Tuning Effect of Plenum Size?

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Nico5999

Nico5999

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Apr 5, 2015
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Hey homies,

I'm wondering if we can get a thread concisely explaining what the advantages/disadvantages are of having a larger plenum? I'll be doing some experimentation with my Air Arms rifles after the competitive season ends and this will likely be an area that I delve into.

Aside from the potential for more power, is there an advantage to having a larger plenum volume in general?

Interested in seeing what the forum comes up with.

Best,
Nico
 
The advantage of a large plenum is straightforwardly the highest energy potential at the lowest possible operating pressure (regulator setpoint).

Disadvantages:
1. Size and weight
2. For in-tube regulators, a large plenum represents a compromise which steals precious reservoir volume.
3. For exceedingly large plenums, there is a potential that firing will consume so little air as to not produce a sufficient pressure drop to cause the regulator to cycle open. The extreme spread may be negatively affected as a result. Being tuned to the velocity knee will ameliorate this concern for the most part.

The general guidance for in-tube regulators is to allocate about 0.5cc per fpe, in order to achieve the best shot count. Otherwise if you can reasonably have as large of a plenum as you want, a 1.0cc per fpe will yield better efficiency. Above 1.0cc per fpe, the law of diminishing returns kicks in pretty hard.
 
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Airfuture said:
Why does this rule not apply to the Altaros M24, which has a plenum smaller than 10 CC, has a regular (balanced) valve?
Click to expand...
High setpoint, high refresh rate regulator (big passage), those things can help a small plenum perform better.

Just adding plenum is not always a guaranteed way to increase performance. I have seen many add more plenum only to learn they could not take advantage of the plenum due to 1) small porting and/or 2) insufficient hammer strike.

it is possible to actually lose velocity after adding plenum, if your gun is on the edge of having sufficient hammer strike. The higher average pressure during the shot cycle requires sufficient hammer strike, else it can reduce dwell, reducing velocity at the same reg setting and hammer strike as before adding plenum.

Adding to what Jason mentioned above, I find it easier to create a consistent tune with plenums of .5cc/fpe than with plenums of 1cc/fpe and larger. Everyone's tuning style and skills are different, that is just what works better for me.....
 
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mackeral5 said:
High setpoint, high refresh rate regulator (big passage), those things can help a small plenum perform better.

Just adding plenum is not always a guaranteed way to increase performance. I have seen many add more plenum only to learn they could not take advantage of the plenum due to 1) small porting and/or 2) insufficient hammer strike.

it is possible to actually lose velocity after adding plenum, if your gun is on the edge of having sufficient hammer strike. The higher average pressure during the shot cycle requires sufficient hammer strike, else it can reduce dwell, reducing velocity at the same reg setting and hammer strike as before adding plenum.

Adding to what Jason mentioned above, I find it easier to create a consistent tune with plenums of .5cc/fpe than with plenums of 1cc/fpe and larger. Everyone's tuning style and skills are different, that is just what works better for me.....
Click to expand...
I think the M24 has a stock regulator and is not big passage or "high flow"! Maybe Skout Epoch has it but it hasn't been explained in detail yet...
 
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nervoustrig said:
The advantage of a large plenum is straightforwardly the highest energy potential at the lowest possible operating pressure (regulator setpoint).

Disadvantages:
1. Size and weight
2. For in-tube regulators, a large plenum represents a compromise which steals precious reservoir volume.
3. For exceedingly large plenums, there is a potential that firing will consume so little air as to not produce a sufficient pressure drop to cause the regulator to cycle open. The extreme spread may be negatively affected as a result. Being tuned to the velocity knee will ameliorate this concern for the most part.

The general guidance for in-tube regulators is to allocate about 0.5cc per fpe, in order to achieve the best shot count. Otherwise if you can reasonably have as large of a plenum as you want, a 1.0cc per fpe will yield better efficiency. Above 1.0cc per fpe, the law of diminishing returns kicks in pretty hard.
Click to expand...
I think that disadvantage number 3 is the must dangerous one for accuracy.
 
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Cranky1 said:
I have actually noticed #3 in my early maverick compact when set to low fpe for shooting in the front yard.
Click to expand...
FYI, that outcome is not the fault of the plenum failing to refresh. Rather it is what happens with any regulated gun when the hammer spring tension is dialed back well below the plateau velocity. It is an artifact of operating at a state of partial valve lock, where a proportionally large amount of the hammer energy is soaked up just cracking the poppet from the valve seat, leaving very little residual hammer energy to create lift and dwell.

It's the principal reason a "power wheel" implemented as a transfer port restrictor (rather than as a hammer spring adjuster) does not produce this undesirable behavior.
 
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nervoustrig said:
FYI, that outcome is not the fault of the plenum failing to refresh. Rather it is what happens with any regulated gun when the hammer spring tension is dialed back well below the plateau velocity. It is an artifact of operating at a state of partial valve lock, where a proportionally large amount of the hammer energy is soaked up just cracking the poppet from the valve seat, leaving very little residual hammer energy to create lift and dwell.

It's the principal reason a "power wheel" implemented as a transfer port restrictor (rather than as a hammer spring adjuster) does not produce this undesirable behavior.
Click to expand...
I was kind of thinking this was what actually was happening, but I did notice that the reg was not refreshing everytime. I know the port restrictor on my streamline was a lot more consistent.
 
