Other Plenums

[Ratzy]

I've been playing with Plenums for about 8 months.

This only applies only to regulated guns.

Plenum supplies additional air volume to help keep the pellet/slug under acceleration. Hopefully for the length of the barrel.

When air pressure drops to a level low enough the pellet stops accelerating. If that is at 67% of the length of your barrel then the effective length of your barrel is 67% of the physical length.

Plenum is the volume of air between the regulator and poppet or equivalent air transfer mechanism.

Optimum Plenum volume appears to be 3-4 times your barrel volume. Diminishing returns at 6x and above.
Volume= barrel length x barrel inside radius * Pi
.25 cal, 600mm barrel
(6.35/2)*600*3.1416= 19CC
19CC barrel volume should have a plenum of 60-80CC.

There really is no such thing as too much plenum.

Some plenum is better than no plenum

If you regulate a non-regulated gun then you may be limiting your power, possibly significantly, without a plenum.

If you have a properly devised plenum then you can reduce the regulator pressure significantly and get many more shots with the same FPS/FPE as before the plenum.

Some exceptions like a very small inlet from the bottle, like my Gauntlet G30, and the plenum will have little or no effect.

Plenums that replace a gauge are also limited by the size of the hole.

Just an observation it seems minimum hole for the plenum is some value larger than your caliber. Have not tested that, need to buy some washers to reduce the flow and see where it becomes an issue.

 

[Ratzy]

I've been playing with Plenums for about 8 months.

This only applies only to regulated guns.

Plenum supplies additional air volume to help keep the pellet/slug under acceleration. Hopefully for the length of the barrel.

When air pressure drops to a level low enough the pellet stops accelerating. If that is at 67% of the length of your barrel then the effective length of your barrel is 67% of the physical length.

Plenum is the volume of air between the regulator and poppet or equivalent air transfer mechanism.

Optimum Plenum volume appears to be 3-4 times your barrel volume. Diminishing returns at 6x and above.
Volume= barrel length x barrel inside radius * Pi
.25 cal, 600mm barrel
(6.35/2)*600*3.1416= 19CC
19CC barrel volume should have a plenum of 60-80CC.

There really is no such thing as too much plenum.

Some plenum is better than no plenum

If you regulate a non-regulated gun then you may be limiting your power, possibly significantly, without a plenum.

If you have a properly devised plenum then you can reduce the regulator pressure significantly and get many more shots with the same FPS/FPE as before the plenum.

Some exceptions like a very small inlet from the bottle, like my Gauntlet G30, and the plenum will have little or no effect.

Plenums that replace a gauge are also limited by the size of the hole.

Just an observation it seems minimum hole for the plenum is some value larger than your caliber. Have not tested that, need to buy some washers to reduce the flow and see where it becomes an issue.

Huben Power has regulators with 25CC, 50CC and now a BAP.

Trajectron has a 151CC plenum AirForce regulator.

You can roll your own with parts from AliExpress, specifically Shan Bao 20CM and 40cm 3000psi tubes and a few other parts. They make 125cc and 250cc plenum.

 

[Motorhead]

Many of us tuners doing this PCP thing for MANY years have found that plenum volumes related to FPE ( Foot Pounds Energy ) of 1/2 to 1cc of plenum volume per Ft Lb of wished for output has worked out very well.

Just an FYI ... as you were

 

[Tricky-Dicky]

We with our limited power in the UK often reduce the plenum volume as most are intended to serve unlimited power application so we end up making fillers for them among other tweaks to reduce wasted air.

 

[EPG]

Does anyone know the plenum volume of a Daystate Wolverine R - bottle version?

 

[Motorhead]

We with our limited power in the UK often reduce the plenum volume as most are intended to serve unlimited power application so we end up making fillers for them among other tweaks to reduce wasted air.

Indeed .. as I've stuffed plenums reducing volume to get a lower power rig to cycle regulator more consistently decreasing ES swings while also helping accuracy by reducing excess air leaving muzzle.

 

[Ratzy]

I saw all that and did the math to optimize the plenum based upon barrel. I was more interested in the fall over point where the additional plenum did not increase energy. Looking for ratio of barrel volume to plenum volume.

Then I set up the my AirForce Condor as the test vehicle. I had 2 caliber, 3 barrels and 4 plenums.
.22 and .25 caliber
12, 18 & 24 inch barrels
25cc, 50cc, 125cc and 250cc plenums

Which is how I got to 3x-4x plenum to barrel ratio.

I also put the 125 and 250cc plenums on my .30 Evanix Sniper as they were overkill on the Condor.

 

[Motorhead]

Do what works for you ... wisdom & experience of others is always there for guidance.

