Plenum Volume Confusion

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Misguided

Misguided

Member
Oct 30, 2024
22
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Highland Michigan
Ive seen this stated many times. More plenum volume gives a higher shot count along with higher velocity. Logic tells me other wise.

I'm pulling these numbers out of my @$$

If you have 600 CC tank and 5cc plenum = 120 shots

Now you add a external plenum which increases plenum volume to 15cc........that extra air in the plenum would certainly increase velocity.....but 600 CD tank divided by 15cc plenum equals 40 shots .....

This in in a perfect world where no more air leaves the tank and goes thru the plenum and with the projectile out the bore

Am I wrong?
 
I would like to know as well i did not add a bigger plenum but i ported my avenger .25 barrel and transfer port and my shot count went down even after changing tune i have gained plenty of fps but if i max it out i can get 5 to 8 shots at full power mild tune is about 30 now ...in my logic better air flow equals more air leaving the tank
 
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You're only using a portion of the plenum for each shot.

A bigger plenum will have a smaller amount of pressure drop as the pellet travels down the barrel. The amount of pressure drop is proportional to the plenum volume versus the barrel volume.

You can make the same power at a lower reg pressure with a larger plenum...So you can achieve more shots on a fill.

Having a higher average pressure will also accelerate the projectile faster in the initial portions of the barrel which can produce added efficiency if you tune to take advantage of this by closing the valve a little earlier.

Mike
 
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I went to a larger plenum on my P35-25 but it had nothing to do with shot count. I wanted to get 34 grain pellets up to 800 fps. The plenum I added is external so it did not reduce the main storage area volume but other options would have. Bigger plenum is mainly a way to add fps. Any impact on shot count would be a secondary effect. If you increase power shot count will decrease. If you make the same power at a lower reg pressure you will gain a few shots but I don't see that as sufficient reason to go to the trouble.
 
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Misguided said:
Ive seen this stated many times. More plenum volume gives a higher shot count along with higher velocity. Logic tells me other wise.

I'm pulling these numbers out of my @$$

If you have 600 CC tank and 5cc plenum = 120 shots

Now you add a external plenum which increases plenum volume to 15cc........that extra air in the plenum would certainly increase velocity.....but 600 CD tank divided by 15cc plenum equals 40 shots .....

This in in a perfect world where no more air leaves the tank and goes thru the plenum and with the projectile out the bore

Am I wrong?
Click to expand...
If this were the case my p35x with a 60cc plenum would give me about 5 shots per fill from a 255cc tank.

The panthera 700mm barrels would only get 3 or 4 shors per fill as well. FX impacts known for getting a plethora of shots would only get 10-15 shots per fill.
 
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The addition of a plenum isn't really about extra shots, it's more about efficiency and making better use of the air.

When the poppet gets knocked off its seat and air pressure sends it right back to the seat, there is a tiny bit of air released that travels through the transfer port and through the barrel port, pushing the pellet down the barrel.

As I understand the concept, with no plenum, the only air available for the shot is inside the air path after the regulator. When the poppet is knocked off its seat, an immediate drop in pressure occurs as the HPA fills the air path and pushes the pellet out with greatly reduced pressure and volume.

Adding a plenum large enough to supply all the air that can pass through the valve when it's knocked open, allows for better efficiency and according to those that study such things, the valve closes earlier, conserving air.

There are a whole lot of other variables and this is not a chisled in stone statement, just a rough guide.
 
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Misguided said:
Ive seen this stated many times. More plenum volume gives a higher shot count along with higher velocity. Logic tells me other wise.

I'm pulling these numbers out of my @$$

If you have 600 CC tank and 5cc plenum = 120 shots

Now you add a external plenum which increases plenum volume to 15cc........that extra air in the plenum would certainly increase velocity.....but 600 CD tank divided by 15cc plenum equals 40 shots .....

This in in a perfect world where no more air leaves the tank and goes thru the plenum and with the projectile out the bore

Am I wrong?
Click to expand...
Excellent question! Now I'm not a physics major be any means, however, what the plenum will do is increase power on a certain setting and give the same shot count. But will allow you to increase your shot count as you lower your power setting in comparision to the guns standard configuration. Why? that I don't know, but that's what usually transpires with my eastern european guns, The valving system on said gun will also react potentially differently, Say, a Huben versus a Vulcan 3. Theoretically, a reg setting of 110 bar with medium HS tension with plenum will yield same power as say, no plenum and reg setting 130 bar. More air but lower pressure gives same end power to projectile.
 
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JaceSpace1369 said:
If this were the case my p35x with a 60cc plenum would give me about 5 shots per fill from a 255cc tank.

The panthera 700mm barrels would only get 3 or 4 shors per fill as well. FX impacts known for getting a plethora of shots would only get 10-15 shots per fill.
Click to expand...
Ok so it doesn't
Firewalker said:
The addition of a plenum isn't really about extra shots, it's more about efficiency and making better use of the air.

