GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on June 09, 2014, 04:04:55 PM
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I have been asked on occasion what is the optimum volume for the valve in a pumper.... There has been a guideline around for a while, from James Perotti, to use a valve that is half the barrel volume, regardless of pressure.... I've never seen the math behind it, so I thought I'd explore it, using Lloyd's Internal Ballistics Spreadsheet.... I set the variables to roughly what you might expect for a 24" barrel in .22 cal which has a barrel volume of 15.0 cc.... I used a 16 gr. pellet, and tried it with 1500 psi (typical for a pumper) and 3000 (just from curiosity).... and the plotted the energy and efficiency versus the valve volume as a percentage of the barrel volume.... Here are the results.... The blue lines are at 1500 psi....
(http://i378.photobucket.com/albums/oo221/rsterne/Millenium%20Pumper/DumpValves_zps8ff30e30.jpg) (http://s378.photobucket.com/user/rsterne/media/Millenium%20Pumper/DumpValves_zps8ff30e30.jpg.html)
The first thing you will notice is that the overall efficiency is very low.... That is because with a dump valve a lot of the air is leaving the barrel after the pellet is long gone, and the larger the valve, the worse that is.... In the case of a valve that equals the barrel volume, think of it this way.... If we start with 1500 psi in the 15 cc (100%) valve, when the pellet reaches the muzzle, that air has expanded to fill the 15 cc barrel as well.... The total volume is 30 cc, and the pressure is down to 750 psi.... Once the pellet leaves the barrel, the energy in that 750 psi of air only produces noise and no longer accelerates the pellet.... If we were to close the valve at the instant the pellet left the barrel, we would retain HALF the air in the valve, and only have to pump half the air back into it for the next 1500 psi shot.... This is how a retained air pumper (also known as an ACP or air conserving pumper) works.... PCPs work in the same manner, and the valve typically shuts when the pellet has only moved 25-33% of the way down the barrel.... Compared to a dump valve, closing the valve when the pellet has travelled only 50% of the barrel length will use only 500 psi of air instead of the whole 1500, and yet the velocity will be nearly as great.... At 25%, the pressure will only have dropped to 1200 psi instead of zero, using only 20% of the air in the valve.... However, the velocity will have only decreased about 10% and the efficiency will be about 4 times as great....
Now, back to the size of a dump valve.... As you can see, there is still a small increase in FPE when you go larger than the recommended 50% valve size, but the drop in efficiency is great.... I can see why James Perotti recommends 50% because going larger uses a lot more air for the additional gains.... It might help if we look at what valve sizes are used in some of todays pumpers.... A 1377, with a 10" long .177 barrel has a barrel volume of only 4.0 cc.... a 1322, same gun with a .22 cal barrel, is 6.2 cc.... The older 2289, with a 14.5" barrel in .22 cal has a volume of 9.0 cc, and the newer one with a 12" barrel is 7.5 cc.... A stock 13XX/XX89 valve is about 1.5 cc, so that works out to 38% for a 1377, 24% for a 1322, and 17-20% for a 2289.... If we look at a Benjamin 397, with a 19" long .177 barrel, the volume is 7.7 cc, and for the 392 in .22 cal the volume is 11.8 cc....The valve in a 39X is 4.2 cc, which works out to 55% in .177 cal and 36% in .22 cal.... During the development of my Millenium Pumper, I made a .25 cal Carbine version with a 20" barrel that had a volume of 16.1 cc.... The first valve I used was 5.5 cc (34%) and the second one I used was 6.7 cc (42%).... From my experience I think a valve of about 1/3rd of the barrel volume is a good balance between power and efficiency.... and I would agree with Perotti that 1/2 is about the maximum, beyond that you will be wasting a lot of air.... The trade off in going to a 50% valve instead of a 33% valve is about 13.5% more FPE at the expense of 50% more air used (and therefore 50% more pumping).... assuming the same pressure in both cases.... Going to a 100% valve increases the FPE only an additional 19%, but needs a 100% increase in air (and double the pumps) over a 50% valve....
Bob
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Thank you for this post. I have long said that an ACP AirRifle is the perfect comprise between MSP and PCP :) .
