Steve in NC, your thoughts?....

I feel indeed fortunate to be able to bounce ideas off you and learn from your vast experience.... You present me with a different way of thinking about things, which is both refreshing and challenging.... The concept of relating bore to valve volume and how that relates to efficiency, is a perfect example.... I think perhaps where we have been running into confusion (or at least I have), is that the size of the valve is only the complete answer in a "pump and dump" situation where the entire contents of the valve disappear during the shot.... With an Air Conserving pumper, or a PCP, that of course doesn't occur, only a portion of the air is used.... In this case, the volume of the valve is still important, of course, but that volume is modified by the reservoir of air feeding the valve (assuming it can get in quickly enough).... In the extreme case, where the reservoir IS the valve (and only the back half with the seat exists), then the main purpose of increasing the reservoir size is to increase the average pressure during the shot.... The graph I posted previously showing why I strive for 1cc per FPE I want is aimed at optimizing that situation....

In order to better understand what is happening, I took my current 2260/2560 regulated HPA project and calculated the volume of 1600 psi air used for each shot, at varying hammer spring preloads, to get the ccHPA/shot.... I then divided that into the bore volume, to get the ratio of bore volume to ccHPA used for the shot.... and plotted the results on a graph which already contained the efficiency in FPE/CI.... Here are the results....

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You will notice immediately the close correlation between the amount of HPA expanding into the barrel during the shot and the efficiency (which is what you told me a while ago).... I didn't record data for the .25 cal at lower preload settings because I wasn't interested in shooting at lower velocities.... You can see in the .22 cal graph that the efficiency peaks once there is only a very tiny pulse of air expanding into a very large volume (bore:volume>10), which could be expected.... The green lines I added to the graphs show that when the efficiency is 1.0 FPE/CI, the bore volume is about 5 times the amount of HPA that is being released by the valve.... It must be remembered, of course, that the average pressure in the valve is over 1500 psi during the shot because of the large (65cc) volume available for the valve to draw from....

When I do future work, I plan to record and plot the bore/shot volume ratio, and see how that relates to the efficiency.... What I'm having trouble with is how the James Perotti 1/2 bore volume rule for the valve size fits in with this data.... I'm showing a much smaller volume of HPA released than half the bore volume.... HELP !!!

Bob