Pellet Velocity Increasing as Cylinder pressure decreasing WHY?

[Sheharyar]

Guys my Walther is not performing well. Here's the data

250bar to 200bar 880,877,875fps


As The pressure drops off from 200 to 199bars the fps are 922,923,914 etc

Can Someone explain what is the issue here ? Walther Gives shot count of about 40 per fill but mine doesn't give accuracy below 200bars.

 

[X27]

What model and caliber?

 

[Sheharyar]

Walther Dominator 1250 .22cal 40joules having shims in it regulator.

 

[Scirroco]

Hi sh
It means you jave gotten to a point when the hammer spring can open the valve longer/quicker,as the pressure drops in the cylinder,
You need to find the optimum fill pressure for un regged guns,dont know whether your particular is regged or not (if its regged you have a problem,,,best way to find optimum fill pressure for unregged guns is to work up from ab 140 bar in 10 bar increments and chronograph it,ypu will.arrive at the point for example say 180 bar when velocity will drop,you have then found your sweet spot for filling,on my old 12 ft lbs daytate 177 (non regged) sweet spot was 160 bar ..at 170 bar velocity was down ab 20fps
Rgds steve
Rgds

 

[Tominco]

Scirocco beat me to it. 
The back pressure helps to push the valve closed after each shot. When that back pressure drops the valve remains open longer, releasing more air behind the pellet which increases velocity. 

 

[BigTinBoat]

"Scirroco"Hi sh
It means you jave gotten to a point when the hammer spring can open the valve longer/quicker,as the pressure drops in the cylinder,
You need to find the optimum fill pressure for un regged guns,dont know whether your particular is regged or not (if its regged you have a problem,,,best way to find optimum fill pressure for unregged guns is to work up from ab 140 bar in 10 bar increments and chronograph it,ypu will.arrive at the point for example say 180 bar when velocity will drop,you have then found your sweet spot for filling,on my old 12 ft lbs daytate 177 (non regged) sweet spot was 160 bar ..at 170 bar velocity was down ab 20fps
Rgds steve
Rgds

The Walther IS regulated and as such I don't believe you have a "problem", and the "fix" is easier (IMHO) then if it were unregulated. Right now your regulator pressure is not in "tune" with your hammer spring. For efficiency you want to tune your gun so that the velocity just slightly raises when it comes off the regulator. Right now you are getting a pretty good raise (48fps). Ideally you want it no more then 15-20FPS and this will only be for a couple of shots and then the FPS will drastically drop. When tuned like this you are as they say "on the curve". Right now you are "over pressured" or "under sprung". Take your pick. If you wan your velocity to stay at the 875 range then you need to drop the regulator pressure. If you want the velocity to be around the 900fps range then you need to increase the HS preload.
Right now as it is tuned you are wasting air, thus the lower shot count you are getting.
The easiest way to tune it would be to raise the velocity. Basically you want to fill the gun and increase the preload on the hammer spring until the starting velocity is right around 900-910fps. Then fill completely and shoot a string. It should stay around 900-910 till the end of string where it will increase to 920 for a couple of shots before falling drastically. You should see the same number of shots but they will be more consistent and they are faster. If you want to increase the shot count then dropping the regulator pressure is the way to go about that.

 

[Sheharyar]

Thanks Guys.Still trying to get consistency and good shot count.The walther doesn't have an adjusted for the hammer spring.There is an allen key for the pressure in the reg i.e how much intake air is needed for the reg.
There are washers.We can adjust the Belleville washers for good fps.But cant get the ideal settings.

 

[BigTinBoat]

"Sheharyar"Thanks Guys.Still trying to get consistency and good shot count.The walther doesn't have an adjusted for the hammer spring.There is an allen key for the pressure in the reg i.e how much intake air is needed for the reg.
There are washers.We can adjust the Belleville washers for good fps.But cant get the ideal settings.

To lower pre-load you can cut a bit of coil off the spring. To increase pre-load you just add a washer between the spring and the spring guide.

 

[Sheharyar]

Ok.I will try and add a washer in the spring guide.I've got 900+ fps but same results, at 200bar the fps increases to 930+

 

[BigTinBoat]

"Sheharyar"Ok.I will try and add a washer in the spring guide.I've got 900+ fps but same results, at 200bar the fps increases to 930+

OK - what you need to do is run an entire string. You need to see what it peaks at. If you just stop at 930 then you might not be at the highest point. Keep shooting till it goes up and then drops back off. When it drops it will be like a rock.
Please post the shot string, it will help to analyze.

Also - in your other thread you asked about the shims. I would not buy any shims until you get this done, and then you open up the regulator and see what your stack looks like now.

 

[Sheharyar]

Actually I've done alot of tests and Now the conclusion is that the Cylinder isn't giving consistent air which causes this.I used another cylinder which was at 70 bar the fps were 850 which i compared the the previous cylinder at 190bar was giving 870fps. So I'll test the gun tomorrow.Then tell y'all.

 

[BigTinBoat]

"Sheharyar"Actually I've done alot of tests and Now the conclusion is that the Cylinder isn't giving consistent air which causes this.I used another cylinder which was at 70 bar the fps were 850 which i compared the the previous cylinder at 190bar was giving 870fps. So I'll test the gun tomorrow.Then tell y'all.

OK - once I see shot stings I will reply back with help.

 

[Sheharyar]

250 bars
926
929
940
941
941
940
941 215bar approx
955
956 200bar approx
951
956
952
951
956
943
944
946
940

 

[BigTinBoat]

"Sheharyar"250 bars
926
929
940
941
941
940
941 215bar approx
955
956 200bar approx
951
956
952
951
956
943
944
946
940

This looks like an unregulated string, but might be because you did NOT post a full string. You need to see the bottom numbers. What happened after the 940? Did you run out of pellets? or did the gun just not shoot anymore.


Really trying to help you here........but you need play along - Please post a FULL shot string. (that means shooting till it is readily apparent the gun is out of regulation or off the curve.

 

[Sheharyar]

250 bars cylinder pressure
847
846
843
847
846
845
851
861
852
855
851
850
851
851
846
852
851
848
851
847
851
850
851
851
851 JSB 16gr. Pellets
855
855
The end latest string after tuning.Thank God got some consistency