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***
The targeted FPE for my setup is 60 fpe.
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
***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***
The targeted FPE for my setup is 60 fpe.
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