How Plenum Volume Effects FPE Output

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ackuric

ackuric

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Mar 10, 2017
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Colorado, United States
Here is a graph demonstrating how plenum volume effects FPE output (In regulated rifles, non-regulated generally don't have this issue). This information is relatively universal in the fact that I think 1/3 cc plenum per FPE to 1 cc plenum per FPE is the most ideal range for a pcp. The lower in power your rifle is the closer to the 1/3 value you can be without seeing much loss, where as the higher power the rifle is the more energy loss would be seen @ 1/3 cc of plenum, still within acceptable margins I'd imagine.

***this graph reflects my setup but the rule of thumb for plenum volume availability being 1/3rd to 1 cc per FPE still stand for most pcp's***

1546147271_7912311995c2855c7167528.83959404_plenum.jpg


The targeted FPE for my setup is 60 fpe.



1546177006_5475073965c28c9eea188c4.26535660_plenum.jpg




FWIW there would be no considerable change in power going from 120 to 240 cc's of plenum in my current setup…but in a very high powered 240 FPE rifle, it would be entirely different. But as the graph suggests, around 50-100% of intended FPE in volume for plenum measured in CC's is most ideal, and 1/3rd acceptable, below that is very poor for performance, and above marginally better. Hope this helps some understand plenum volume and its importance. I have verified my mathematical model with 3 real world plenum size changes that correlate 100% to the graph.



The issue with in-tube regulators is they take available space away from the HPA reservoir, where as bottled guns can have a optimally sized plenum designed into the rifle and need not worry about HPA volume availability. My personal work around to increase my plenum from 20 cc's to 53 cc's involved fabricating an external plenum of 33 cc's that had air flow greater than all forward porting so air was as ready as quickly as the barrel demands. With that said if you find yourself wanting lots of power but are trying to do such with very little plenum, there is likely a need to have a custom internal or external plenum configured into your rifle by a pro provided you lack the technical knowledge and skill required to do so, especially if you start teetering below the 1/3rd cc's per fpe value.



-Matt
 
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Matt,

thank you - very educational, at least for me. Based on above one can calculate also the optimum power for a rifle, taking current plenum size as a start, and efficiency as a priority.
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Glad to help. Sizing the plenum correctly certainly can help with improving shot count or efficiency. Plenum is a key part of the equation however, barrel length, caliber, port size, and average shot pressure (a result of plenum volume / starting pressure) are all key factors in determining energy output, as well as efficiency at particular power levels. Heres a graph for effects on fpe with various port sizes on my same rifle @ current reg pressure (1,950) and at 3,000 psi. Graph speaks for itself!



1546175265_3056607135c28c321b4e652.93745575_portsize.jpg

 
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Matt,

Of course you are correct, but unfortunately available physical space gets in the way of any correction. The net result is that it is infinitely better to sell the problem rifle and buy another. It will be much less expensive.
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Depends. If internal capacity is non-issue and a sacrifice of shot count for power is more desired than a custom larger plenum can be fabricated cheaply, alternatively much like my external plenum, that can be fabricated rather cheaply as well. Its rare these days for a manufacturer to get all things @ their most optimal level of operation. Commonly power and plenum size are sacrificed or even more likely short sighted in many designs. A Rather simple fix if a sub optimal plenum volume is found to be your only problem with a particular rifle. Certainly more so if you modify your rifle way beyond the manufacturers intended FPE outputs...such as I have by a potential 100+% (stock 45 fpe, modified 98 fpe unregulated and 60+ fpe regulated.)
 
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good stuff matt.no one on here nor at fxusa could tell me the the plenum cc. volume on a fx crown, fx guesses 25-30.would you know for certain? tks
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Ernest Rowe may know, not sure. I measured mine by removing it and filling it with water and measuring the ml's (1 ml = 1 cc). I don't have a Crown or I would. 25-30 sounds about right though, which isn't too bad unless you're trying for 100+ fpe out of a .30 cal



Total plenum volume is from reg to valve seat so valve volume + reg volume.
 
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Here is some math behind how plenum volume works. 

Say you start with 135 bar pressure, such as I am at now, thats roughly 1,950 psi. My plenum volume is 53 cc's. With a 20 FPE tune my rifle may only use around 1 cc of air, but at 50+ FPE my rifle is using 4 cc's or more of air per shot. We'll use the 4 cc figure (rounded down) to dramatize the pressure drop during the shot cycle.



The volume from my valve seat to pellet base is just shy of 1 cc, we'll round up to 1 cc to make things simple.



1 cc + 4 cc = 5 cc

53 (my plenum volume in cc) * 1950 (starting reg pressure) / 58 (combined volumes from Plenum, Volume from seat to pellet base and the additional volume in barrel that was created while pushing pellet during valve open) = 1781

(1781 + 1950) / 2 = 1865 (avg pressure @ valve seat) = 169 total pressure drop



Now lets run that same formula back for 20 cc's instead of 53

20 * 1950 / 25 = 1560

(1560 + 1950) / 2 = 1755 (avg pressure @ valve seat) = 390 total pressure drop.



With the above 20 cc plenum configuration, to hit the power levels I hit now with 53 cc's I would need to increase my regulated pressure 110 psi (roughly) to keep the average pressure at the valve seat the same. 



I hope the above math further helps people understand how the math behind plenum volume works..why its important, and how the diminishing returns on going bigger apply, as well as how dramatically small plenums can hurt/reduce power and efficiency due to large pressure drops.
 
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I would guess plenum size is one of the reasons big bore airguns often is unregulated? They basically need all the air available in the main reservoir to obtain a high pressure all the way down the barrel?
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Correct, it is difficult and not really feasible to have a plenum size required for many big bores, although it is possible, especially if designed eloquently. 
 
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