell curve down that 100 psi.... I suspect that since the peak velocity is about right with a stock port (834 fps with 14.3 gr. pellets) that the latter is the case.... All strings were shot with JSB Express pellets, and here they are....



The first thing to notice is that there is hardly any difference in power between the stock 9/64" (0.140") transfer port and 1/64" larger (0.156") or smaller (0.125").... This is to be expected, because the barrel port, which is 0.134" is influencing the flow available to the pellet to a large degree.... Even so, there is about 10 fps difference in the strings, and the shot count goes from 12 shots with the largest port to 13 stock, to 14 shots with the 1/8" port.... The efficiency of those strings is running about 1.27 FPE/CI, pretty typical for a Disco.... Once the port diameter drops below 1/8", however, the velocity and energy starts to drop significantly, and in addition the noise level also starts to drop as well....

With the 7/64" (0.109") port, power is right at 20 FPE for 15 shots.... With a 3/32" (0.094") port, it drops further to 17.7 FPE and the shot count increases to 19, this being the first string that stayed within the 4% ES at the full 2000 psi.... I didn't have a 5/64" (0.078") port, I'll make and test one later.... However, I did have a 0.070" port from a Canadian 1377, and with that fitted the string was 23 shots at an average of 12.5 FPE, starting at 2000 psi and ending at 1050.... Interestingly, with that small a port fitted, I can start at 2200 psi and add another 5 shots to the string for a total of 28 shots within 4%.... Remember that with a Disco tube, all these shot counts would DOUBLE.... I plan to get some Challenger transfer ports, drilling one out to 1/16" (0.063") and testing the other at the stock 3/64" (0.047"), and possibly one in between as well.... When the ports get tiny, just a small change has a large effect on the velocity and energy.... The efficiency was slightly less with the smaller ports, the worst one I tested being the smallest port, which still had an efficiency of 1.10 FPE/CI....

I hope you find this information helpful.... As an example, if somebody wanted to set a Disco up for 12 FPE, using the 0.070" transfer port and backing the hammer spring preload down a small amount should be a perfect solution.... A Disco set up that way should deliver about 50-60 shots at 12 FPE within a 4% ES.... and it's very quiet as well....

Bob