Though seriously I like the fact that an ACP design is much more effecient than a MSP.
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Other than the weight, and the added complexity and cost, an ACP is a very nice solution.... It has to be heavier and more expensive than an equivalent PCP because of the addition of the pump and check valve.... On the other hand, you don't have to carry a pump or pony tank separately.... My Millenium Pumper is a good example of what can be accomplished by combining the PCP components of a Disco with the pump parts of a Benji.... plus a few custom parts as required....
http://www.gatewaytoairguns.org/GTA/index.php?topic=39328 (http://www.gatewaytoairguns.org/GTA/index.php?topic=39328)
It incorporates a fill fitting so that it can be filled from a tank when one is available and then just topped up in the field between shots.... again a good compromise.... You can call it an ACP or a PCP with onboard pump.... either way, it's a great combination....
Bob
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Other than the weight, and the added complexity and cost, an ACP is a very nice solution.... It has to be heavier and more expensive than an equivalent PCP because of the addition of the pump and check valve.... On the other hand, you don't have to carry a pump or pony tank separately.... My Millenium Pumper is a good example of what can be accomplished by combining the PCP components of a Disco with the pump parts of a Benji.... plus a few custom parts as required....
http://www.gatewaytoairguns.org/GTA/index.php?topic=39328 (http://www.gatewaytoairguns.org/GTA/index.php?topic=39328)
It incorporates a fill fitting so that it can be filled from a tank when one is available and then just topped up in the field between shots.... again a good compromise.... You can call it an ACP or a PCP with onboard pump.... either way, it's a great combination....
Bob
Is nice though I personaly would prefer one of the simpler ACP's that are based on the 13xx platform, and do not have an external fill of any kind. The ones that a few have made that have a valve chamber volume of aproximately equal to 2 times the barrel volume, using a modified 13xx valve with a small external chamber added. Then tune the springs and you have a good light weight ACP that does not weigh that much more than the 13xx of the same configuration would as a full dump pumper.
Though that is just me. And that is the reason I am going to do an ACP based on the 66, all of the mods needed to do it on the 66 will transfer dirrectly to the same mod on a 13xx platform gun.
OT Question: By the way do you know any one that makes a low cost .25 cal barrel that fits the 13xx as far as the OD is conserned (I can cut my own lead in, and make a bolt no problem)?
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You cannot modify a 13XX valve internally to increase the volume to 2X barrel volume.... You can add an auxilliary chamber, such as by using a spent CO2 cartridge, that is what I did on my .25 cal Carbine.... The fact remains, the resulting gun is going to be heavier than a PCP of the same size and performance, because of the bulk and weight of the pump.... If you don't want a fill fitting on an ACP, and enjoy pumping to fill from empty (my .25 cal took 80 pumps), then simply leave off the fill fitting.... You will need SOME method of measuring the valve pressure to achieve the same velocity shot to shot, whether that be a gauge or a moving pin type pressure sensor....
There are no .25 cal barrels that are 7/16" OD that I am aware of, but you can use a Lothar Walther 12mm OD barrel or a Green Mountain (MRod) 1/2" OD barrel and turn the OD down to fit the steel 13XX breech.... The MRod barrel would have to be shortened and either barrel would need the transfer port drilled, in addition to the chamber and leade, and in the case of the LW barrel crowning and bluing..... You will also need a much larger valve than the 13XX valve to handle the much larger barrel volume, of course....
Bob
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bob,
thanks for the post. more to figure out on how these toys actually work. is there somewhere i can fine more information about lloyd's internal ballistics spreadsheet?
i was hoping to try & match the numbers for the 1377 that i got with the valve volume reduced. right now i got my valve at 1/3 of the barrel.
actually i'm trying to figure out how to get valve volume size & pressure for a selected muzzle velocity. kind of like working from a selected muzzle velocity back through the barrel & to the valve & pressure. any way to figure that?
thanks,
plinker99
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Lloyd's spreadsheet is proprietary, he was nice enough to give me a copy.... it is not generally available....