 

[Ratzy]

Not saying you are wrong, pointing out my objective was different. I was looking for optimized plenum volume not how much energy. Yes, energy, FPS, was how I determined fail over but the actual FPS value was a secondary concern. Only the point at which it no longer increased with plenum.

I will say that a 50cc plenum on a 24" .22 makes a huge difference. I can run the reg at 1500psi and get over 900FPS with H&N Baracuda Match 21gr.

.25, JTS 25.39gr, 24" & 50cc plenum running regulator at ~1750psi. Should be at least 60CC plenum for a 19CC barrel volume but 50CC is what I have.

 

[Motorhead]

Charts and fancy graphs mien little ... reread post #8

 

[Ratzy]

Charts and fancy graphs mien little ... reread post #8

You are explaining something that I was not looking for.

Optimization of plenum volume based upon barrel volume. It has zero to do with your point. I wanted to find out where a plenum stops being effective. Increasing plenum size until there is no or limited improvement. Then I could determine minimum fully effective plenum size for a specific barrel and caliber. Which I did. Then I posted the results and some of the things I learned along the way.

The specifics of how much energy is gained is not of relevance to my objective.

See post #1 for reference

 

[AlanMcD]

You are explaining something that I was not looking for.

Optimization of plenum volume based upon barrel volume. It has zero to do with your point. I wanted to find out where a plenum stops being effective. Increasing plenum size until there is no or limited improvement. Then I could determine minimum fully effective plenum size for a specific barrel and caliber. Which I did. Then I posted the results and some of the things I learned along the way.

The specifics of how much energy is gained is not of relevance to my objective.

See post #1 for reference

I think you are missing Scott's broader point . . . .

It is all well and good to try to figure out how to optimize plenum size around the volume of the barrel, but in the end there still has to be consideration given for what one is doing with a given barrel - there will be a different optimal answer for any given barrel between one set up for lighter pellets at lower speeds and one set up for heavier pellets at higher speeds. In the end, air usage is not fixed for any given barrel, and optimization will need to consider that - and power (in terms of FPE or Joules, whichever one prefers) will be a good element to consider on that front.

I think there is clearly opportunity to improve on the traditional guidance Scott referenced - after all, the guidance range has the high value as double the low value. I have used that guidance many times myself as as a starting point, but would love to see it refined. I don't think barrel volume on it's own is any more helpful, but maybe together they could complement each other . . .

I do disagree with your statement that there is no such thing as too much plenum - in my experience, too much plenum will lead to a larger ES in speed as the refill cycle becomes more variable (at the extreme, there might be almost no refill on some shots).

 

[Ratzy]

I didn't miss his point, it had nothing to do with what I was working on.

As for your points quite possibly valid but I need a larger sample size and a larger test bed group to build up a proper testplan. Neither of which I have. Now if someone wants to donate rifles with multiple barrels I could be convinced to expand my efforts .

I have to disagree with the too much plenum statement. In my reading on plenums I ran across a Big Bore thread where it was asked if a Big Bore could be regulated. So I built up a plenum using the 125 and 250CC tubes. All totalled with the various parts around 400CC. Ugly, ungainly and heavy but it was possible. I put it all on the .25 Condor and did not see any degradation on the chrony numbers.

No different than a unregulated rifle with a 400cc bottle. I can't see the physics of how what you reported occurs. As long as the flow is above the minimum required to maintain barrel pressure then it acts just like an unregulated rifle.

It could be the issue was the regulator not refilling the plenum fully before the next shot. I have seen some slow regulators over the years. But not here. For all my experiments I used the same Huben Power regulator body from a Huben Power MAX-ATH (50CC top half) except for the 25CC, that was a Huben Power Dream Universal. The 125 and 250CC Shan Bao tubes have a M28x1 thread and the Huben Power regulator body (without the 50cc plenum) is also M28x1.

This was the big bore regulator and plenum.
The 125cc ran from the U adapter to the Tee junction. The 250cc ran parallel to the gun frame. The tophat was on an elbow from the Tee junction. It was a pain to get everything tight and clocked. Real world you would want a manifold from a single billet in place of the Tee and Elbow. 1.6L bottle too. Turns out my nephews neighbor in Oregon has a .510 Texan so when I go up there next I will reassemble this and see if it works on a big bore.

And the math was close to minimums for volume. Where it gets twitchy is accounting for big bore pellet weights and the 3000psi limit on the tubes. Mr. Newton and his silly laws may make it non viable.

 

[Ratzy]

Big bore pellet weights, I haven't done any research there but I imagine it takes a much higher starting pressure to accelerate those pellets to reasonable exit velocity vs .25 that i was working with. So it may be that 3000 psi max is a non-starter.

 

[Ratzy]

Also, haven't delved into volume limitations. The Condor was set up with high flow tophat, heavy hammer and spring. I'd like to have calculated the flow but dont have sufficient data (or knowledge) to account for all those variables.

