I had earlier posted my puzzlement that at max power wheel and max preload screw on my Impact, the velocity was actually a bit lower than at slightly lower preload settings. Glem.chally and Vetmx replied that this was because too much hammer causes the valve to rebound hard and fast, prematurely closing the valve and lowering velocity. Vetmx said he never tunes this way, and instead starts with low preload and works up to find peak velocity.
So I started with low preload and sure enough, I hit my "max" velocity well below maximum preload, then a second time as I continued increasing preload. And at higher preload settings, every 10-shot string would have one shot 20 or more fps slower than the rest, usually the 7th or 8th shot, then the remainder would be faster.
Reasoning that adjustable devices are most consistent near the middle of their adjustment range, I decided that I needed more reg pressure so I wouldn't need extreme preload adjustments. Went from 95 bar to 115 in steps until I stopped getting the low speed outliers, then began adjusting the valve knob until I got acceptable ES. For a 23 gr .22 slug I ended up at 920 fps or so with reg at 115 and ES 10-14. Now if the wind drops I can begin working for accuracy at 70 yards again. So that was a worthwhile discussion, and thanks to you both.
So I started with low preload and sure enough, I hit my "max" velocity well below maximum preload, then a second time as I continued increasing preload. And at higher preload settings, every 10-shot string would have one shot 20 or more fps slower than the rest, usually the 7th or 8th shot, then the remainder would be faster.
Reasoning that adjustable devices are most consistent near the middle of their adjustment range, I decided that I needed more reg pressure so I wouldn't need extreme preload adjustments. Went from 95 bar to 115 in steps until I stopped getting the low speed outliers, then began adjusting the valve knob until I got acceptable ES. For a 23 gr .22 slug I ended up at 920 fps or so with reg at 115 and ES 10-14. Now if the wind drops I can begin working for accuracy at 70 yards again. So that was a worthwhile discussion, and thanks to you both.