MSPs have so many possible solutions, because you can pump them a different number of strokes, there is really no "one" correct answer that will be enough better than other solutions to make it worth trying to figure it out mathematically.... I think a valve volume of 1/3rd the barrel volume is a decent place to be, and not larger than 1/2.... If you go for a smaller valve, you need to increase the pressure for a given muzzle velocity.... Just look at the graph and extrapolate between the solid lines.... The efficiency of the shot increases as you use a smaller valve for a given velocity, but the gun gets harder to pump because the pressure required is higher.... This in turn decreases the pumping efficiency....
Bob
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Hey Bob any chance you could run a 20 grain .25 pellet in a 18.75" barrel @ 1100 and 1500 Psi?
I know I am getting about 17 fpe with ~.13 c.i. valve,
my gut, says I could hit ~725 fps with a .16 valve and close to 800 with .22 valve.
just curious to see if it comes close to matching.
I wonder what assumptions he made in the spreadsheet?
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Assuming you are already getting 17 FPE (618 fps) in .25 cal with a 20 gr. with your 0.13 CI valve, the spreadsheet says ~ 19.2 FPE (657 fps) with a 0.16 CI valve and ~ 22.7 FPE (715 fps) with a 0.22 CI valve.... All those are at 1500 psi, with an 18.75 barrel in .25 cal, having a volume of 0.92 CI.... Your valve volumes work out to 14%, 17%, and 24%.... so they are all pretty small for a .25 cal.... I would try and get up to about 0.30 CI if possible.... That should put you at about 26.2 FPE (769 fps) with a 20 gr. pellet.... I looked at my initial results for my .25 cal Disco pumper carbine, which had a 20" barrel, and at 1500 psi it shot 665 fps with 25.4 gr. pellets (25 FPE).... It had a valve that was 5.5 cc and a barrel volume of 16.1 cc, so that worked out to 34%.... That also means that my estimate of 26.2 FPE at 1500 psi with an 18.75" barrel is probably slightly too high....
The basic variables in the spreadsheet are caliber, pellet weight, valve dwell, reservoir pressure and volume, transfer area volume, barrel length, pellet drag and starting force, the gas used (air, nitrogen, helium, CO2), and an efficiency factor which usually runs between 55-70%.... The normal procedure is to input the known quantities, use default quantities for some of the minor ones if they aren't known, and then balance the dwell and efficiency until it matches what the gun is doing (or what is normal).... For a dump valve, the dwell is simply selected so that the valve is still open when the pellet leaves the barrel.... I had to use an efficiency factor of 67% to get your gun to deliver 17 FPE with the 0.13 CI valve, and that is not unreasonable for such a small valve.... The calculations are just straight F = MA, done every 0.000001 sec. as the pellet moves down the bore until it exits, correcting for the new pressure as the air expands behind the pellet.... It takes into account the weight of the gas behind the pellet accelerating as well.... The spreadsheet has had MANY revisions and refinements, and I feel it represents a VERY good model of what is going on inside a pneumatic airgun (CO2, pumper, or PCP)....
Bob
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Assuming you are already getting 17 FPE (618 fps) in .25 cal with a 20 gr. with your 0.13 CI valve, the spreadsheet says ~ 19.2 FPE (657 fps) with a 0.16 CI valve and ~ 22.7 FPE (715 fps) with a 0.22 CI valve.... All those are at 1500 psi, with an 18.75 barrel in .25 cal, having a volume of 0.92 CI.... Your valve volumes work out to 14%, 17%, and 24%.... so they are all pretty small for a .25 cal.... I would try and get up to about 0.30 CI if possible.... That should put you at about 26.2 FPE (769 fps) with a 20 gr. pellet.... I looked at my initial results for my .25 cal Disco pumper carbine, which had a 20" barrel, and at 1500 psi it shot 665 fps with 25.4 gr. pellets (25 FPE).... It had a valve that was 5.5 cc and a barrel volume of 16.1 cc, so that worked out to 34%.... That also means that my estimate of 26.2 FPE at 1500 psi with an 18.75" barrel is probably slightly too high....
