Author Topic: Longer plenum for Huma regulator for Mrod  (Read 1387 times)

airgunstocks

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Longer plenum for Huma regulator for Mrod
« on: April 21, 2017, 10:46:34 AM »
I made a longer plenum for the Huma regulator in my .22 Marauder.  The gun went from 30 fpe to 36 fpe.  For those with a lathe it's no big trick to copy the dimensions from the original plenum, but if you want it I could send a photo of the dimensioned sketch I used.  Probably not going to take the time to draw it in Solidworks; it's a simple piece of lathe turning.  I left out the gauge hole to make it easy.  One thing that will save you some effort is the thread for the regulator that you have to cut in the plenum is M22x1.0mm  I got a tap off eBay for about $20.



Michael McKeown
www.airgunstocks.com


  • Napa, CA

rsterne

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Re: Longer plenum for Huma regulator for Mrod
« Reply #1 on: April 21, 2017, 11:03:10 AM »
Nice job!.... Hard data is always good for the non-believers that plenum volume is important.... do you happen to have the inside volume of the two?....

Bob
  • Coalmont, BC

airgunstocks

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Re: Longer plenum for Huma regulator for Mrod
« Reply #2 on: April 21, 2017, 12:42:54 PM »
original plenum measured with water: 11.67 cm3
valve measured with water: 4.38 cm3
total original volume: 16 cm3
30 fpe = .53 cc/fpe

new plenum calculated volume: 24 cm3
plus valve: 4.38 cm3
total new volume: 28.38 cm3
36 fpe = .78 cc/fpe

.78 cc/fpe has this huge caveat: the gun needs to be tuned for the new power.  It calculates that it should do more.  I sized the plenum hoping for 41 fpe.  The plenum volume may not be ideal for a good e.s. which initial testing doesn't look all that great.  The first couple of shots seem to go out close to 40 fpe, but then settles down for the rest of the magazine's worth at about 36 fpe.  I don't know how many shots it'll provide.  If calculated based on the measured 40 fpe it does for the first couple of shots, assuming that a tune can be found where it sustains that performs over a magazine's worth, that would be .7cc/fpe.  If I can find 41 fpe, that would be .69 cc/fpe.

I also don't wonder that a slightly smaller plenum would be more efficient.  Hopefully more people out there will make some plenums and report back.  It probably only takes about an hour to turn it out of some round stock; if you have a lathe, it's a simple project.  I turned the o.d., cut the o-ring groove, drilled out the middle .6875" through (to clear the bottom of the o-ring groove), parted it off, turned it around, drilled for the tap .8125 up to a safe area before the o-ring groove, used a boring bar to cut the recess for the regulator's o-ring, and tapped the thread.  I didn't bother getting in past the threads with a boring bar and enlarging the i.d. like the original.  I sized it based on just the drilled hole.  And as I previously mentioned, I left the gauge hole off.  That makes the project much easier.

The regulator is set at about 1800 psi.  More might tighten up the e.s. and provide that last bit of power I was looking for.

The other numbers are .187 transfer ports, WAR's twin springs attached to a SteveNC Freeflight hammer design, but my own variation (his breaks the little aluminum screw extension so I'm using a longer screw without the extension).  I'm not using the WAR twin spring end cap (I don't think it fits my Gen 1 gun, but I wouldn't have used it anyway as the freeflight system doesn't need it...mainly I wanted the springs).  The valve is a WAR valve, but it only provided about 2 fpe more than my already drilled out stock valve, but 2 fpe is 2 fpe, so it's good.  The WAR valve has a lighter poppet spring than the original, and they open up both the transfer hole and the poppet hole, my original valve I only opened up the transfer hole.  The gun has a Marmot Militia hammer forged barrel on it.

I"m sure there's more performance to be found, but I'm going to set it aside for a while.  Gotta get back to work and stop playing :-)
Michael
  • Napa, CA

rsterne

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Re: Longer plenum for Huma regulator for Mrod
« Reply #3 on: April 21, 2017, 02:34:24 PM »
A larger plenum will actually move your velocity further from the plateau (assuming you are already somewhere on the knee, ie below the plateau).... This is because the average pressure during the shot will be higher, so with no other changes (same preload, etc.), the valve will actually close a bit sooner, reducing the dwell.... So that 36 FPE you are seeing is occurring with a bit less dwell than before.... so it should be easy to tweak a bit more power with just a small amount more preload....

The efficiency in FPE/CI should be higher with the larger plenum, but your shot count may be a bit less, because you have less air on the HP side of the regulator now (by 12 cc).... Nice job, anyways, and thanks for the data....  8)

Bob
  • Coalmont, BC