I took some time today to experiment with the hammer travel / valve lift settings to see what effect it had on shot strings and efficiency.... I chose a power level that is at the upper end of what would be practical with JSB Kings, around 1000 fps average.... I started with a fill pressure of about 2000 psi and 4 turns in on the hammer travel and found I got a decent 7 shot string with the RVA set at 7 turns out filling to 1900 psi.... I then turned the hammer travel out 1 turn, the RVA out another half turn, and got another 7 shot string starting from 1950 psi.... One more turn in on the hammer travel same RVA setting, and I got a good string starting from 2200 psi, but only 6 shots.... I had to use the higher fill pressure to overcome the limited lift imposed by that setting, however.... In each case, I played with the RVA and fill pressure to get the best (ie flattest) bell-curve.... Here are the results....
In practical terms, there is little difference between the first two strings, and in fact they had the same efficiency, 1.08 FPE.CI.... At the greatest hammer travel, however (the black line), the valve can only open a maximum of 0.084" before the hammer hits the bumper on the back of the valve.... First of all, you can see that the string is shorter and steeper.... Instead of getting 7 shots within a 4% ES like in the first two strings, I only got 6 shots within a 6% ES.... However, the efficiency was quite a bit better, at 1.33 FPE/CI.... more power from less air used, but a shorter string.... Some of that extra efficiency, however, could be from the higher pressure required to overcome the severely limited lift at that setting.... I therefore wanted to try more settings in between 2-3 turns in on the hammer travel to explore what was happening.... I did several more strings, at 2.25, 2.5, and 2.75 turns in on the hammer travel, and played around with the RVA and fill pressure (in all cases between 1900-2000 psi) to optimize the shot strings at the same power levels.... Here are those results....
Now a clear trend is emerging.... as the maximum lift on the valve decreases, the shot string shortens and drops off very quickly after the peak.... The black line (0.095" max lift) shows that very clearly.... The red line (0.116" max lift) shows the flattest shot string, and the blue line (0.105" max lift) is in between, with the velocity just starting to turn down on shot 7.... All three of these settings produced better efficiency (1.14-1.16 FPE/CI) than the "unlimited lift" curves from the first chart, but not as good as the "severely limited" curve.... My conclusion was that once the hammer contacts the bumper (which seems to occur at about 0.105" of lift at roughly 1500 psi) the velocity then drops off on subsequent shots because the valve can no longer open further.... That very contact limits the amount of air wasted on those shots, increasing the efficiency.... but when carried too far causes the velocity to drop like a stone, shortening the shot string by a full shot (a big percentage out of 7 shots)....
I've never used the bstaley O-ring buffer mod, but from what I understand, these results would appear to contradict what he finds.... I can only assume that the energy absorbing qualities of the bumper is the reason.... The 70 durometer O-rings he usually uses may act a lot less like a lift "limiter" than a lift "cushioner", but that's just a guess.... In any case, the best shot strings seem to be as the bumper JUST starts to come into play.... When the hammer just starts to touch it on the last shot or two it seems to be increasing the efficiency slightly without truncating the shot string.... The roughly 6-7% increase in efficiency is small but measureable.... However, I'm not so sure that at these power levels it is worth the added complication of having the extra adjustment to worry about....
From previous experience, and from using Lloyd's spreadsheet to provide an estimate of how the efficiency is changing over the shot string.... I'm guessing that for the best setting the first shot is about 1.5 FPE/CI, and the last shot is about 0.8 FPE/CI.... That would mean that running the gun on a regulator at 2000 psi should return about 1.5 FPE/CI at 55 FPE, the practical limit for these pellets.... At some point I will tether it to a small tank to confirm that hypothesis....
Bob
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