The basic variables in the spreadsheet are caliber, pellet weight, valve dwell, reservoir pressure and volume, transfer area volume, barrel length, pellet drag and starting force, the gas used (air, nitrogen, helium, CO2), and an efficiency factor which usually runs between 55-70%.... The normal procedure is to input the known quantities, use default quantities for some of the minor ones if they aren't known, and then balance the dwell and efficiency until it matches what the gun is doing (or what is normal).... For a dump valve, the dwell is simply selected so that the valve is still open when the pellet leaves the barrel.... I had to use an efficiency factor of 67% to get your gun to deliver 17 FPE with the 0.13 CI valve, and that is not unreasonable for such a small valve.... The calculations are just straight F = MA, done every 0.000001 sec. as the pellet moves down the bore until it exits, correcting for the new pressure as the air expands behind the pellet.... It takes into account the weight of the gas behind the pellet accelerating as well.... The spreadsheet has had MANY revisions and refinements, and I feel it represents a VERY good model of what is going on inside a pneumatic airgun (CO2, pumper, or PCP)....
Bob
Man that's way worse than I thought I really did think .22 ci would get me at least 750 fps... well at least I know .22 will get me to my original goal of a 700 fps .25 with just Crosman parts...
I knew I was being optimistic but ouch that barrel likes to eat...
.3 ci that's about 33 pump strokes to to 1500 psi with a 2100 pump assuming about 65% efficiency for the pump.
39 or so pumps for a 66 pump... will not even post 13xx pump.
I am beginning to think that 619 fps at 19 pumps was a good compromise.
But
If you look back my original self intro thread my goal was,
"Dreaming of self contained .25 and/or custom up to .30 self contained pumper 20-25 pump max, 650- 700 fps with mostly commonly available parts... (crazy?)"
http://www.gatewaytoairguns.org/GTA/index.php?topic=59377.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=59377.0)
well at least I got that about right @ about .2 ci with a 2100 pump this is predicting about 700 fps in 25 pumps, currently it maxxes at 22 pumps and 630 fps. I just need to add a .07 ci gauge mount ;).
Do you actually think I am building 1500 psi with a old metal piston (with wiper ;D)and fitted pump cup?
I thought 1300 psi might be optimistic. ( the math says 1891 psi in 19 strokes @ 60% eff. pump, 2207 at 70%)
I do remember hoping for at least 1400 psi before maxxing the pump out.
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Part of the problem with pumpers is that the pumping losses increase as the pressure increases.... What starts out at 80% efficient ends up being 50% or less.... Add to that the loss of efficiency when the dump valve gets to be a large percentage of the barrel volume, and you can quickly see why a retained air system starts to look pretty good....
Bob
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@rsterne:
A question on 25 cal for my 2289's (on 10 pumps on full dump currently right around 14FPE, would have to look at my notes for what the one is doing an ACP [I think it is right around 10FPE for two shots with air left after second shot with 29 pumps from a completely empty valve]):
Considering the current performance with a .20ci valve, and 18 inch barrel in both:
What performance could I expect out of these as is converted to .25 cal, with an 18 inch barrel?
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If you are asking what changing the caliber ONLY will do, I can give you a rough idea.... I need to know the pellet weight and caliber currently and in .25 cal.... and the pressure.... From your post I will assume both have an 18" barrel and 0.20 CI valve.... The current FPE levels as a dump shot and for 2 shots MAY be enough information, but doing the dump shot shouldn't be too hard....
Are you REALLY getting 14 FPE on a 2289 pump with only 10 pumps?.... The best I did with an FTP, 14.5" barrel and 14.3 gr pellets was 541 fps (9.3 FPE).... That was quite a while ago, but 50% more than that is quite astounding, even with an 18" barrel....
Bob
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Currently using CPHP 14.3 grain pellets, in .22 cal.
Thank you.
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If you are asking what changing the caliber ONLY will do, I can give you a rough idea.... I need to know the pellet weight and caliber currently and in .25 cal.... and the pressure.... From your post I will assume both have an 18" barrel and 0.20 CI valve.... The current FPE levels as a dump shot and for 2 shots MAY be enough information, but doing the dump shot shouldn't be too hard....
Are you REALLY getting 14 FPE on a 2289 pump with only 10 pumps?.... The best I did with an FTP, 14.5" barrel and 14.3 gr pellets was 541 fps (9.3 FPE).... That was quite a while ago, but 50% more than that is quite astounding, even with an 18" barrel....
