Today I decided to drill out my transfer port in my P-rod to 7/64ths (.109) to maximize efficiency and to gain some power. I was only looking for a 30 to 40 fps increase to bring me up to 700 fps average spread out over 3 clips. Before doing this, I was getting two clips with an average of 672 fps from about 2500 psi to 1800 psi with a variation in velocity of 20 fps from my highest to my lowest speed. If I went up to 2800 psi, I could get 3 clips with a similar average, but a steeper curve, as the shots at the beginning of the string and the ones at the end were a tad slower than the rest. Anyhow, I figured if I drilled out the port, I would gain about 50 fps on average, and I could just dial the springs back to my target of 700 fps, with a nice, smooth curve and plenty of shots to boot. Well, I did some chronying today, and I didn't gain 50 fps - I gained 80, without changing anything but the ID of the transfer port. Not only that, the nice 20 fps variation was gone - with some settings, I was getting up to 70 fps of variation, which plays havoc with my groups. I tried dialing back the hammer spring to 2 turns - no dice. I then tried shortening the hammer throw. This helped a little, but not a ton. For about 7 shots, I had a nice, smooth string of shots ranging from about 690 fps to 715 fps, and I was happy. But then my shots jumped up to the 740's. Then the 750's. Then the 760's. Finally, it peaked at 779 fps and began to drop. With a 2400 psi fill, I started at 690 fps, peaked at 779 fps at around 1800 psi, and when I finally got too frustrated to shoot anymore, I was shooting 770 fps at 1600 psi. It's almost like I have two plateaus depending on pressure - one in the low 700's, and one in the 770's. What I'm really looking for is that nice, smooth curve in the lower 700's. I don't need a ton of power - I got this gun to be my backyard pest shooter, and 700 fps for 2 to 3 clips was all I wanted. Can anyone out there offer me some suggestions for spring tension vs. hammer throw vs. fill pressure that will get me into the range I'm looking for? Any help is appreciated.
Area of a circle goes up faster than it seems...hard to intuit.. that radius squared thing. So while drilling from .1 to .15 seems a small increase (like 50%), it is .0079 square inches vs vs .0177 square inches (about 125% increase). Know that's not the size you drilled,just illustrating how the change...that "little bit larger" always seems to work out to a bigger change in performance than expected.
Have tried various ways of "shrinking" a too large transfer port...mostly they don't work long term. You could try filling with good epoxy and drilling it smaller, but the thin cloating tends to flake off unless the hole is roughened and totally degreased.