My Benjamin 392

[rsterne]

I have never had a Benjamin 392 pumper before, and Eric at Scopes and Ammo was clearing them out last fall, so I had my wife pick one up for me for Christmas, and I opened it today.... She banished me to the shop to stay out of her way for a couple of hours this afternoon, so I did some baseline work to see what I am starting with.... As you can imagine I don't plan to leave it stock....  …. Here is the velocity at each number of pumps, using the new 15.9 gr. JSB Hades pellets, which I thought might be a good weight for it....



What really surprised me is that it retained a tiny puff of air at 7 pumps, and enough at 8 pumps to deliver a shot of over 200 fps.... By the time I had pumped it 10 times, the second shot was over 450 fps....  …. This means that at 8 pumps and over, some extra power is lurking inside.... I know the valve spring is pretty stiff, it may only take replacing that with a weaker one to find that lurking FPE....

Next I used 8 pumps and tried a wide variety of pellet weights, from the 11.9 gr. RWS Hobby all the way up to the 34 gr. JSB Beast.... I shot 2 shots and recorded the average, and between every shot I cocked the gun and fired it to dump all the air.... The "puff" of air of the 2nd shot was minimal with the lightest pellets, but increased with pellet weight, as you might expect.... The fact it is retaining air indicates insufficient dwell to dump it all before the pellet reaches the muzzle, and heavier pellets, going slower, haven't moved as far from the breech, so the valve should have released less air (and therefore retained more)…. Here is a plot of velocity and energy vs. the weight of the pellets....



The behavior was much as expected, with the heavier pellets having more FPE.... What was interesting was that the rate of increase of FPE changed drastically at a pellet weight of about 19-20 gr.... Below that, you gain 2 FPE (17%) by increasing the pellet weight from 11.9 gr. to 18.1.... but above that the gain is less than that (1.8 FPE = 13%) at 34 gr.... You are still gaining energy with heavier pellets, but considering that the velocity is dropping below 600 fps, IMO there are two reasons not to use pellets heavier than about 20 grains.... BTW, I didn't have a pellet between 25.4 and 34 gr., so I used a 29.6 gr. BBT slug, as cast, and it fit right into the data.... This shows that a slug can deliver velocity and energy exactly the same as what you would expect from a pellet of the same weight.... 

Anyways, this is a good baseline to have so that I will know what is happening when I change things.... I think the first change will be an O-ring spacer inside the valve to reduce the headspace, and likely the 2-spring mod. to reduce the pumping effort, or simply a lighter valve spring....

Bob

[Techie]

and likely the 2-spring mod. to reduce the pumping effort

Haven't heard of this mod, and too much pumping effort is my main complaint with my Steroid 392 with billet lever from Mac1, plus with the wood stock 397 I just bought. 

Very interested in watching what you do to this gun.  I think retaining air at the number of pumps you mention is unusual in these guns.  Very curious as to what you discover will solve that.  I can pump my Steroid 392 16 times and still not retain air, but it has Timmy Mac's mods (whatever they are), plus I am at high (6300 ft) elevation.    I had that problem with an old Benji 312 and increasing hammer spring did not solve it.  Duy has it now and totally redesigned the internals.

[RBQChicken]

I am VERY much looking forward to this thread. Thanks!

[melvin]

Subscribed. Excited to see where this goes.

[Hoosier Daddy]

Subscribed. Excited to see where this goes.

Ditto!

Can't believe it took you this long, but something tells me this going to be great.

[superchikn]

I have my first 39x and am looking forward to this as well.

[Hobbyman2007]

Merry Christmas Bob . I will be following this one, I’ve been contemplating adding a small plenum like the millennium pumper for a while. The rifle I have has had the full steroid tune done prior to me getting it and it tops out at near 30 FPE ( from what I remember) I really like the light weight of this gun . Have you by chance checked to see if it’s an accurate shooter? I’m sure you already know about the overspray in the barrel.

[Mole2017]

I am surprised you've never had a 392 before, but you sure got to work on getting to know it. Looking forward to what comes next!

[darkcharisma]

Mike, 16 pumps and no air retained sounds like a dream mod from Timmy Macs. I am interested in seeing whats done inside.

  i still have air retained at 10 pumps but the second shot only net 200 fps from a 14.3grains while the first pump goes 640fps. ill get some more time to play after the holidays are over

[superchikn]

Mike, 16 pumps and no air retained sounds like a dream mod from Timmy Macs. I am interested in seeing whats done inside.

  i still have air retained at 10 pumps but the second shot only net 200 fps from a 14.3grains while the first pump goes 640fps. ill get some more time to play after the holidays are over

Hmmm, for the life of me, other than for bragging rights i cannot figure out why I would want to put so many pumps into a gun for a single shot.  Unless I needed to.
My Freedom pumps much more easily and is a repeater.
My 880 pumps much more easily too.

[rsterne]

Generally, increasing the hammer spring will cure the retained air.... but lightening the valve spring can help as well.... The 2-spring mod replaces the long, stiff valve spring with two, shorter springs and a washer to separate them, that sits on the shoulder down inside the front half of the valve.... The front spring is much lighter, as all it needs to do is hold the check valve closed for the first pump....