Bob
Yes I am realy getting 14FPE on 10 pumps with a 2289.
CURRENT:
18 inch .22 cal barrel.
0.20ci valve volume.
slightly bored and angled TP.
.22 cal.
And yes I would like to know about where it would be with just a change to .25 cal.
I do thank you much.
As to the current power I have nearly completely eleminated head space with a stuffed piston and:
- Teflon tape around the valve between O-Ring and face.
- Shaved about 4 thousandths off of nose.
- Shimmed pump cup (washer under cup, and O-Ring around outside).
- Stuck some eighth inch rod in check to mostely fill the inlet.
- Weakened valve spring by over compressing.
- Bored out transfer port to barrel, and barrel.
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DOUBLE, oops.
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and the pressure?.... If you are using 10 pumps on a 2289 pump, which has a swept volume of 18 cc, that is a total input of 180 cc.... If your valve is 0.20 CI = 3.3 cc, at 100% pump efficiency that works out to 180 x 14.7 / 3.3 = 802 psi.... At 70% efficiency (typical), that is only 561 psi.... At 802 psi, that requires an internal efficiency of 90% to achieve 14 FPE.... At 100% efficiency, the minimum pressure to deliver 14 FPE in an 18" barrel would be 718 psi.... That requires a pumping efficiency of 718 / 802 = 90%....
I repeat, are you SURE you are getting 14 FPE on 10 pumps with a 2289 pump?.... If so, you have achieved greater efficiency than anyone else has ever managed....
OK, read your response, and since you are SURE, then with a caliber change ONLY, and assuming a 20 gr. pellet (I asked but you didn't answer) you should get 610 fps (16.5 FPE).... I would strongly suggest you carefully document all that you did to achieve 90% efficiency with your 2289.... as you should be starting your own airgun company.... Obviously you don't need my help any more....
Bob
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and the pressure?.... If you are using 10 pumps on a 2289 pump, which has a swept volume of 18 cc, that is a total input of 180 cc.... If your valve is 0.20 CI = 3.3 cc, at 100% pump efficiency that works out to 180 x 14.7 / 3.3 = 802 psi.... At 70% efficiency (typical), that is only 561 psi.... At 802 psi, that requires an internal efficiency of 90% to achieve 14 FPE.... At 100% efficiency, the minimum pressure to deliver 14 FPE in an 18" barrel would be 718 psi.... That requires a pumping efficiency of 718 / 802 = 90%....
I repeat, are you SURE you are getting 14 FPE on 10 pumps with a 2289 pump?.... If so, you have achieved greater efficiency than anyone else has ever managed....
I guess I may have to re do the chrony. I thought I was sure. It is also possible that I made an error in calculating the valve volume (do not have the notes on that on hand), if that makes any difference.
OK, read your response, and since you are SURE, then with a caliber change ONLY, and assuming a 20 gr. pellet (I asked but you didn't answer) you should get 610 fps (16.5 FPE).... I would strongly suggest you carefully document all that you did to achieve 90% efficiency with your 2289.... as you should be starting your own airgun company.... Obviously you don't need my help any more....
Bob
Thank you for that, I did forget to mention a pellet mass, and 20 grain works well.
I will definitely have to run them over a chrony again as I do not think I am getting 90% efeciency, I did a lot of work on them after getting it to where everyone else has theres to attempt to get it a bit better, though I would have expected maybe 75% to 78% at best.
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I can see 80-85% eff at the pump as possable because of the lower operating pressure but 90% does seem a bit much. I know I had to figure my valve volume a couple times because of mistakes...
The first time I forgot to subtract the poppets volume ::) from the valve volume...
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Bob, I was very worn out last night and forgot to say thanks...
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Ok I have egg on my face. I just did a quick 3 shot string at 10 pumps, and:
1: 629FPS
2: 633FPS
3: 631FPS
Using CPHP 14.3 grain pellets.
So I just figured at 631FPS =12.6FPE.
Sorry to ask again though would you please check one more time for a what that would be in .25 with a 27.8 grain pellet? Sorry. Off to update my sig ??? .
