Question for Bob on plenum volume.
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Question for Bob on plenum volume.
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Topic: Question for Bob on plenum volume. (Read 990 times))
Scotchmo
GTA Senior Contributor
Posts: 2333
Real Name: Scott Hull
Question for Bob on plenum volume.
«
on:
August 08, 2018, 09:43:27 PM »
Bob,
I know you have done a lot of research into this subject. This should be simple enough. I'm building a project with some dimensional limitations. It's .177, probably 8.4gr or 10.3gr pellets. Regulated pressure is 1200psi. I want 12fpe. I have about 10cc of plenum without some substantial changes. Is this enough? I know that I could bump up the pressure to compensate some. If so, how much?
Thanks,
Scott Hull
«
Last Edit: August 08, 2018, 10:21:03 PM by Scotchmo
»
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Los Osos, California
Morro Bay Airgunners
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Question for Bob on plenum valume.
«
Reply #1 on:
August 08, 2018, 09:54:57 PM »
Yep, that will work just fine.... I recommend 1 cc per FPE, but that is close enough.... Besides, there is probably some additional volume inside the valve.... Make sure that the passage between the plenum and the valve inlet is larger than the valve throat, so that the entire plenum volume is available for the shot....
Once you assemble the gun, max. out the hammer spring preload, shoot a couple of shots, and then back it down a turn and repeat.... Keep going until the velocity has dropped about 20%, and plot out the velocity vs. preload curve.... It should look like this....
Provided you have enough hammer strike available to max. out the velocity (ie find the plateau), you will find that heavier pellets require a bit more preload to get to the beginning of the plateau.... Determine what the plateau velocity is, and back off the hammer strike to reduce the velocity 3-5% and you will be getting the best balance beween power and efficiency.... If you have too much power, you can back off the preload to reduce the velocity about 10% below the plateau.... but any lower than that and when you hit the regulator setpoint the velocity will start to climb because the gun is acting like it is unregulated below the setpoint, and you will get a bell-curve.... If that is your situation, reduce the setpoint and try again.... If your plateau is not high enough, you will need to increase the setpoint....
Bob
Different pellet weights will produce different curves, as shown....
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Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
Scotchmo
GTA Senior Contributor
Posts: 2333
Real Name: Scott Hull
Re: Question for Bob on plenum volume.
«
Reply #2 on:
August 08, 2018, 10:25:20 PM »
Quote from: rsterne on August 08, 2018, 09:54:57 PM
...
Different pellet weights will produce different curves, as shown....
Most of your examples use higher pressures (1800-2000psi).
So even at
1200psi
and 10cc, you think 12fpe is doable?
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Los Osos, California
Morro Bay Airgunners
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Question for Bob on plenum volume.
«
Reply #3 on:
August 09, 2018, 12:40:11 AM »
Yep should not be a problem.... I do very little with .177, but enough to know that should work just fine.... Here is a regulated QB in .22 cal on 1200 psi....
An 8.4 gr. pellet in .177 cal has a lower SD than the 15.9 gr. in .22 cal above.... which means with equal barrel length and pressure you should be able to at least match the velocity.... 12 FPE with an 8.4 gr. is only 800 fps....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
Scotchmo
GTA Senior Contributor
Posts: 2333
Real Name: Scott Hull
Re: Question for Bob on plenum volume.
«
Reply #4 on:
August 09, 2018, 12:45:47 AM »
Thanks,
I did forget to include barrel length - about 14.5". It is an air pistol.
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Los Osos, California
Morro Bay Airgunners
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Question for Bob on plenum volume.
«
Reply #5 on:
August 09, 2018, 01:25:38 PM »
That will make a significant difference.... but I think you should still be OK, Scott.... A larger than 10 cc plenum size will not really help, if you can't get your 12 FPE you may have to use slightly higher pressure.... I would make sure the ports are 75-80% of caliber area all the way through from valve seat to pellet....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
wimpie swart
Shooter
Posts: 41
yes
Re: Question for Bob on plenum volume.
«
Reply #6 on:
August 17, 2018, 09:51:00 AM »
Just something interresting I picked up when servicing a CZ200 rifle, it was a .177, 12fpe rifle, and had a standard plenum of 3.5cc with a 18gram hammer......
The RSA Veldt series built by my friend for HFT competition use, all have a smaller than 4cc plenum with about 26gram hammer, with internal SSG fitted. My rifle is then setup to run the 8.44gr @790fps, but if I turn the SSG in max, it delivers 949fps with the same same plenum.
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South Africa
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Question for Bob on plenum volume.
«
Reply #7 on:
August 17, 2018, 03:30:09 PM »
Is the 3.5 cc plenum including the volume inside the valve.... ie the total volume of air available to make the shot?.... That is the important volume; 1 cc/FPE is great, you can get away with 1/2 cc/FPE..... less than that and you have to increase the regulator setpoint to compensate....
You can certainly use smaller plenums and make the gun work.... The question becomes at what point does the shot count suffer because you have to use too much regulator setpoint to maintain the average pressure inside the valve during the shot.... Compared with using 1 cc/FPE, if you cut that in half you will need to increase the setpoint pressure a bit to compensate (maximum 10% for most PCPs)…. If you cut it in half again, you might need 25% more pressure to get the same FPE.... You can certainly do that, but at the expense of "headroom" (ie the pressure difference between fill pressure and setpoint)…. That costs shot count….
Of course if the regulator is inside the tube, increasing the plenum decreases the volume of the HP reservoir.... That is a situation where 1/2 cc/FPE may well give you a higher shot count, even with the increase in setpoint required.... Smaller plenums are less critical on low powered applications.... because the shot count is usually high anyways.... Bottom line is any plenum will work, but some will work a lot better than others....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
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All Springer/NP/PCP Air Gun Discussion General
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,
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Question for Bob on plenum volume.