So there is quite possibly a limit to the amount of air from the plenum that can be present in the barrel.

Then there is the mechanical difference between poppet valves and AF Tophat valves. Probably something I can at least empirically compare between all my rifles.

Just need to design a plenum configuration that can be used by both types of rifles.

 

[AlanMcD]

OK - I can see how there could be value there if going just for higher power out of whatever you have on hand . . . but it is not "universal" in that it does not apply as well to intentional lower power tunes and shooting. I think there (like 500+mm barrelled .22 calibers shooting under 20 FPE as an example) Scott's rule is likely better - at least more usable and applicable. Again, there may be a way to combine them into something even more usable for a "universal" decision guide.

For lower power shooters, I think your observation could be misleading. For example, with your rule, for a given caliber a short barrel needs less plenum than a long barrel, even if the owner wants the short barrel to shoot at higher power and the long barrel at lower power. That's what happens when one ignores actual air usage - that's where I think Scott's guidance is more likely to result in a better tune (at least in terms of air efficiency and likely shot cycle).

Since I'm not one to go where you are testing I'll leave it alone and for you to figure out (I set one of my .25 cals up for over 90 FPE once, but found no good use for it so brought it back down into the 40s). As for the too much plenum observation, I found that on that very gun when I tuned it back down - the ES dropped a good bit when I dropped the plenum volume back into the range of Scott's guidance and retuned the gun for that level.

Peace out . . . moving on . . . happy testing and shooting!

 

[coastal drifter]

Indeed .. as I've stuffed plenums reducing volume to get a lower power rig to cycle regulator more consistently decreasing ES swings while also helping accuracy by reducing excess air leaving muzzle.

I inquired about an air gun I was interested in mostly for shooting pellets. The gun was the Wildcat MK3. I was told for pellets particularly in .22 the tube version was a better choice over the bottle as it was valved better for consistency. I believe the smaller (plenum) also effects it. This is how it came to me. I have since not touched it as I don't think I could improve it. I would like to add this is the compact version, shot count is 54+ shots on a 225 bar fill.

 

[Ratzy]

OK - I can see how there could be value there if going just for higher power out of whatever you have on hand . . . but it is not "universal" in that it does not apply as well to intentional lower power tunes and shooting. I think there (like 500+mm barrelled .22 calibers shooting under 20 FPE as an example) Scott's rule is likely better - at least more usable and applicable. Again, there may be a way to combine them into something even more usable for a "universal" decision guide.

For lower power shooters, I think your observation could be misleading. For example, with your rule, for a given caliber a short barrel needs less plenum than a long barrel, even if the owner wants the short barrel to shoot at higher power and the long barrel at lower power. That's what happens when one ignores actual air usage - that's where I think Scott's guidance is more likely to result in a better tune (at least in terms of air efficiency and likely shot cycle).

Since I'm not one to go where you are testing I'll leave it alone and for you to figure out (I set one of my .25 cals up for over 90 FPE once, but found no good use for it so brought it back down into the 40s). As for the too much plenum observation, I found that on that very gun when I tuned it back down - the ES dropped a good bit when I dropped the plenum volume back into the range of Scott's guidance and retuned the gun for that level.

Peace out . . . moving on . . . happy testing and shooting!

Again, not what I was doing. Whether or not this applies to low or high power was not a criteria. It was, as I have stated, optimizing the volume of a plenum to the volume of a barrel. My objective was, within my small sample, to determine at what point increasing the size of the Plenum provided diminishing returns in energy.

I had ignored plenums for years. When I started playing with AirForce rifles and a Huben MAX-ATH regulator and its 50CC plenum I discovered why they are important.

So I decided to find out just how far you could go with a plenum. Where in fact was the optimal size for a specific caliber and length of barrel.

Whether anyone uses what I've found is up to them. I use the data to set the size of plenum and then use the regulator to set the energy level.

Properly optimized plenum allows for a lot more shots with a given bottle size. Because it allows you to set your regulator pressure to the absolute minimum for whatever power you are looking for.

The example I shared earlier. .25, 25.4gr pellet with consistent 927fps and a regulator at ~1750psi. I put non Huben adjustable regulator on that gun and had to set the reg to, I think, 2400psi to get similar results.

As for low power, no reason an optimized plenum could not be used. Depends on lower bounds of the regulator and the various control mechanism in the gun. I can see where regulated pressure could get too low for the gun to operate. Pretty sure the lower bounds on the Airforce style hammer and tophat guns is below the general poppet style.

But I've never gone the low power route. As I noted the Condor has high flow valve, heavy spring, and heavy hammer. I have 2 setups, the 927fps and ~1100fps. 50cc plenum and 125cc plenum with different reg settings.