And thank you very much.
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I can see 80-85% eff at the pump as possable because of the lower operating pressure but 90% does seem a bit much. I know I had to figure my valve volume a couple times because of mistakes...
The first time I forgot to subtract the poppets volume ::) from the valve volume...
yeah, I still am not sure if I have correctly calculated the volume, though I am sure that I have messed up with the chriny measurements.
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To get 12.6 FPE still requires 82% efficiency overall.... That could be 82% pumping efficiency and 100% in the gun (and I've never seen anything hit over 75%) or 100% in the pump and 82% in the gun.... As a comparison, my Uber-Carbine, which is a 2289 with a 2200 pump (50% more swept volume), an FTP with minimal headspace, and a bored out valve of 0.13 CI, with bored out and streamlined ports of 0.166" diameter and a lighter valve spring.... does 610 fps on 10 pumps (the equivalent of 15 pumps with a 2289 pump).... It has been my experience that really large valves only pay dividends when you pump enough to get quite a lot of pressure, not at 700-800 psi, so that means you are doing about 50% better than what is considered excellent performance....
Regardless, you asked for an estimate of that gun in .25 cal with a 27.8 gr. pellet.... Assuming your 82% overall efficiency level can be maintained, you should get 497 fps (15.2 FPE).... All I can say is "good luck"....
Bob
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I do thank you. And I am still double checking all of my testing. I actualy had a slightly reduced volume valve in my main 2289 for a long time, then decided to try a larger valve volume not to long ago (about a month), though I am now questioning all of the numbers that I have recently gotten, as there is definitely something that is not quite adding up.
And I do thank you as well for getting me to review my data. I would not have known about the errors if you had not.
Also thank you very much.
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.13 valve @1500 psi
0.22
Barrel Vol Barrel PSI
1 0.03799 1160.756
2 007599 946.657
3 0.11398 799.239
4 0.15198 691.548
5 0.18997 609.432
6 0.22796 544.748
7 0.26596 492.476
8 0.30395 449.358
9 0.34195 413.183
10 0.37994 382.398
11 0.41793 355.882
12 0.45593 332.805
13 0.49392 312.539
14 0.53192 294.599
15 0.56991 278.607
16 0.60790 264.262
17 0.64590 251.322
18 0.68389 239.590
.2 valve at 750 psi
0.22
Barrel Vol Barrel PSI
1 0.03799 630.268
2 0.07599 543.502
3 0.11398 477.734
4 0.15198 426.165
5 0.18997 384.645
6 0.22796 350.497
7 0.26596 321.917
8 0.30395 297.647
9 0.34195 276.780
10 0.37994 258.647
11 0.41793 242.744
12 0.45593 228.684
13 0.49392 216.163
14 0.53192 204.942
15 0.56991 194.828
16 0.60790 185.666
17 0.64590 177.326
18 0.68389 169.704
.2 valve @ 1000 psi
0.22
Barrel Vol Barrel PSI
1 0.03799 840.357
2 0.07599 724.669
3 0.11398 636.979
4 0.15198 568.221
5 0.18997 512.860
6 0.22796 467.329
7 0.26596 429.223
8 0.30395 396.863
9 0.34195 369.040
10 0.37994 344.863
11 0.41793 323.659
12 0.45593 304.912
13 0.49392 288.217
14 0.53192 273.255
15 0.56991 259.771
16 0.60790 247.554
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Those are barrel pressures as the pellet travels, right?.... If we compare a 0.13333 CI valve at 1500 psi with a 0.200 CI valve at 1000 psi, they both start with the same amount of air inside, and in theory it took the same number of pumps to put it there.... (Actually, since pump efficiency drops with pressure, it might take an extra pump to get to 1500 with the small valve, but you get the idea).... However, if you look at the pressure as the pellet moves down the barrel, the smaller valve at higher pressure gives the pellet a harder initial "kick", so the velocity would be greater.... I ran the numbers, assuming a 14.3 gr. pellet and 60% internal efficiency, and the .200 CI valve at 1000 psi produces 11 FPE, while the 0.13333 CI valve at 1500 psi produces 13.3 FPE.... This is why stuffing a valve works IF the larger valve isn't being filled to the pressure capacity of the pump....
