Benjamin 392 power limit

[Psipumper]

I wonder what the maximum power a 392 could possibly make at 14 pumps.
I have reduced the volume of my valve to 4.9cc and 14 pumps is 2100 psi in the valve. I get 954 fps with the crosman pellets and 766 with the JSB monsters.
So about 33 fpe.
My thinking as of late is the 392 should not be pumped over 14 . Just the principle of it. It’s a 392.
That being said, I did get 839fps with the monsters  before reducing valve cc size.

[35 shooter]

Wow, 33 fpe from a 392 is awesome... i’m Amazed at what some of you guys accomplish with my favorite air rifle!

Is your accuracy the same as stock at that level, or do the groups start opening up compared to stock?

I’m asking because i’m considering having my 392 steroided, or attempt some power modding myself.

[Psipumper]

Wow, 33 fpe from a 392 is awesome... i’m Amazed at what some of you guys accomplish with my favorite air rifle!

Is your accuracy the same as stock at that level, or do the groups start opening up compared to stock?

I’m asking because i’m considering having my 392 steroided, or attempt some power modding myself.

The accuracy is the same. I shoot the 18g Jsb pellets in this one at about 800 fps. Just put it back together but It would take about 12 pumps to get that velocity.
I bought a second 392 and it seemed like a red Ryder out of the box compared to this one.

[Psipumper]

The second 392 I bought is my favorite. It has a smaller 3.4cc valve. 8 pumps has it wound up pretty tight. I get 830 fps with the crosman 14.3 pellets. I use the 15 grain jsb in it. Haven’t even checked their velocity. Very accurate. I have ordered a tin of the new Hades pellets for it. If they are the same accuracy it should be a good combination. This gun has two .5 cc spacers that could be removed to increase valve volume to 4.4cc. This would increase the number of pumps and top power. I haven’t tried it or want to.
This gun is noticeably quieter and more efficient.
High power gun is named “K1”
Lower power gun is named “K2”
K1= 766@ 8 pumps
K2= 830@ 8 pumps

[35 shooter]

Awesome performance!

[RBQChicken]

What's the pumping effort?  Seems like you'd need hemorrhoid producing effort to get that kind of fpe from a pumper

[Psipumper]

What's the pumping effort?  Seems like you'd need hemorrhoid producing effort to get that kind of fpe from a pumper

The last few pumps are heavyweight. Kinda like the old Star Trek phaser; you don’t set it on kill that often. Usually a 10 to 12 pump permanent stun.

[Psipumper]

Testing  the gun at higher pressures with 5.75cc valve volume . I was able to get right at 40 fpe with the heaviest pellets I have. Probably would be 42 fpe with 32 grain pellets. The valve at 2400 psi would start to retain air here losing power. A balanced type valve would allow more pressure and be faster off the seat. If someone wanted to, I think 50 fpe is in reach. Is it practical or would the gun last?
No. It would just be bragging rights.

[Rob M]

another way to skin the cat ( from a mechanical standpoint )
sleeve the inner tube down to lets say .625 ( like a 1377 piston _
you would be able to pump over 2800 psi no issue , the effort would be less than it is now on pump 14.. The downside is youd have to complete about double the number  of strokes.. Elongating the pump handle would be number one priority were i going for max power on a 39x.

[Rob M]

aside from that crazy idea. your numbers are the best i can remember seeing on a 39x platform/

[Psipumper]

aside from that crazy idea. your numbers are the best i can remember seeing on a 39x platform/

Thanks Rob. That is one reason I backed the gun down to 14 pumps. To make apples to apples comparison.

[Tom Tucker]

Fantastic work!

I tend to favor the bigger valves.  To me the power limit of a pumper is the amount of power it can make at the safest pressure that it can take over and over again without significant wear.  "Significant wear" is subjective but to me if I can go through a few tins of pellets without my piston and valve moving away from each other detectably, I'm pumping to a safe pressure.  So I prefer to increase the valve volume quite a bit and then fill it to whatever I consider max pressure.  Number of pumps doesn't really matter because they become easier to do, as long as I don't run out of fingers and toes to count on.

Of course, it's hard to talk about valve volume without considering barrel volume.  I think that if someone were to de-solder the stock barrel and replace it with a 26-inch maximus barrel, or something .25 cal, they could really see some high muzzle energy.

[Psipumper]

I plan on trying to convert to a blow open balanced valve for this gun. Not solely to increase power but to improve accuracy as well. A lighter hammer and spring should have less jump on release and a smoother trigger.

[Rob M]

I plan on trying to convert to a blow open balanced valve for this gun. Not solely to increase power but to improve accuracy as well. A lighter hammer and spring should have less jump on release and a smoother trigger.

thats brilliant  and makes perfect sense.. part of the reason the 880 daisy does so much with soo little is the blow open valve design..

[RBQChicken]

Wouldn't you have a too-heavy trigger with a blow-open valve and that kind of pressure?

[Psipumper]

This  type of balanced valve is not like the Daisy 880. The rear of the poppet is vented to atmospheric pressure. If I can design it keeping the valve volume the same size is my goal. Not sure what the bias size of the balance chamber to throat diameter should be. Maybe 75% to 80% if possible.

[Rob M]

This  type of balanced valve is not like the Daisy 880. The rear of the poppet is vented to atmospheric pressure. If I can design it keeping the valve volume the same size is my goal. Not sure what the bias size of the balance chamber to throat diameter should be. Maybe 75% to 80% if possible.

ok .. a balanced valve..when u said blow open i was thinking SSP or 880 style ( literally blow open )A balanced design would be awesome .. There was a thread where i think wyoman or gipetto ( no it was tackdriver ) made a balanced valve for his 1377 .. It was the simplified version where the stem literally exhausts back to the hammer area. I would think for this application a small pen spring creating the differential would work.. In other words the atmospheric front chamber could house a small spring, Since efficiency will not be an issue when going for full exhaust  of the valve.

[Rob M]

to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.

[Psipumper]

to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.

If you were to remove the spring, yes it would remain open. Since the rear of the poppet vents to atmosphere instead of the pressurized throat during firing, the poppet is forced to the rear or blown open when it leaves the seat.
A sufficient valve spring still has to be used to seal the poppet on the first pump as usual. My concern would be if 75% bias would apply enough additional seal pressure on the poppet seat at 2100 psi.

[Rob M]

to clarify.. there is no remaining force to close the valve after the shot.. SO a return spring would be needed for the next shot.. You could have an effective balance of 100% - the spring force , which generally might be 5 lbs tops.. this way youre reset for the next shot.

If you were to remove the spring, yes it would remain open. Since the rear of the poppet vents to atmosphere instead of the pressurized throat during firing, the poppet is forced to the rear or blown open when it leaves the seat.
A sufficient valve spring still has to be used to seal the poppet on the first pump as usual. My concern would be if 75% bias would apply enough additional seal pressure on the poppet seat at 2100 psi.

i see what you mean.. I think bob said 67% was always a safe bet.. and yes unlike a pcp this has to build from 1 pump, nothing will work if it wont seat during the first pump.. I CHEATED  a few years ago on my second 50 caliber co2 rifle. I oringed the seat and made a huge poppet, this way almost no sealing force was required.. Granted it was an odball build but keep in mind if the valve is balanced the poppet diameter in not important ( so long as the balancing is correct