Author Topic: Plenum Size  (Read 1343 times)

sixshootertexan

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Plenum Size
« on: July 26, 2016, 02:34:48 PM »
I know I read that you what 1cc of air for every FPE you would like to achieve right?

So if I have a plenum with a diameter of .687 x 2.75" long.  3.1415 * .3435² *2.75 = 1.019ci or 16.69cc

Is that correct?

Thank Ricky
« Last Edit: July 26, 2016, 02:42:48 PM by sixshootertexan »


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rsterne

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Re: Plenum Size
« Reply #1 on: July 26, 2016, 02:41:35 PM »
That depends on if the plenum is 2.5" long (as in the text), or 2.75" long.... but you have done the calculation correctly....

You can get away with about 1/2 cc per FPE if you increase the setpoint about 10% to compensate.... I do not recommend less than that....

Bob
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sixshootertexan

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Re: Plenum Size
« Reply #2 on: July 26, 2016, 02:45:05 PM »
Corrected that typo.

Will caliber affect this?  Say .177 vs .30.
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rsterne

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Re: Plenum Size
« Reply #3 on: July 26, 2016, 02:55:54 PM »
Not really.... There is a slight effect from the efficiency of the PCP, a really efficient tune can get away with a bit smaller plenum.... but mostly it depends on the FPE required, because the amount of air you use depends on the power output.... Also, a higher setpoint pressure can get away with a slightly smaller plenum.... but the rule of thumb works for nearly anything.... There isn't much point in going larger than 1 cc per FPE (you don't lose, but the gains are small).... The power and efficiency drop off fast below 1/2 cc per FPE.... Between those sizes, you can compensate with about a 10% change in setpoint pressure....

Bob
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sixshootertexan

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Re: Plenum Size
« Reply #4 on: July 26, 2016, 03:43:24 PM »
Thank you Bob.
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sixshootertexan

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Re: Plenum Size
« Reply #5 on: June 13, 2018, 09:31:39 AM »
Bob I have another plenum size question. If you have a tethered rifle regulated at over 3000psi shooting 200FPE will you still need a 1cc /FPE plenum?
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rsterne

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Re: Plenum Size
« Reply #6 on: June 13, 2018, 02:23:33 PM »
That is what I would recommend.... If you go smaller, you will have to increase the tethering pressure to get the same FPE.... and your efficiency will drop.... My latest two PCP project guns are actually larger than 1 cc per FPE.... the 6mm is 150 cc, regulated at 2800 and producing 120-130 FPE.... and the .257 which is unregulated has a 300 cc reservoir, but when I tether it at 3800 psi it shoots just shy of 200 FPE, and I can get a few shots off tether with it....

Bob
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sixshootertexan

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Re: Plenum Size
« Reply #7 on: June 13, 2018, 05:09:46 PM »
If I do this project I guess I'll get one of those China CF bottles for it.
CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177