My Benjamin 392

[rsterne]

Charles, that is certainly a good alternative, I will consider it.... I have used the Hobby tubing to bush up Disco valves from the 5/32" hole to 1/8" when fitting an MRod poppet, and it works well.... If using your method, I would press fit a solid rod into the 0.236" hole in the back of the valve, and then drill it afterwards in the lathe for the stem.... If it pressed in crooked, it wouldn't matter that way....

Do you really think you have to undercut the 1/8" stem in a 0.236" throat for a .22 cal valve?.... Maybe if building for slugs, and maximum FPE, but for pellets?....

Tom, I went back and read those threads about the skeletonized valve by James Perotti…. I also tried to make sense of the part about making 37 FPE with stock barrel length at 12 pumps....  …. I put those numbers into my "lofty goal" spreadsheet (for a reality check), and it spit out a minimum pressure of over 1200 psi, assuming .22 cal.... Then further down the page, Steve in NC stated that was with a .32 cal barrel.... and it all made sense.... 

Bob

[Psipumper]

The stem guide is completely finished before pressing into valve. The angle would have be machined after the .125 hole is drilled  due to drill deflection. The angle shape reduces dead air space in the throat.
The undercut on the stem probably doesn’t make a difference. I didn’t see any gain going from .156 to .093 stem diameter. I expected some magic but didn’t see it. Later on when I went to higher power maybe it helped.

[rsterne]

I was thinking of drill the stem hole from the back of the valve, then the angled face inside won't affect it....

On my Disco builds, I noticed a difference going down from 0.156" on the stem to 0.125", but no difference going smaller, with a 0.236" throat, in .22 cal.... If you are trying to flow more air, for a larger caliber, then with a 1/8" stem you may need to go over 1/4" on the throat, of course.... I did build one gun with a 3/32" stem, so that I didn't have to go as large on the throat and poppet, and hence hammer strike.... It seemed to work fine....

Bob

[Psipumper]

I was thinking of drill the stem hole from the back of the valve, then the angled face inside won't affect it....

On my Disco builds, I noticed a difference going down from 0.156" on the stem to 0.125", but no difference going smaller, with a 0.236" throat, in .22 cal.... If you are trying to flow more air, for a larger caliber, then with a 1/8" stem you may need to go over 1/4" on the throat, of course.... I did build one gun with a 3/32" stem, so that I didn't have to go as large on the throat and poppet, and hence hammer strike.... It seemed to work fine....

Bob

You can try doing it that way but with the tight tolerance , the hole will have a joggle as the drill breaks through the angle. Maybe a small file could fix.
I pressed mine in with a machinist precision vice and it worked out well.
Like I said in that previous post , the angle section has to be a slip fit or it will deflect since it engages half the hole. The rest was .003 press.
It does need to be very accurate to squarely seal at the seat.

Charles

[Tom Tucker]

Tom, I went back and read those threads about the skeletonized valve by James Perotti…. I also tried to make sense of the part about making 37 FPE with stock barrel length at 12 pumps....  …. I put those numbers into my "lofty goal" spreadsheet (for a reality check), and it spit out a minimum pressure of over 1200 psi, assuming .22 cal.... Then further down the page, Steve in NC stated that was with a .32 cal barrel.... and it all made sense.... 

Bob

I don't think it's possible to get that kind of power with 12 pumps and a 14.8cc valve, even in .32 cal.  There just wouldn't be enough pressure when you consider the swept volume.  Maybe it is possible that this was accomplished with a certain number of pre-charge pumps and then 12 re-charge pumps per shot?