When the poppet gets knocked off its seat and air pressure sends it right back to the seat, there is a tiny bit of air released that travels through the transfer port and through the barrel port, pushing the pellet down the barrel.

As I understand the concept, with no plenum, the only air available for the shot is inside the air path after the regulator. When the poppet is knocked off its seat, an immediate drop in pressure occurs as the HPA fills the air path and pushes the pellet out with greatly reduced pressure and volume.

Adding a plenum large enough to supply all the air that can pass through the valve when it's knocked open, allows for better efficiency and according to those that study such things, the valve closes earlier, conserving air.

There are a whole lot of other variables and this is not a chisled in stone statement, just a rough guide.
Click to expand...
"The addition of a plenum isn't really about extra shots, it's more about efficiency and making better use of the air."


Firewalker, exactly my understanding of what more plenum space is supposed to do. I kept seeing statement after statement that " I will get increased shots" I couldn't wrap my head around that
 
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Misguided said:
Ok so it doesn't

"The addition of a plenum isn't really about extra shots, it's more about efficiency and making better use of the air."


Firewalker, exactly my understanding of what more plenum space is supposed to do. I kept seeing statement after statement that " I will get increased shots" I couldn't wrap my head around that
Click to expand...
The reason for this is likely because they arent stating the whole truth. If your gun without a plenum is regged at say 110b and your shooting 18gr fx at 900. The addition of a plenum will increase the speed of said projectile. BUT if you lower the reg you can acheive the same speed as before, thus giving you more shots since less reg pressure is being used.
 
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You will get more shots per fill with a bigger plenum. It's basic common sense. Nothing to do with math or all the sht. Why would you get more shots per fill if bottle is the same. Example your gun have a 480cc bottle and 10cc plenum. Reg set to 130 bar getting 25gr pellet at 850fps with 100 shots per fill. Now you add a 20cc plenum. Since the larger plenum will give you more power, you can turn down your reg for the same power as before. Now your set up will be 480cc bottle, 20cc plenum, 100 bar reg, 850 fps, and 120 shots per fill. Since your reg is now lower you will get more shots per fill till it hits the reg. Simple common sense.
 
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Misguided said:
Ok so it doesn't

"The addition of a plenum isn't really about extra shots, it's more about efficiency and making better use of the air."


Firewalker, exactly my understanding of what more plenum space is supposed to do. I kept seeing statement after statement that " I will get increased shots" I couldn't wrap my head around that
Click to expand...

It's actually an unexpected consequence, you add 25cc of extra volume, you will get 25cc of extra shots so both statements appear to be true.
 
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JaceSpace1369 said:
The reason for this is likely because they arent stating the whole truth. If your gun without a plenum is regged at say 110b and your shooting 18gr fx at 900. The addition of a plenum will increase the speed of said projectile. BUT if you lower the reg you can acheive the same speed as before, thus giving you more shots since less reg pressure is being used.
Click to expand...

That is my understanding as well. 😊
 
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PigeonMan said:
You will get more shots per fill with a bigger plenum. It's basic common sense. Nothing to do with math or all the sht. Why would you get more shots per fill if bottle is the same. Example your gun have a 480cc bottle and 10cc plenum. Reg set to 130 bar getting 25gr pellet at 850fps with 100 shots per fill. Now you add a 20cc plenum. Since the larger plenum will give you more power, you can turn down your reg for the same power as before. Now your set up will be 480cc bottle, 20cc plenum, 100 bar reg, 850 fps, and 120 shots per fill. Since your reg is now lower you will get more shots per fill till it hits the reg. Simple common sense.
Click to expand...

I do believe it has a lot to do with math. There are consequences for any change made.
 
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So just an FYI, this is a non linear problem. If you've taken a differential equations you can relate this to a tank mixing problem.

The other thing to remember is you are dealing with a compress able fluid. Not water. When you up the pressure of air, the amount of air does not scale linearly. Conversely a plenum is regulated. This you fix the pressure and increase the tank volume. But the gas volume does not increase proportionally to the tank volume. To maintain the same pressure of air across a larger volume you need a MUCH larger quantity of air.

Thus to tie it back to the original question. The key is the compression of the gas that yields higher shot count.

An easy anecdotal thing you can try out is get two air rifles with different tank sizes. A tank size of 150 CC does not take half as long to fill up as a tank size of 300 cc
 
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Thomas air explained it well in his post, I think it was the second post. A larger plenum raises the average pressure released when you shoot. That either gives you higher velocity or allows a regulator reduction with the same velocity and thus a few more shots. But I still maintain any change in shot count is secondary. Main effect is higher velocity at a given regulator setting.
 
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