FX Dreamline plenum size for custom build

[airgunstocks]

I'm doing a custom build: FX Dreamline .22 tuned to 26 ft. lb.s. This is the latest model gun that came with the new regulator that is a separate part that threads onto the action on one end and the airtube threads on the other end. I'm removing that regulator and using the longer air cylinder from the .25/.30 versions, not drilled for the sideways adjustable factory regulator, with a Lane regulator.

The plenum space that came from FX measures a paltry 3.6cc (.7795" dia x .4625 long). It did produce my goal of 26 ft. lb.s

The plenum that came with the Lane regulator was likely sized to place the regulator so as to overlap the factory hole in the air cylinder from the previous model of the gun with the FX in-tube regulator, and not necessarily sized for performance. This plenum measures 15.8cc (.8615 dia. x 1.6535 long). This fits the oft recommended .5 cc/ft.lb.

The tiny factory plenum size did produce my goal of 26 ft. lb.s, perhaps at a higher than ideal pressure, but it wasn't significantly so.

I don't want to unnecessarily give up reservoir volume with a plenum longer than necessary (high shot count is a goal of this build).

What do you experienced plenum tuners recommend for a plenum size for my build? (I have a lathe and can make any size).

Thank you,
Michael McKeown

 

[Vetmx]

I don’t think you have a choice because of the atmosphere hole in the tube. Personally, I would assemble it with the Lane and new tube then test. With a hopefully lower reg setting, the slight loss in volume might balance out. But you will always have gas in the tank if you decide to push the gun down the road if the latest craze strikes your fancy.

 

[airgunstocks]

Thank you for your response. As I mentioned, the tube I will be using does not yet have a breath hole. I will be drilling the breath hole based on where the regulator ends up with whatever size plenum I end up using.

 

[airgunstocks]

Ah, I realize I failed to consider the volume in the action/block ahead of the valve.

 

[airgunstocks]

I don't play with tuning very often, carving gunstocks is my world. I measured the volume in the block as 19 1/2 cc (I filled with water from a syringe), plus the 3.6cc of plenum they provided = 23 cc

So maybe I go with the size plenum FX used, which is 11.75mm long?

 

[xyon]

If you are making your own plenum spacer, you may be able to make one that does not require drilling of the tube. Then you can start larger and shorten it as required.
Also if you are keen on optimizing the shot count and/or measuring the air use per shot. You can shoot a string with the original tube and reg. Then do the same with a larger plenum and presumably a lower reg pressure and see which is better.

 

[airgunstocks]

Once I realized why the plenum from Lane was so long (to fit the previous version of my gun with the sideways adjustable regulator's hole in the tube), and I measured the volume in the block, I decided to make a plenum the same size as what was in the gun from the factory. I could have just used the factory's plenum, but someone on here might want my factory regulator.

As an aside, the longer air cylinder I'm using is longer than the barrel (well, shroud), so I made a shroud extension, or basically a moderator that is the same diameter as the shroud. At 26 ft. lb.s it wasn't loud to begin with, but it oughta quiet the gun down even further. And of course I carved a custom stock for it. The gun is going to be super cool I'll post about it when it's finished and I find out what my shot count is.