I have a rough spreadsheet that I use for predicting power levels in my pumpers and CO2 guns.  It's not perfect, but I've relied on it for some time now, and I've based the calcs on information that I've learned here.  I get the following numbers for a 392:

Stock 392:  4.2cc valve volume, 14.3 grain projectile, 19.5 inch .22 cal barrel

800 PSI:  13.75 FPE @ 50% efficiency

Skeletonized 392:  14.8cc valve volume, 14.3 grain

400 PSI:  7.07 FPE @ 50% efficiency
600 PSI:  10.86 FPE @ 50% efficiency
800 PSI:  14.31 FPE @ 50% efficiency

In .32 cal with a 50 grain projectile, those numbers rise to:

400 PSI:  14.14 FPE @ 50% efficiency
600 PSI:  22.46 FPE @ 50% efficiency
800 PSI:  30.53 FPE @ 50% efficiency

It doesn't hit 37 FPE until 965 PSI.

[rsterne]

Sounds about right.... My point was that the 37 FPE wasn't in .22 cal, which was "kind of" implied.... I'm not even sure that it was done with the 14.8 cc skeletonized valve, to be truthful.... Nor do I know (yet) how pump strokes relate to pressure in a 392, although my .25 cal Carbine had only slightly less swept volume (0.745" ID instead of 0.777", same stroke), and the valve volume was 6.1 cc.... The chart for that is below....



As you can see, at 10 pumps I got 1020 psi, and at 12 pumps I got 1170 psi.... Mind you, that was with an F-T-P....

Bob

[rsterne]

Here are a couple of "calculated" charts for the 392 (4.2 cc valve) that I looked at when working on my .25 Carbine pumper....



I wonder how they stack up against what one actually does?.... Do you have that data for a stock 392?....

Bob

[Tom Tucker]

Sounds about right.... My point was that the 37 FPE wasn't in .22 cal, which was "kind of" implied.... I'm not even sure that it was done with the 14.8 cc skeletonized valve, to be truthful.... nor do I know (yet) how pump strokes relate to pressure, although my .25 cal Carbine had only slightly less swept volume (0.745" ID instead of 0.777", same stroke), and the valve volume was 6.1 cc.... The chart for that is below....



As you can see, at 10 pumps I got 1020 psi, and at 12 pumps I got 1170 psi....

Bob

Ahhhhh... I went back to the thread, and I see what you're talking about.  Apparently the 37 FPE at 12 pumps was accomplished with a Sheridan that was converted to .32 cal. and had a teflon piston seal.

Post #4 "converted Sheridan, uses the stock piston dia., stock stroke lenght, stock barrel lenght, stock valve volume, skinny probe, and porting, and makes 37fpe. on 12 pumps from empty. james"

Post #6 "James was referring to his full-custom .32 caliber, steel barrel-and-tube, teflon-piston Benji look-alike. Typical projectile weight (ball and handmade pellets) is 40 to 45.5gr."

That's the "fun" of researching old threads.  There is great info to be found, but parts of the conversation are often missing, implied context is often lost, and images are broken more than half of the time. 

So, looking at your chart for the .25 Carbine...  that's about 55cc swept volume, right? 

If so, the pressure seems to correlate with your 13XX flat-top piston testing.  This gun has 3.7 times the swept volume and 4.2 times the valve volume of a 13XX, so it should be building pressure at about 88% of the rate.  I recall you reaching 1100PSI on 10 pumps in a 1322, so this is right in the ballpark, perhaps a hair more efficient.

[Tom Tucker]

Here are a couple of "calculated" charts for the 392 (4.2 cc valve) that I looked at when working on my .25 Carbine pumper....



I wonder how they stack up against what one actually does?.... Do you have that data for a stock 392?....

Bob

Charles "psipumper" put a gauge on a stock 392.  I would have to dig up the thread, but as I recall, it hit 800PSI on 8 pumps and 1000 PSI on 12 pumps...

Here's the thread (2nd reply), 800PSI on 8 pumps, 1200PSI on 16 pumps

https://www.gatewaytoairguns.org/GTA/index.php?topic=133027.0

[rsterne]

IIRC, the swept volume on my .25 cal Carbine was 54.5 cc (7.625" stroke)….

