How far down can you tune a disco?

[Martis 1]

Ok this is gonna sound counter productive, but I'm curious.  When I first installed a PA in my disco, I also used a spring from a 2260.  When I ran some 18.3 grain pellets throught the chrony, the best I could get with that spring cranked down was just under 600 fps.  That was with the spring really cranked down to nearly binding.  I shot a couple pellets, realized that wasn't gonna do it for me, and installed a heavier spring.  I now have the gun shooting around 700 fps with good accuracy. 

It got me to wondering though, for a back yard gun, could I install the lighter spring and detune the gun to, say, 500 fps?  Would that cause any problems? 

Ron.

[Monkeydad1969]

I do it.  No issues what-so-ever.  I use a 13XX spring when I really want to shoot slow.  PM me your address and I can send you a spring.  It helps if you have a RVA that will allow you to change out springs w/o removing the entire end cap.

But...be warned, an addiction takes hold, you find yourself in a dark place, and you never come back...from the Darkside.



Joe

[Martis 1]

Thanks Joe.
I was kinda curious as I have a lighter spring that I initially started with.  Depending on how bad the ground squirrels get in my yard this summer, I may install it and down tune.  Unfortunately, the Crosma PA I'm using has to be removed in order to replace springs.  That's something I shoulda taken into consideration when I selected a PA.  Kinda not worthy having to remove the scope every time I want to play with the springs...
Ron.

[Ribbonstone]

Realize that with a lighter spring, will run its best at a lower pressure.  Will be slow with a normal 2K fill (or even not shoot at all) but start gaining speed as the pressre decreases (and the striker force better opens the valve).

Really haven't tried to down tune that far with the .22 Disco.  Have brought a .177 Challenger PCP up and down in power, and it's pretty much running on a Disco valve.  Crosman's way of lowering speed/energy is by way of a very small transfer port.  Guess the idea there is that the gun still runs at normal pressures but the small transfer port limits the amount of air the valve can release.

So if you want a quick-change between low and high power, then switching striker springs and changing fill pressures might do it for you...but I suspect that even with a light spring, velocity would still increase to 700fps or more once the pressure gets low enough to match the striker's force on the valve.

[Martis 1]

Realize that with a lighter spring, will run its best at a lower pressure.  Will be slow with a normal 2K fill (or even not shoot at all) but start gaining speed as the pressre decreases (and the striker force better opens the valve).

Really haven't tried to down tune that far with the .22 Disco.  Have brought a .177 Challenger PCP up and down in power, and it's pretty much running on a Disco valve.  Crosman's way of lowering speed/energy is by way of a very small transfer port.  Guess the idea there is that the gun still runs at normal pressures but the small transfer port limits the amount of air the valve can release.

So if you want a quick-change between low and high power, then switching striker springs and changing fill pressures might do it for you...but I suspect that even with a light spring, velocity would still increase to 700fps or more once the pressure gets low enough to match the striker's force on the valve.

This is some great info!  It's got me wondering.  When I ordered my PA I also ordered a 2260 spring, which is shorter and much lighter than the factory disco spring.  To my surprise, the Challenger PA also came with a challenger spring, which appears to be the same wire thickness as the disco spring, but about 4-5 coils shorter. 

I initially installed the 2260 spring and gave in an arbitrary starting tension.  I pumped the gun to 2k per the pump's gauge and fired one or two rounds over the chrony.  They were around 545-550 fps.  I stopped and cranked the PA down till the spring nearly bound up, re-filled the gun, and tried again.  That only got me 599-600fps on two shots.  I stopped and switched to the heavier challenger spring. 

Now I'm wishing that when I had that 2260 spring installed, I had shot a full spring.  Probably would have climbed dramatically once the pressure dropped.  Be interesting to see if I could get a decent shot string at around 700 fps with the lighter spring and lower pressure.  Maybe a good project when I get bored...

Thanks!
Ron.

[Ribbonstone]

See if this makes sense to you.


Non regulated PCPs are a balance between fill pressure and striker force.


The air pressure  presses the valve stem closed,   Lets just say the valve stem seal is 1/3” in diameter (just a number).  That’s  about .086 square inches.  With a 2000psi fill, the force holding the valve closed in (.086X2000)  is about 172 pounds of force.


Opening against that 172 pounds of closing force is the hit of the striker to the valve stem.  That can be stronger or lighter depending on the weight of the striker, the length of the striker stroke, or the power of the striker spring.  With a Discovery, the only think you really can easily change is the strength of the striker spring.


So the idea is that at the start, the force of the striker hit to the valve is a bit LESS than the closing force of the air pressure.  So the valve just “blips” open for a very short time, letting lose a little blast of 2K air.

With each shot taken, the closing force of air pressure DECREASES as the air is released.

So the balance between opening and closing force changes.  With less air pressure, is less closing force, so the opening strike opens the valve farther, releasing more air.

Which means that the speed INCREASES as the two forces (opening and closing) come into a kind of odd balance.

Eventually that balance shifts as air pressure drops farther and farther.  The opening force stays constant, but there is only so much air compressed into the gun and the closing force just can’t keep up.


So what happens with a weaker spring is that the  speeds start slow.  The opening force is just too weak to really knock the valve open wide enough to get a decent blast of air behind the pellet.


But with each shot, the air pressure decrease…so the opening force knocks that valve open farther and farther…and speed increases as closing pressure drops.

 So what normally happens with just weakening the striker hit is that the peak speed happens at a lower pressure. Won’t be as fast as a higher pressure, but will be considerably higher than when the opening force is much weaker than the closing force (when the fill pressure is considerably higher than the force of the striker hit to open the valve).


SOOO..one way to keep opening and closing force in balance at higher pressures is to  NOT change the spring tension, but to change the transfer port.  Will still be "balanced" between opening and closing force, but a small transfer port just won't let as much air get behind the pellet.

[Martis 1]

It took me awhile to understand that.  When I first looked at pcp's it seemed like your velocity would decrease with each shot, since each time you fire, you're air pressure decreases.  Once I understood the striker force and how it relates, it made more sense.  The different diameter transfer port and it's effect seems logical as well. 
I still may, at some point, install the lighter spring and play around, but right now it's shooting very accurately at 700 fps with 18.3 grain pellets.  Still not sure why I'm only getting 20 or so good shots per fill, but I can certainly live with it. 
Thanks again for all of the great information!
Ron.