Benjamin 392 power limit

[Tom Tucker]

No follow up shot - unless you tuned the gun to conserve air and used the tech to get two full power shots instead of one double-power shot.  I wonder how long it would take to heat the air in the valve with 50-100 watts?  The first shot would take the longest to prepare because first the housing itself would have to get up to operating temp.  There would also have to be some fins inside the valve to transfer heat to the air more rapidly. 

Sorry, Charles, I didn't mean to derail your thread - I am eagerly awaiting the next phase for your 392.  I think this is one platform that hasn't gotten as much attention as it deserves.

[Psipumper]

Works that way some times. I think  we have answered the question of max power. 38 to 42 fpe. Another question of interest would be the .177 caliber version and the hypothetical .25 caliber.
Would calculations  using the baseline efficiency of my gun at 40 fpe (2300psi and 5.75 cc) be usable for that?
My guess would be in .177 -1025@8 pumps with 3.4 cc valve.
In .25 caliber, I don’t know. They make a pellet the same weight (25.4)

[Hobbyman2007]

Great thread guys . I absolutely love my steroid pumper , too bad it only gets used as a truck gun for fall bird hunting . Pretty awesome that you got that kind of power from this platform . If you can get your hands on some 30gn BBT cast bullets they are worth a try . That’s all I feed mine with and I haven’t missed a bird with it yet . 

[rsterne]

Charles, it is of course easier to get FPE in .25 cal and harder in .177.... because of the difference in the base area of the pellet that the air pushes on.... In addition, you should use a smaller valve volume in .177 and larger in .25.... For a 50% valve volume, with a 19" barrel that works out to 3.8 cc in .177 cal, 5.9 cc in .22 cal and 7.6 cc in .25 cal.... A stock Benji valve, at 4.2 cc, is already large enough for a .177....

My .25 cal Disco Carbine, with a 20" MRod barrel (16 cc) and a stock Benji 392 pump linkage in a Disco tube, got 750 fps (32 FPE) with the 25.4 gr. Kings at 1500 psi with a 6.1 cc valve (pump and dump, 38% valve volume)…. That was the extent of my testing with a small valve, after that I changed to a 27cc valve using retained air for multiple shots.... At maximum hammer spring preload I hit 40 FPE at 1500 psi, but the valve was retaining enough air (1000 psi) for a 760 fps 2nd shot with 550 psi left after the 2 shots.... However, the velocity of the 1st shot appeared to be plateauing, so I don't think more hammer strike would have increased the velocity of the first shot any more, just wasted more air....

Bob

[Psipumper]

Charles, it is of course easier to get FPE in .25 cal and harder in .177.... because of the difference in the base area of the pellet that the air pushes on.... In addition, you should use a smaller valve volume in .177 and larger in .25.... For a 50% valve volume, with a 19" barrel that works out to 3.8 cc in .177 cal, 5.9 cc in .22 cal and 7.6 cc in .25 cal.... A stock Benji valve, at 4.2 cc, is already large enough for a .177....

My .25 cal Disco Carbine, with a 20" MRod barrel (16 cc) and a stock Benji 392 pump linkage in a Disco tube, got 750 fps (32 FPE) with the 25.4 gr. Kings at 1500 psi with a 6.1 cc valve (pump and dump, 38% valve volume)…. That was the extent of my testing with a small valve, after that I changed to a 27cc valve using retained air for multiple shots.... At maximum hammer spring preload I hit 40 FPE at 1500 psi, but the valve was retaining enough air (1000 psi) for a 760 fps 2nd shot with 550 psi left after the 2 shots.... However, the velocity of the 1st shot appeared to be plateauing, so I don't think more hammer strike would have increased the velocity of the first shot any more, just wasted more air....

Bob

Ok. A hot .177 or .20 caliber is no problem. The power ratio to a 392 would be the same as it has been in the platform. The .25  looks like it could have issues in a pumper. Just not enough pressure and volume  to do it right.

[rsterne]

A bit more barrel length, and a bit more pressure, will give satisfactory results in .25 cal.... My Millennium Pumper did a single shot of 50 FPE, 2 equal shots of 45 FPE, or three of 40 FPE, using retained air, a 25" barrel, 28.5 cc valve, and 1800 psi.... I don't know how large a valve you can get inside a 392, that will be one of the limiting factors, along with the pressure you can pump to without buckling the pump linkage....

Just increasing the caliber from .22 to a .25 cal 19" barrel should take that 39 FPE 392 with the 5.75 cc valve to 45 FPE (nearly 900 fps with the same weight 25.4 gr pellet) at 2300 psi.... If you can squeeze in a 7.6 cc (50%) valve, you should be tickling 50 FPE at the same pressure.... A lot more pumping to get there, of course....

Bob

[Tom Tucker]

Charles, you've done the 3 o-ring mod, right?  I wonder if you could drill out or cut away some of the center of the valve and use the tube walls to hold the air?  That would sure increase the volume.

[rsterne]

The guys on the Yellow Forum used to "Skeletonize" the 392 valves to do just that.... I don't know what volume they achieved, though.... Extreme version....



