Tuning Plenum Volume

[bowdude]

In the search for answers to figure out how to degas my Caiman X I came across a YouTube video of the Caiman(shorter version) being disassembled. During the disassembling I noticed that the guy removed a vinyl piece out of the plenum. The only thing I could think of is to reduce the plenum volume. Does reducing the plenum volume reduce fps/fpe? I’m trying to detune my Caiman X to 20fpe, now it’s running around 35fpe & I don’t need that much power. I tried to detune it before with just the hammer spring but it was way inconsistent but I got 210 shots from a fill. I started tuning the hammer spring till I got consistency and it settled at 35fpe with 17.5gr slugs for 75 shots.

sorry back to the question: does plenum volume effect fps/fpe?

 

[biohazardman]

If you can get to the back of the hammer, on your gun, you can degas it by resting a rod, of some sort, on the hammer and repeatedly tapping it with something. Know that, when you do this, the air has to go somewhere like out the barrel or the transfer port if it is exposed. So take care when doing this.



If the gun is tuned for higher power then reducing the plenum size substantially will likely reduce the power some. If it is tuned for a medium or lower power you would need to reduce the volume quite a bit to lower the power that way.

Edit;

Probly the best way to reduce power,in an unregulated gun, "other than with the hammer spring tension", is to restrict the transfer port between the valve and the barrel. Just a small bushing affair with a smaller hole in it than the original port size will reduce your power.

Just get a piece of Delrin rod, other plastics will werq also, and drill a hole, for your reduced port size, in the center. Put the piece in your drill chuck and hold a file to it until it is the right diameter for the port. Cut it off and smooth it out and you have what you need.

Paintball macroline, and many other smallish but strong plastic or metal tubes, would werq great as well.

Stock steel Crosman port, for the 22xx guns, on the left. Delrin is the black one on the right. The center is polypropylene refrigerator line, it is soft, and only good for the pressures associated with Co2 guns unless it is supported by something stronger. 

 

[nervoustrig]

does plenum volume effect fps/fpe?

Yes, it affects the average pressure delivered to the pellet during the time the valve is knocked open. A very small plenum may be nearly depleted of air when you fire, whereas a large plenum will experience almost no pressure drop.

An overly simplified example:

• small plenum: starts at 2000psi, drops to 500psi -> average pressure delivered to the pellet is (2000+500)/2 = 1250psi
• large plenum: starts at 2000psi, drops to 1500psi -> average pressure delivered to the pellet is (2000+1500)/2 = 1750psi

Edit: Stuffing a plenum to reduce its volume is a poor way to decrease energy because it adversely affects efficiency and shot count. I would treat it as a last resort...far better to reduce both the regulator’s setpoint and hammer spring tension if possible. Next best would be restricting the transfer port (or valve exhaust or barrel port).

 

[Scotty1]

I wonder if the vinyl piece is a de-pinger of sorts?