If your memory is correct, Charles' results on a stock 392 would indicate about 70% efficiency with 0.5 cc headspace.... At least that is the curve that most closely matches.... 

Bob

[Tom Tucker]

IIRC, the swept volume on my .25 cal Carbine was 54.5 cc (7.625" stroke)….

If your memory is correct, Charles' results on a stock 392 would indicate about 70% efficiency with 0.5 cc headspace at 8 pumps.... but a bit less than where that curve lies at 12 pumps.... Likely the cup is compressing and the headspace is increasing more after 8 pumps?....

Bob

I corrected my post so the data is actually at 8 pumps and 16 pumps, not 8 and 12, but yes, at 16 pumps it matches your chart for 70%/0.5cc at 15 pumps.  The pump cup must be compressing a bit.

[rsterne]

Yeah, I corrected my post before you added your comments.... *LOL*…. Pretty close agreement anyways.... and a baseline to know what I'm doing.... Assuming my stock gun was the same, it was retaining air at about 700 psi... but with the 2-spring mod and lighter valve spring I have increased that by 50% to about 1050 psi....

Bob

[rsterne]

By, it's been a long time since I looked at those calculated charts for the 392.... It REALLY shows the importance of minimizing headspace in reducing the number of pumps to reach a given pressure.... 

Bob

[Tom Tucker]

By, it's been a long time since I looked at those calculated charts for the 392.... It REALLY shows the importance of minimizing headspace in reducing the number of pumps to reach a given pressure.... 

Bob

Yes, it sure is.  There is a ton of "wasted" effort in a pumper, but minimizing headspace and moving away from a single-shot dump valve can go a long way towards improving on that.  I think the ultimate design goal of most modified pumpers is to maximize energy output from the gun as a percentage of the total pumping effort from the shooter.  A secondary goal is probably to maximize energy output from the gun as a percentage of the PEAK pumping effort from the shooter on the final pump.  A third goal, for a more radical build, would be to also increase the safe maximum operating pressure of the gun...

[Psipumper]

By, it's been a long time since I looked at those calculated charts for the 392.... It REALLY shows the importance of minimizing headspace in reducing the number of pumps to reach a given pressure.... 

Bob

I see the charts and with the two variables being the headspace and pump efficiency. I don’t understand how pump efficiency is calculated. Also headspace is impossible to calculate exactly. The cone shape cup appears to do a good job looking at my latest data from my 392.
I measured the cc of the valve at 4.9 cc.
8 pumps =1200.  14 pumps = 2200
If the chart showed .1cc at 100 percent pump efficiency it may fit the chart.
If the valve size were 4.2 , I think the pressure would be close to 1500 @8 pumps.
I checked the velocity with 14.3 CPHP pellets from 4 to 14 pumps:
4=595     6=708      8=796      10=862
12=915    14=963
Even at the best condition, it will take a lot of pumping to fill a extra large valve to 1500 psi. Assuming it takes 8 to fill a 4.2cc valve.
I also found in my notes that a 5.75cc valve took 16 pumps to reach 1800 psi.
To reach the power level you desire, I don’t know how big the valve should be . With the check valve using the captive  oring , maybe you can get 8cc without skeleton? I would not want to pump much more than that. Should be close to the power you want. I hope you can use some of this to estimate what you can expect.

Charles

[rsterne]

Charles, the pumping efficiency is the pressure increase as a percentage of what it "should" be if all of the air in the swept volume is compressed into the volume of the valve plus headspace (ie 100%)…. If you have 8 pumps of a 58 cc swept volume, and no headspace, filling a 4.2 cc valve, the pressure should be (8 x 58) / 4.2 = 464 / 4.2 = 110 bar = 1602 psi.... At 100% efficiency, with no headspace, the pressure vs. pumps graph would be a straight line....