Once you start working at a fixed pressure, however, the larger valve will produce higher velocities (but require more pumps).... as per the graph at the beginning of this thread.... If you want higher efficiency (more velocity per pump), use a smaller valve.... If you want more pure FPE, use a larger valve, but be prepared to pump more.... Even then, going past a valve volume of 1/2 the barrel volume isn't really worth it, as shown above....
BTW, the numbers in your chart is basically the math that Lloyd's spreadsheet uses.... You take those pressures, turn them into a force, and use that force to accelerate the weight of the pellet (and the air, once the valve closes) as the pellet moves along the barrel.... If has various corrections, but that is the basis of how it works.... It does one calculation for every millionth of a second instead of every inch of travel, however.... so several hundred calculations in the length of the barrel....
Bob
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Those are barrel pressures as the pellet travels, right?.... If we compare a 0.13333 CI valve at 1500 psi with a 0.200 CI valve at 1000 psi, they both start with the same amount of air inside, and in theory it took the same number of pumps to put it there.... (Actually, since pump efficiency drops with pressure, it might take an extra pump to get to 1500 with the small valve, but you get the idea).... However, if you look at the pressure as the pellet moves down the barrel, the smaller valve at higher pressure gives the pellet a harder initial "kick", so the velocity would be greater.... I ran the numbers, assuming a 14.3 gr. pellet and 60% internal efficiency, and the .200 CI valve at 1000 psi produces 11 FPE, while the 0.13333 CI valve at 1500 psi produces 13.3 FPE.... This is why stuffing a valve works IF the larger valve isn't being filled to the pressure capacity of the pump....
Once you start working at a fixed pressure, however, the larger valve will produce higher velocities (but require more pumps).... as per the graph at the beginning of this thread.... If you want higher efficiency (more velocity per pump), use a smaller valve.... If you want more pure FPE, use a larger valve, but be prepared to pump more.... Even then, going past a valve volume of 1/2 the barrel volume isn't really worth it, as shown above....
Bob
Interesting, this makes me wonder if I still have the volume reducing shim in my full dump valve. I guess I will have to open it up to find out, I do not know what the actual volume would be with the shim as I never took the time to figgure it out. Maybe my numbers are not quite as far off as I had thought?
I will look in that valve later today.
Though what volume would it take for a 2289 with 18 inch barrel to reach 12.6FPE on 10 pumps with aproximately 72 to 74% effeciency (I think it reasonable to assume that is the ball park of where I am)?
And thank you much.
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That still seems unreasonably high for FPE, as I was only getting 9.3 FPE with 10 pumps and a 14.5" barrel (then add about 1 FPE for an 18").... and that with stock valve volume (~0.09 CI).... If we assume 80% pumping efficiency, and 10 pumps moving 18 cc per pump, then you are putting ( 10 x 18 x 0.8 ) = 144 cc of air into ~ 1.48 cc, so call it 100 bar (1450 psi).... With an 18" barrel, to reach 12.6 FPE, you would need 73% internal efficiency, which is very high.... I was only achieving about 60% internal efficiency, and that with very careful flow and porting work....
The larger you make the valve above stock, the less likely you are to achieve over 600 fps on 10 pumps....
Bob
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That still seems unreasonably high for FPE, as I was only getting 9.3 FPE with 10 pumps and a 14.5" barrel (then add about 1 FPE for an 18").... and that with stock valve volume (~0.09 CI).... If we assume 80% pumping efficiency, and 10 pumps moving 18 cc per pump, then you are putting ( 10 x 18 x 0.8 ) = 144 cc of air into ~ 1.48 cc, so call it 100 bar (1450 psi).... With an 18" barrel, to reach 12.6 FPE, you would need 73% internal efficiency, which is very high.... I was only achieving about 60% internal efficiency, and that with very careful flow and porting work....
The larger you make the valve above stock, the less likely you are to achieve over 600 fps on 10 pumps....
Bob
I do thank you. And yes the shim is still in that valve (I am going to leave it for now until I can get better poower numbers). So it is actualy reduced volume as it sits.