Today, after much prompting by Travis, I started modifying my Cothran valve, to try and improve it's efficiency in my .25 cal regulated BRod.... It is important to realize that these changes may not work (in fact are unlikely to work) in an installation that is prone to hammer bounce.... In my gun, I have an energy absorbing bumper made from "Iso-Damp" on the back of the valve, an extremely light MDS hammer of only 28 grams, a recessed striker that only pushes the valve stem about 0.050", and an SSG.... This allowed me to consider removing the small metering rod (counter-piston) which lives inside the valve stem.... There is a debate as to the purpose of that device, but I believe it to be to prevent the valve from re-opening on a second, or subsequent hammer strike, by acting as a "leaky check valve", slowing the HPA that fills the poppet head during the cycling of the valve from dumping back too quickly into the exhaust port.... I have been told that removing it will cause everything from uncontrollable hammer bounce, to various burps or flutters, to extreme air use, to even destruction of the valve itself.... However, since Travis agreed to supply me with any parts I destroyed I decided to remove it.... I tethered the gun at 1900 psi and started testing.... What happened?.... Exactly NOTHING.... Zip.... NADA....
The valve functioned exactly as before, the same velocity, sound, air use, everything.... I was shocked, but it does reinforce my belief that its primary purpose is anti-bounce.... which I have already addressed by a combination of four different methods.... I did notice one thing that changed, if I leaned on the hammer strike by adding preload (instead of gap) to the SSG.... thereby vastly overdriving the hammer.... the velocity DROPPED slightly, accompanied by a high-frequency "flutter".... I don't know how else to describe it.... It sounded like hammer bounce, but much faster.... The strange thing was, it didn't use more air.... As I said, removing the metering rod may not work in many installations, if they have any possibility of hammer bounce.... but in my gun, set up the way it is, it made no difference, when the SSG was adjusted normally, with a bit of gap.... The velocity was stable until you got to the cliff, and then dropped like a stone, same as always....
OK, so the other thing Travis (and others) have tried is drilling out the tiny vent hole in the valve stem.... This hole lives in the exhaust port, and allows the hollow head of the poppet (which is the diameter of the "force reduction" piston attached to the front of the valve) to be at atmospheric pressure between shots.... This "blanks off" the equivalent area of the poppet, drastically reducing the opening force required to crack the valve.... When you fire the gun, the pressure in the exhaust port rises, HPA flows through that hole, through the valve stem (past the metering rod) and pressurizes the inside of the poppet, providing a greatly increased closing force for the valve.... Without this vent, the valve would stay open, and likely vent the entire reservoir.... With it, the valve cycles in a timely manner, allowing lots of air to escape and produce copious amounts of power.... The way Don builds the valve, I would estimate the dwell time at roughly 2 mSec.... and if you have way too much hammer strike, it can be even more.... Limiting the valve lift keeps that more constant, preventing the valve from being a real air-hog, which is why I like that modification....
In stock form, that vent hole is tiny, only about 0.020".... Logic dictates that enlarging it will make the valve close faster, reducing the dwell.... and that should lead to increased efficiency.... The stem is HARD, and Travis warned me not to try drilling it, and I didn't have small enough drills anyways.... so I did like he did, and used a thin pointed diamond burr in my Dremel and "countersunk" the hole, gradually opening it up at the same time.... The smallest diameter, doing it this way, is just at the very inside of the vent hole, and I found I was causing tiny burrs inside the stem, which I had to keep cleaning out with a #31 twist drill (0.120"), turning it by hand.... I did this in case I had to drop the metering rod (0.118" diam.) back in the hole, I didn't want it to get stuck.... Anyway, I used a couple of sewing needles with the points ground off as go-nogo gauges, and when one that was 0.028" would fit through the hole (but one measuring 0.030" wouldn't) I quit grinding.... I chose 0.028" because it has TWICE the area of the original hole.... I have no idea it that is optimum, but I wanted to start smaller than what others had used, which was 1/32" (0.031") or larger.... I left the metering rod out, reassembled the gun, and repeated my testing.... All I can say is WOW !....
First of all, it took a bit more hammer strike to open the valve.... not a lot, but a couple of turns less gap on the SSG.... Secondly, instead of a completely flat plateau, with only a few fps drop just before you got to the cliff, and then nothing.... I actually saw a "knee" on the curve.... It wasn't long, the sensitivity to SSG gap was still there, but compared to stock it was day and night.... Here are my results....
Note that the blue (stock) and black (no metering rod) lines virtually lie on top of each other.... At 3 turns of gap on the SSG, the velocity is at maximum, at 4 turns, it has only dropped a few fps, and shortly after that the velocity became unstable (400-500 fps ES) and at 5 turns out it was in the basement.... With the larger vent hole, compared to the velocity at zero gap, there was about a 10 fps loss at 1 turn out, and then about another 15 fps in the next turn.... At 2 turns of gap, the velocity was 25 fps below the peak, and the ES was still less than 1%.... At 2.5 turns of gap, the average velocity dropped another 40 fps, to what in a conventional valve would be the middle of the knee, but the ES had increased to about 2%.... At 3 turns of gap, the velocity was down 200 fps, but the shots were all over the place, with an ES of over 5%.... The valve was becoming unstable, although nowhere near as bad as a stock one.... At 4 turns out, the valve was no longer cycling properly, it was in the basement....
I then tethered the gun to my 500 cc bottle, regulated at 1800 psi, for some efficiency testing.... I left the bottle connected to the gauge on my Great White (valve closed) so I could record the pressure drop for a 8-shot mag. and tested each half turn out, starting at zero gap.... At that setting, the velocity was maximum for this pressure, at 968 fps, and the drop 280 psi, for 0.97 FPE/CI.... This was basically the same as I had before, with the 34 gr. JSB Heavies.... At 1 turn out, the velocity was 962 fps (220 psi) and 1.22 FPE/CI.... At 1.5 turns, 951 fps (210 psi) and 1.24 FPE/CI.... and at 2 turns of gag, 943 fps (200 psi) and 1.28 FPE/CI.... This is far better efficiency than I have ever had before with the Cothran valve in my .25 cal BRod.... At 2.5 turns out, the velocity (809 avg.) was becoming unstable, and it was much worse at 3 turns out (701 fps), so I didn't record the efficiency, figuring it was meaningless....
So, what are my conclusions?.... Well, for one thing, if you have cured all chance of hammer bounce, I think you can try removing the metering rod, I don't think it is necessary.... Secondly, doubling the area of the vent hole, by increasing the diameter to 0.028", decreases the dwell at a given hammer strike, and creates the beginning of a knee to the velocity vs preload/gap curve.... It gives you significantly more control of the velocity just above the cliff, and softens the cliff into a usable, although sensitive knee.... While the operating pressure is still by far the dominant factor in the velocity, you can now detune the velocity just a bit, saving air in the process.... I haven't got all the answers, but I think we're on the right track....
Bob