At 70% efficiency, that would drop to 0.7 x 1602 = 1121 psi.... If you look at the lower graph, for 70% efficiency with a 0.1 cc headspace, you will see it is just under 1100 psi.... As you introduce headspace, you have to calculate the pressure at the end of each pump stroke, which is why the lines become lower, and more curved, as the headspace increases....

As you can see from the chart on my .25 cal Carbine, I reached 1480 psi at 16 pumps.... That was with a 54.5 cc swept volume (0.745" ID x 7.625" stroke) filling a 6.1 cc valve, using a flat-topped piston.... That is not far off what you got, filling a 5.75 cc valve to 1800 psi with the larger pump swept volume of a 392.... probably about a 1 pump advantage to your setup, but with much higher effort (838 lbf. on the piston vs 645 lbf.)....   

I may develop 2 versions of this project gun.... or at least try one and the progress to the other, since I have 2 valves to experiment on.... I think ultimately I would like to have the largest possible valve volume, using a 2-piece valve, and after finding out what the FPE is with a dump shot, tune it for 2 equal shots, likely with the 18.1 gr. JSB Heavy.... Since the barrel volume is 11.8 cc, using more than half that amount of air per shot is very wasteful of pumping effort.... Who knows, I might be able to get 3 shots?.... 

Bob

[Psipumper]

Bob, thanks for the explanation and formula. I checked my numbers to the formula and got about 90 percent efficiency at 8 and 14 pumps with the 4.9 cc valve. Since the 90 percent was constant at the higher pressure as well, I think this would indicate a small headspace. Am I right to think the 10 percent pumping loss would be caused by the cup not sealing quickly at the top of the pump stroke ? Oil or a faster speed during pumping may improve ?

This is some good stuff to know. I can now calculate the pressure in my other 392 with the 3.4 cc valve that doesn’t have the pressure gauge.

Charles

[Psipumper]

Thanks again. I hope I haven’t gone off topic too much. After all the title is “My 392”. I will be following your project.

[rsterne]

I don't know what really affects pumping efficiency.... Obviously things like flexing in the linkage (or anything else) are bad, but that may mostly be because it increases the headspace, particularly at higher pressures.... Possibly the force required to open the check valve even affects it?.... Adiabatic heating during pumping, and then the loss in pressure when the charge cools again are probably a contributor as well.... Most likely the two things (headspace and efficiency) are inextricably linked through sorcery and black magic.... 

One thing you may, or may not have noticed.... On my Uber-Carbine, which is tuned for 2 equal shots, that occurs at 19 pumps if I am shooting continually during a testing session.... However if I pump 19 times and then let the gun cool back to room temperature, the first shot is faster than the second.... I have to add another pump to get the pressure back up to what it needs to be to get 2 equal shots.... 

Please, continue your comments and input.... I will undoubtedly learn from your experience on 392s, no worries if the thread gets sidetracked a bit along the way.... 

Bob

[rsterne]

Work progresses slowly on my 392.... I wanted to fit an MRod poppet, which has a smaller stem, 1/8" instead of 5/32".... At the same time, I decided to enlarge the valve throat from 7/32" (0.219") to 15/64" (0.234")…. This will increase the throat area from the equivalent of a 0.155" hole to that of a 0.198" hole.... which is a 63% increase in area.... I also wanted to enlarge and angle the exhaust port to ease the flow from the throat into the exhaust port.... Part of the plan to install the MRod poppet was to bush the hole in the back of the valve with K&S brass hobby tubing, but Charles (Psipumper) suggested that I drill the back of the valve straight through at 0.234" and then press in an insert which was drilled 1/8" for the stem of the poppet.... By grinding an angle on the front of the insert, it would fill some of the vacant space from the original vertical exhaust port, and the volume behind that in the throat....