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I am glad to read that other people have so much knowledge and more even are willing to share to each of us who like to scratch their head thinking why and how things happen.

Like many before me i have also put some testing into plenum size about a year ago and am happy to report that indeed increasing plenum alone touching nothing else made the valve cycle shorter (when prior tuned at knee) but alowed for higher energy potential (if no other bottleneck is present).

I didn't yet analyse the general rule but one thing that i observed in my testing was that by increasing working pressure the plenum capacity should also increase for a benefit in potential energy. I cannot however say by how much or come by a general rule myself because there are only 3 volume arrangements i had at the time.

The tests were made in a FX maverick with different barrel arrangements and hammer was adjusted in each case for max energy respectively at 2 different working pressures.
P stands for the hammer spring position and had enough travel to accommodate all barrels plateau levels.

Maverick energy @120bar
120bar .177 700mm
Zan 20gr plenum 32cc 910 p15 49j
Zan 20gr plenum 89cc 970 p16 56j
Zan 20gr plenum 146cc 970 p16 56j

120bar .22 800mm
HN 30gr plenum 89cc 980 p16 86j
HN 30gr plenum 146cc 980 p16 86j

120bar .25 600mm
Zan 41gr plenum 89cc 856 p17 90j
Zan 41gr plenum 146cc 870 p17 93j

120bar .30 700mm
Zan 68gr plenum 89cc 823 p17 138j
Zan 68gr plenum 146cc 845 p18 146j

Maverick energy @135bar
135bar .177 700mm
Zan 20gr plenum 89cc 1000 p15 60j
Zan 20gr plenum 146cc 1007 p15 61j

135bar .22 800mm
Zan 40gr plenum 89cc 905 p19 98j
Zan 40gr plenum 146cc 930 p19 104j

135bar .25 600mm
Zan 41gr plenum 89cc 900 p19 100j
Zan 41gr plenum 146cc 907 p18 101j

135bar .30 700mm
Zan 68gr plenum 89cc 845 p19 146j
Zan 68gr plenum 146cc 868 p20 154j

Sorry if for most barrel arrangements there is a test with the smallest 32cc plenum test missing but at the time I was aiming to discover what was the smallest vilume i needed for each barrel.
 
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I'm not sure what your data says, but it seems to me that with the exception of the 68gr .30, you always had too much plenum with the 146cc size. Wouldn't we need the shot string numbers to see the fluctuations to see if that were so? This is a great learning moment, and an awesome teaching one for you professors out there. I will be following this thread closely. I would love to have a clear understanding of this.
 
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I like seeing all of that data collected, great stuff.

Oddly this is an area.where I go more by feel, by sound than anything else. Coupled with chronograph readings, shot cycle largely tells me where I'm at vs where the gun wants to be. I collect and analyze a decent amount of data professionally and try not to let the hobby feel like work to me, lol.

Any time exceeding 1-1.5cc/fpe, ive found everything must be in perfect order to yield low ES--- flawless, responsive, regulation, perfectly consistent hammer strike, usually accompanied by restricted transfers and leaning on the hammer spring to make hammer strike more consistent.

I enjoy tuning at the opposite end of the spectrum---->80% of bore area porting, light hammers, less hammer spring. This method is more tolerant from a ES perspective when coupled with smaller plenums, more like .5-1.0cc/fpe, but even as small as .25cc/fpe.

Not attempting to argue which method is "better", just sharing thought processes. IMO, the method involving restricted transfer and more hammer strike is the more robust method, the more tolerant/damping of variation, from an ES perspective. It just is not the method I enjoy.
 
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To me the first step is to see how your current plenum falls in the ".5-1 cc per fpe" range. If the plenum is outside this range on the low size, a bigger plenum will probably give you more velocity and power. If it is already in that range, it may not help. If it is in the high portion of that range or beyond then it would seem like it is almost certainly not going to give you much. I put a 35cc plenum on my P35-25 which came with a plenum of less than 15cc. I wanted to get to 800 fps or a little more with JSB 33.95 grain (which in my experience are really more like 35 grain). About 50 fpe. So 15 is low outside the recommended range and 35 is within it. I gained substantial additional velocity with the plenum change. The plenum I used is external so I did not loose air storage capacity. I had previously drilled out the path from the valve to the barrel port and the barrel port from about 75% of bore size to 85%. That gave me a few more fps but the plenum was a much bigger change.
 
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The biggest advantage of a large Plenum is when tuning for high fpe. Example my FX crown has a rather small plenum so when you want to shoot slugs it soon hits max.... And its also getting airhungry.. The promblem is you set the reg. To like 150 bar. With a small plenum and high fps you hit the valve hard with the hammer.... the problem is the plenum pressure drops a lot. And by the low plenum pressure there is reduced helping force from the remaining air pressure in the plenum to close the valve again. This means the valve is open too long time.... this means waste of air and further pressure drop, and dirty air behind the projectile.... the double size Plenum means the remaining air pressure will only drop less than half for the same slug speed.... But this differece is less notible with air riflees with balanced valves ....
 
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nervoustrig said:
3. For exceedingly large plenums, there is a potential that firing will consume so little air as to not produce a sufficient pressure drop to cause the regulator to cycle open. The extreme spread may be negatively affected as a result. Being tuned to the velocity knee will ameliorate this concern for the most part.
Click to expand...
Jip, that is what I experienced before when shooting lighter pellets than it was tuned for and turned down the hammer without reducing regulator pressure.
 
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