Influence of Port Size on FPE in a Discovery

[rsterne]

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

[Doc Sharptail]

Got a picture of the test platform?

I'm going to get back at that 20-78 tube in a couple of weeks, and the info you've presented here is a good starting barometer. I won't be able to use fill pressures quite that high...

Got a 1701-P on the way in, so I'll probably be busy with that for a while...

Regards,

Doc Sharptail

[rsterne]

I received some Challenger and Canadian 1377 Transfer Ports yesterday, and this evening I finished the testing.... All shot strings started at 2000 psi, and stopped when the velocity fell to 96% of the peak for that string.... The upper strings were shot previously.... One string, for the 0.070" port, was re-shot as the previous data seemed to high, and when I measured the port, it was, in fact slightly oversize, so I used one of the new 1377 Canadian ports.... Here are the shot strings....



Remember that the reservoir in this gun is from a 2260, so the shot count would double for a Disco with the regular 135 cc reservoir.... The only string which had a complete bell-curve starting from 2000 psi within the 4% ES was the 0.094" transfer port.... The larger ones only required an 1800 psi fill, likely the result of the PRod gauge port which flows more air into the valve, keeping the average pressure during the shot a bit higher.... As you would expect, using the larger gauge port with a stock Disco hammer spring and preload didn't increase the performance of the gun, it just moved the strings to the right.... All of the smaller ports started within 2% of the peak velocity, and in fact the bottom two strings started quite near the peak.... That indicates that either a higher fill pressure could be used, or the hammer spring preload backed off a bit.... Since the efficiency is always higher during the first part of the shot string, I feel that the gradually declining efficiency as the port got smaller and smaller may well be due to the loss of the first part of the shot string....

The efficiency with the larger ports ran between 1.22 - 1.28 FPE/CI, pretty much usual air usage for a Disco.... As the size dropped below 0.082" the efficiency gradually declined, and with the smallest port was just under 1.0 FPE/CI.... As I said, however, the first (and most efficient) half of the string was missing.... It wouldn't surprise me that properly tuned the efficiency would stay in the 1.2 FPE/CI range regardless of port size and FPE output.... I also graphed the FPE vs. the port size, and here is the result....



Basically, once the transfer port is smaller than the 0.134" barrel port, the FPE drops pretty much linearly with the transfer port diameter, and in fact you can extend the trend through zero on both axis.... The upper part of the curve is nearly a plateau because of the flow being limited by the barrel port.... These results are consistent with what I have seen before, both in previous testing with a Disco, and in other PCPs....

I hope that this information is helpful to anyone considering detuning a Disco for target or indoor use.... Using smaller transfer ports is an excellent way to flatten the shot string at the same time as you reduce the FPE of the gun.... A transfer port of 0.055" is close to optimum for the <500 fps Canadian rules, and one of about 0.070" (as used in the Canadian version of the 1377) is about perfect for the 12 FPE rules in the UK or for Field Target shooting.... Incidently, here is the test bed used....



For these test, however, a standard Disco rear spring seat plug was used, not the Challenger RVA shown in the photo....

Bob

[rsterne]

As I mentioned yesterday, I suspected that the efficiency of the shot strings with the smaller ports was suffering because of the loss of the first (high pressure) portion of the string.... This morning I replaced the Disco hammer spring with one from a 1377, still using the stock Disco rear plug as a spring seat.... Here are the results....



The red line is from yesterday, using the Disco hammer spring, and the green line from today using the 1377 hammer spring.... The first two shots were just below 550 fps, placing them outside the 4% ES range, so they weren't included.... but it still serves the purpose of the experiment.... The shot count went from 24 to 39, and the average velocity dropped from 605 fps to 563 fps, but the efficiency jumped up from 1.13 FPE/CI to 1.37 FPE/CI.... even better than I suggested last night.... As a comparision, the string using a 0.063" port and a Disco hammer spring only had 30 shots averaging 545 fps with an efficiency of 1.09 FPE/CI.... You can see how important the first half of the shot string is to the overall efficiency and shot count.... It reinforces how important an RVA is to tuning a PCP as without being able to set the preload on the hammer spring it is much more difficult to get the best results.... Had I used an RVA with the Disco spring and reduced the preload, similar results would have been obtained.... Remember, a full sized Disco tube would double the above shot count, giving nearly 80 shots at 10 FPE....

Bob