Charles suggested that I drill the hole for the stem first, to insure it is straight and square to the valve seat.... but since I have a lathe, and I have made lots of my own valves, I figured I could drill the hole afterwards from the back.... First, however, I decided to drill and tap holes in both sides for 8-32 low profile SHCSs to better locate the valve in the tube.... I was concerned about the single valve screw, not so much for the strength of the screw itself, but the load on the thin brass tube.... I calculated the safety margin on the support given by the tube wall to that screw, which was the only thing keeping the valve from sliding back.... To my surprise, I found that at 1200 psi it was only 1.02:1.... 

That means that if the pressure exceeds 1200 psi, you can expect the screw to start deforming the brass tube on the back of the hole.... IF that happened, the valve would move back, and the exhaust port would no longer line up with the port in the receiver.... Since it is always nice to have a 3:1 safety margin, I decided to add a screw to each side of the valve, set down against a flat spot milled in the valve body.... Once I drill holes in the sides of the brass tube, I will triple the ability of the brass tube to prevent the valve moving back.... The holes are in the same plane as the port and existing valve screw, and the head is slightly smaller than the countersink for the valve screw.... so they won't interfere with the installation of another O-ring ahead of the screws, in case I want to skeletonize the valve....

I set the valve up in a square indexing block (against an end stop) with a 5C collet, using a #20 drill to line up the exhaust port with the headstock.... Then by simply turning the block I could drill the side screw holes in the same plane and at 90 deg. to it and the existing valve screw hole.... Once those were drilled and tapped, and the flats for the heads milled, I turned the block so that the exhaust port was lined up again with the headstock.... I then rotated the milling attachment 20 deg. and plunged a 5/32" mill through at the location of the existing exhaust port.... I examined the inside to confirm that there was lots of room, changed the angle to 25 deg. and milled the front of the angled port.... I then raised and lowered the valve 0.012" to widen the exhaust port and make the hole where it meets the receiver round.... It ended up at 3/16"....

I removed the valve from the collet block, and chucked it in the lathe, with the exhaust port lined up with the chuck key hole.... That means I can put it back into the 3-jaw chuck in the same orientation each time.... I used a 3/16" stub drill and drilled along the hole for the stem, from the back of the valve.... I then went to 7/32", which was the size of the existing throat (it lined up perfectly), then a #1 drill (0.228") and finally the 15/64" drill to finish it to the size of the new throat.... I then made an insert out of 6061-T6 aluminum, about 0.002" over, ground an angle on the front (to match the exhaust port) and heated up the valve body to press in the insert.... Long story short, it didn't work, and stuck about halfway in.... I had to cut it off, machine it off flat with the back of the valve, and then drill it out again.... There was no damage to the valve, so I made a new insert, the same size as the hole in the back of the valve.... polished it a thou smaller and then pressed it in with Loctite 638.... It ain't going anywhere.... 

I then used a small spherical burr in my Dremel, and working through the exhaust port, smoothed out the front of the insert and faired the exhaust port into the throat.... Now, when you look into the exhaust, you can see part way through the throat of the valve, and the passages are nice and smooth.... An 11/64" (0.172" drill) wobbles around loose in the exhaust port, so it is larger than that.... The last step was to drill the 1/8" hole for the stem of the MRod poppet.... Here is the finished back half of the valve, along with the MRod poppet and spring....



The insert in the back of the valve covers the 2 side screw holes, and the original lower valve screw hole, sealing them off.... They are only under pressure during a shot, so no danger of the valve leaking down anyways.... One added bonus of the insert is that it supports a greater length of the valve stem, so it doesn't wobble around like the stock one did.... I haven't drilled the holes in the tube for the side screws yet, nor machined the extra O-ring groove.... but I think I will test the gun at this stage.... I have not yet drilled out the receiver/barrel port, nor turned down the bolt probe, so I will be able to see what difference that makes, if any, after testing the new valve port and poppet.... Incidently, I think there is room to angle the exhaust port 30 deg. instead of 25.... but it might get pretty close to the valve seat.... BTW, the MRod poppet seems to seal up perfectly, just blowing in the front of the valve it has no leak detectable....

Bob