Pcp guns 101

[Scottycoyote67]

I don't know much about pcp guns but I'd like to learn.  Can anyone point me to a good resource on the ins and outs? My question right now is, why is it on guns like the marauder pistol, the fill limit is 3000, but the sweet spot starts around 2600?  Is there an easy way to increase the sweet for more consistent useable air?

[Ribbonstone]

Think of it as a balance between two forces.

On one side we have the force of the striker hitting the valve stem.

On the other side we have the force of the air pressure holding the valve closed.


On a full 3K fill, the striker force is worth less than the air pressure force holding the valve closed, so the valve opens just a little bit and the shot is slow.  (If the striker hit is way-low, the valve might not open at all and you get no shot…called “valve lock”).

Each shot uses up some of the air pressure, so the force holding the valve closed grows less and less with each shot and the valve opens farther and farther. Pellet speed increases.


At some point they get in balance (this is usually called the “sweet spot”). The two forces kind of compliment each other, and the pellet’s speed stays stable for some number of shots.

Sooner or later the air pressure drops off and velocity starts to decrease with each shot.    NO matter how much more the striker can open the valve, there just isn’t enough air to produce a full power shot.

SO…

If you were to increase the striker force to the valve, you’d need to increase the air pressure’s force holding the valve closed, and the sweet spot would happen at a higher pressure.  Reverse is also true: light striker hit will find its sweet spot at lower pressure.


By experiment, could find just the right setting for spring tension to have the START of the sweet spot happen at the MAX pressure of the gun, which would give you the most shots at th most speed (assuming power was you goal).


THEn it gets complicated.

Are several ways to change the striker's force.  Heavier or lighter weight, more spring tension or less, longer or shorter stroke (stroke being the distance the striker travels to hit the valve stem).

The passage ways between valve and barrel (normaly called transfer ports) can restrict air movement (think of it as a variable kink in a garden hose).

Are ways that the air pressures shutting force changes as well.  It's based on pounds per square inch, so the fatter the valve stem seal, the more force holding it closed.

[Scottycoyote67]

thx ribbonstone, that helps.  I can see a chrony purchase in my future

[Ribbonstone]

No doubt about it, chronograph is the way to go.


Can make some educated guesses without one, but they’re just assumptions.

Can find the sweet spot by shooting groups on identical targets.  Does illustrate the first posts (slow at first, level "sweet spot", and the fall off as air pressure drops).



Then cut out the targets and line them up like this:



Now this assumes that faster = higher impact.  Is a good assumption, but you won’t know how fast the gun is shooting.

BTW: do have a chronograph, and it did work out that the above gun had about 20-22 “good” shots (“good” being shots inside of 25fps in this gun’s case).