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PostPosted: Wed Nov 02, 2016 12:20 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8989
Location: Coalmont BC
Many of you know that I have a rule of thumb for plenum size in regulated PCPs of 1 FPE/CI.... It's a bit arbitrary, but I didn't just pull it out of my butt.... Here is a graph that may hope you understand the influence of plenum size on FPE....


This graph makes a few assumptions.... It assumes that the unregulated gun is tuned to shoot 45 FPE at 2000 psi, and that the efficiency is 1.0 FPE/CI, which was typical for a stock MRod, before the SSG or the use of short, stiff springs.... and without hogging out the ports.... Yeah, they did a bit better than that, but not a lot.... The red dot shows that compared to having the whole 215 cc reservoir available, as it is in an unregulated MRod, if you have a plenum of 45 cc, the pressure loss during the shot will drop you down to about 43 FPE, a loss of only about 5%.... The next blue dot to the left represents a plenum of 1/2 cc per FPE, or in this case 23 cc, and the FPE has dropped to just over 40 FPE, a 10% loss compared to the unregulated gun at the same 2000 psi.... If you use a plenum even smaller than that, the FPE drops off very quickly....

The potential FPE of a PCP, leaving all other things equal, is proportional to the pressure at the valve seat.... The reason for the losses in the graph above is that as you decease the plenum volume, the pressure during the shot drops more, because the regulator is not capable of adding air to the plenum during the 1-2 mSec. that the valve is open.... The air in the plenum is all you have to produce the shot.... So, if you are going to lose 5% in FPE due to reducing the reservoir of the unregulated gun down to 1 cc per FPE, then you need about 5% more pressure to get back to square one (45 FPE).... That is only 100 psi, for our 2000 psi example..... With a 45 cc plenum, using 2100 psi should put you back to your original 45 FPE, with no other changes to the gun.... However, if you use a plenum half that size, you will need 10% more pressure than you started with, or about 2200 psi.... If you cut the plenum in half again, down to 11 cc, to get back to 45 FPE you will have to increase the setpoint pressure by about 25%, to 2500 psi.... You can see how important it is to maintain a large enough plenum for the FPE you want the gun to produce....

Since I did that graph, there have been significant improvements in PCP efficiency, by eliminating hammer bounce through using an SSG or a short stiff spring.... In combination with using larger ports, which allows more power at lower pressures, it is not uncommon to get 1.5 FPE/CI now instead of 1.0 FPE/CI, which means we are getting the same power using 1/3 less air.... This flattens out the above curve, so that you need less pressure increase to make up for smaller plenums.... Instead of needing a 10% increase in pressure when using a plenum that is 1/2 cc per FPE, you will only need about 7%.... Using larger ports can more than make up for that requirement, so when you combine all these tuning techniques, we are seeing regulated MRods using plenums of less than 1/2cc per FPE and producing the same FPE at the same, or even slightly less pressure, than what can be achieved with a stock unregulated MRod....

When you use an internal regulator, ie one that fits inside the reservoir, there is a compromise between setpoint pressure and plenum size to be addressed.... The larger the plenum you have, the lower the required setpoint pressure for a given FPE.... However, a large plenum means less high pressure air volume in the reservoir.... On the other hand, if you reduce the plenum to increase the reservoir volume, you need a higher setpoint, which then reduces the amount of high pressure air that is USABLE because the pressure range between full and the setpoint is less.... With current technology, it seems that 1/2 cc per FPE is a good place to start with an in-tube regulator.... For low powered applications, requiring less pressure, you can go even smaller than that, and simply accept a higher setpoint.... For high power applications, and particularly for larger calibers.... it is unlikely you will be able to get a large enough plenum inside an existing air tube.... That is the reason there will be an effective caliber limit when you want to build a regulated PCP on an existing platform.... For high power applications, using a regulated bottle fed or tethered PCP.... I still recommend 1 cc per FPE of course....


Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!

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