Tuning Peculiar observation tuning my Impact

[Khornet]

Was fiddling with my tune and decided to try a slightly higher reg pressure of 100 vs my usual 95. MkII PP 700 mm .22. Did the usual thing of opening the valve knob past 4 lines, wheel on max, turn in the preload screw til the wheel won't move then back off a hair til it just moves, to find max velocity for the new set point.

I was getting lower velocity at that point than I did when I backed the screw off a quarter to half turn. Then the usual pattern of little or no velocity change as I backed out the screw until I hit 31/2 to 4 turns out. So what was happening? With the screw maxed out, is the spring distorted somehow so that energy is lost to "un-distorting" it before it begins to expand? Found the same phenomenon at higher and lower reg pressures.

 

[Glem.Chally]

Why do you need so much preload for such a low reg pressure? 

Its overdriving the valve.

 

[Khornet]

Well, unless I'm misunderstanding Bob Sterne's writings, in tuning you should seek 95-97% on the max velocity for a given reg pressure. So you max out preload and open the valve adjuster, note the resulting velocity, and then begin decreasing preload until you're at or near that 97% value. Then you can use the valve adjuster to find the point in that 95-97% range where you get best efficiency and accuracy. So I'm just doing what the gun and chrono tell me. They tell me, unless I'm missing something, that at 100 bar on my reg my max velocity possible is about 956 fps for a 23 grain H&N .218 slug. 97% of that is around 927 fps, which occurs when I've reduced preload by about 4-5 turns, always remembering that it's hard to know exactly how many turns with that little screw and the limited throw available with that little Allen key in that narrow slot.

When I get to about 935 fps I begin turning in the valve adjuster by fractions. When I reach an average of 926 or so, I get a quiet gun with about 90 or 100 shots from 200 bar down to 100, with extreme spreads of 7 to 10 fps. If that's not the way to do it, I'd love to know, because all I want is accuracy and efficiency.

I have tuned to lower reg pressures with much lower preloads, and gotten some tight ES strings with what I consider superb accuracy, but they are unstable. They always deteriorate after a few days, accuracy falls off, and chrono shows a change from the velocity/ES I got when tuning. I'm thinking that at the lower preloads there are more things which can change hammer force enough to affect velocity/ES/harmonics, things like temperature swings. So I'm seeking a tune with a little more hammer, so that a small change in the hammer blow is not enough to affect things.

Anyway, my question was WHY is there a point, near max possible preload, at least in MY gun, where increasing preload DECREASES velocity? Is something out of whack?

 

[dannya1a]

if I'm correct. there is a point with a regulated gun that as you dial in more hammer strike power, the velocity begins to fall off. As Glem said "you are over driving the valve at that point, and your wasting air as well." OR, it could be that you're hitting the valve so hard that it is only able to pop it open so quickly and it is closing so quick due to the hard hit that you don't get enough air to flow through and get reduced FPS.

When tuning it is all a balancing act.

 

[Glem.Chally]

if I'm correct. there is a point with a regulated gun that as you dial in more hammer strike power, the velocity begins to fall off. As Glem said "you are over driving the valve at that point, and your wasting air as well." OR, it could be that you're hitting the valve so hard that it is only able to pop it open so quickly and it is closing so quick due to the hard hit that you don't get enough air to flow through and get reduced FPS.

When tuning it is all a balancing act.

Bingo.

Hammer is hitting the valve so hard its bouncing back and slamming shut (no dwell, lower power). If you want to tune that way go for it. Your gun is fine.

 

[Khornet]

Aaaaand...there's my answer. Thanks a lot. So I was getting an incorrect starting point in my tune because of the bounce. And of course that's not the way I want to tune, but you do need a starting point and I DID want to know why the velocity went down like that.

 

[Corvid_hunter]

The regulator may not be designed to work well under 100 bar. Someone more knowledgeable than I will know the operating range for the regulator.

 

[TMH]

The regulator may not be designed to work well under 100 bar. Someone more knowledgeable than I will know the operating range for the regulator.

My brand new MkII PP 700mm .22 came from the factory (or from Baker) with the reg set at 80. I was surprised at that setting but I guess that Impact PP is pretty effective.

 

[Vetmx]

This is why I don’t do the turn my wheel to max then turn the screw to max thing. Yes, I set my wheel on max, then I just keep shooting over the chronograph and turning the screw until I hit my true Max. I have a gun that has two plateaus when adjusting the HS for a particular reg setting. Operating on that second plateau is not a good place to be so I always take the long tedious way to find the exact spot my guns, Impact included, peak.

 

[Khornet]

I emailed FX a few weeks ago about minimum PP reg settings. They said there's no harm from low settings, e.g. no valve rod damage, but below 80 the reg doesn't work very well.

 

[Khornet]

I just measured the maximum rotation I can get with the Allen key on the preload screw. It's 120 degrees. 1/3 of a full rotation. So when I thought I was 4 turns out, I was over 5. Now I will try Vetmx's way of finding plateau velocity.