You must retain the check valve portion separately, obviously, in this version.... If you used a "captive O-ring" check valve, where the only moving part is the O-ring, held on a flat head screw with a groove in the shank, that is threaded into the air entry port, you could shorten the front housing to just behind the new O-ring groove....

Bob

[Psipumper]

I didn’t realize the velocity in .25 would improve that much. Looking at the available pellets weights to slow it back down to about 850 would add more fpe.
I would be happy at 5.75 cc .
I think the crosman custom shop should offer the barrel assembly. Model 395PA sounds good.

[rsterne]

Here is the captive O-ring check valve.... The idea came from Steve in NC on the Green Forum....



The flat head screw is threaded into the (tapped) air intake hole in the nose of the valve.... It has a small groove cut along one side of the threads for an air passage, up to where the head starts.... The O-ring is captive on the screw, and snug enough to just pull into the "V" formed by the underside of the flat head screw and the valve body.... When you pump, the air stretches the O-ring to allow air to enter the valve.... The O-ring then pulls back into the "V" to seal.... The O-ring is the only moving part.... no springs or retainers are required....

Bob

[rsterne]

Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....

Bob

[Tom Tucker]

Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....

Bob

I think the biggest limitation the 392 has is the width of the pump tube.  Sometimes I think it's TOO wide.  My 392 and 1300KT both do about 16 FPE - at 10 pumps in the 392 and 20 pumps in the 1300 - and it's actually easier to get there in the 1300, even though it's developing about 33% more pressure and has a shorter pump arm.

[Psipumper]

Crosman would not go out there for sure.
To me they should add .20 and .25 to the lineup with no internal changes. I think  the sales would be good.

Crosman would also have to make a 5.75 cc valve with big ports, however, and a linkage strong enough for 2300 psi.... Neither are going to happen, IMO.... That pumping effort is just too much, and too hard on the gun.... You might be OK with it, but the average guy, no way.... Don't forget, Crosman recommend only 8-10 pumps....

Bob

[rsterne]

What weight pellet for the 392 at 16 FPE at 10 pumps?.... Any mods, or is that dead stock?.... I can start from there and predict what a .25 would do....

Bob

[Psipumper]

What weight pellet for the 392 at 16 FPE at 10 pumps?.... Any mods, or is that dead stock?.... I can start from there and predict what a .25 would do....

Bob

Crosman rates the 392 with 14.3 g @ 685.
In reality they are 640 fps stock. 8 pumps maximum.

[Tom Tucker]

14.3 grain pellets.  The only modifications are a valve that is unscrewed by about 1/16" to bring it closer to the piston and to slightly increase volume, and a check that has been shortened by 3/16."  Volume is about 4.55cc.

Completely stock it did 12.9 FPE at 8 pumps.  With the modifications it does 14.9 FPE at 8 pumps and 16.3 FPE at 10 pumps.

[rsterne]

If a stock 392 with 14.3 gr. is 640 (13 FPE) at 8 pumps…. with no changes other than a .25 cal barrel, and using JSB Kings (25.4 gr.) I would expect about 540 fps (16.4 FPE)…. Personally, I think that is too slow for most guys, although it is still over the 500 fps limit in Canada.... 

If mild mods get you to 16 FPE at 10 pumps, that gun should hit about 600 fps with the 25.4 gr. in .25 cal (just over 20 FPE)…. IMO I'd sooner have a .22 cal shooting 15.9 gr. pellets at 680 fps.... Using the new Hades pellets it should perform as well as a .25 for hunting, with a far flatter trajectory.... I think the stock valve is just too small for a .25 cal.... JMO....

Do you use a larger CS O-ring when you unscrew the valve 1/16" ?.... I have an aluminum 392 valve here with a 0.102" CS O-ring, and when you screw the two halves together, you end up with about a 1/16" gap and can see the O-ring.... Is that how you did yours?.... What is the LOA of the valve, or the distance between the original 2 O-ring grooves?....

Bob

[Tom Tucker]

That's exactly it... a -113 o-ring in the center with some teflon tape on the threads for good measure.  There is little interference if any between the pump cup and the valve even with this adjustment.  It's amazing how much pump efficiency was left on the table in the stock form.  I guess Crosman/Benjamin really didn't want anyone to break the gun by over-pumping.

My valve is brass, but probably the same dimensions as the aluminum one you have.  I don't have any measurements, though.

[Rob M]

the best thing that Bob, James perotti and Phil riggs did with their pumper designs was getting away from the constraints of the 392 main tube and soldered barrel design..James was the designer of a 63 fpe 32 caliber pumper many many  moons ago.. Phil designed and still uses a 13.5 fpe SSP with 1.375 piston..Bob as we know used a disco platform ..
 back to the conversation i just read, leaving the valve as a solid unit and adding a 16gram ( perpindicular )  threaded co2 cart would also add tremendous volume with little effort.

[Rob M]

heres a link to some of James work.. an ssp in this case.

https://www.tapatalk.com/groups/thegreencrosmanforum/here-is-james-perotti-s-ssp-pictures-stunning-t44499.html

and the pumper

32 caliber pumper james by murphyrobert9, on Flickr