Think about it this way (numbers for example only)…. If the check valve seat is 0.25" in diameter, and the valve spring is pushing on that check valve with a force of 10 lbs.... it takes 10 / (0.25 x 0.25 x PI/4) = 10 / 0.049 = 204 psi to open the check valve to let ANY air into the valve.... The pump tube ID is 0.78", so the force required on the piston to develop that 204 psi is 204 x (0.78 x 0.78 x PI/4) = 204 x 0.478 = 98 lbs.... The pump linkage has to push that hard on the piston, just to overcome the force holding the check valve shut.... Worse than that, if you have 1000 psi in the pump tube, between the piston and the valve, you will only put (1000 - 204) = 796 psi into the valve....  …. Not only that, but the extra pressure required inside the pump causes the linkage to flex, and the pump cup to deflect, both of which increase the headspace, and reduce the maximum pressure you can reach.... and in addition the pumping efficiency.... 

Bob

[jackpine]

Generally, increasing the hammer spring will cure the retained air.... but lightening the valve spring can help as well.... The 2-spring mod replaces the long, stiff valve spring with two, shorter springs and a washer to separate them, that sits on the shoulder down inside the front half of the valve.... The front spring is much lighter, as all it needs to do is hold the check valve closed for the first pump....

Think about it this way (numbers for example only)…. If the check valve seat is 0.25" in diameter, and the valve spring is pushing on that check valve with a force of 10 lbs.... it takes 10 / (0.25 x 0.25 x PI/4) = 10 / 0.049 = 204 psi to open the check valve to let ANY air into the valve.... The pump tube ID is 0.78", so the force required on the piston to develop that 204 psi is 204 x (0.78 x 0.78 x PI/4) = 204 x 0.478 = 98 lbs.... The pump linkage has to push that hard on the piston, just to overcome the force holding the check valve shut.... Worse than that, if you have 1000 psi in the pump tube, between the piston and the valve, you will only put (1000 - 204) = 796 psi into the valve....  …. Not only that, but the extra pressure required inside the pump causes the linkage to flex, and the pump cup to deflect, both of which increase the headspace, and reduce the maximum pressure you can reach.... and in addition the pumping efficiency.... 

Bob

What? I assume all would hold true for a 397, so I'm watching this thread too (but I'm in for the mechanical aspects-what size spring and where  )
I thank you for diving into this with booth feet and sharing!

[Tack Driver 10]

It'll be interesting to see what you do with this gun.

I find one of the biggest limiting factors with these long, fat stroke guns is leverage or lack of it.
To make this gun easy to pump would require a pump arm over 20" long.

[DanD]

and likely the 2-spring mod. to reduce the pumping effort

Haven't heard of this mod, and too much pumping effort is my main complaint with my Steroid 392 with billet lever from Mac1, plus with the wood stock 397 I just bought.
...

Here's a diagram from the old Yellow...

[rsterne]

Thanks for that diagram, DanD…. I didn't have one handy....

Bob

[anti-squirrel]

I'll be following along this as well.

Later today I'll do some testing on my 392PA at 8+ pumps.  I never pump over 5 except the very occasional longer-range squirrel-buster, but squeezing out some efficiency and reducing pump effort are both wins in my book. 

[Techie]

Hmmm, for the life of me, other than for bragging rights i cannot figure out why I would want to put so many pumps into a gun for a single shot.  Unless I needed to.

You are absolutely right to question that kind of craziness Ray.  It's a tremendous amount of effort just to get somewhat close to PCP power levels.  The only reason I ever did it was to verify Timmy Mac's claim that it can be done.  If you're looking for that kind of hand-pumped power (actually more), it's much easier to work a hand pump and fill a PCP.

[rsterne]

I agree with the basic premise that pumpers from an efficiency point of view (effort versus results) are kind of like self-flagellation.... It feels so good when you stop.... 

However, they do present a kind of perverse pleasure in that you can see directly the results of your work in the effect it has on the target.... 

I started with Springers, and progressed through MSPs and CO2 to PCPs, and I have found what I truly enjoy.... However, once in a while I get drawn back to my roots for a while.... and this is one of those side trips.... I have built several pumpers, including my 2289 Uber-Carbine with a 2200 pump that gets 2 shots at just over 600 fps with 14.3 gr. pellets (12 FPE) on 20 pumps.... I have a .177 Uber-Pumper, also with a 2200 pump, that will shoot 7.8 gr. at 500 fps on just 3 pumps, and can hit over 900 fps (in fact I have broken Mach 1 with it with light pellets)…. I mostly use it for shooting Grasshoppers which it's Bug Buster scope is perfect for, as it can focus down to 3 yards.... I built, but no longer have, a .22 cal Uber-Pumper, based on a 140 pump tube, and it was a beast, shooing 14.3 gr. pellets over 900 fps, but it was easy to grow tired of pumping it.... I also built a .25 cal Carbine and the .25 cal Millennium Pumper, which were both amazing projects, the latter would deliver 3 shots at 45 FPE without repumping.... On the other hand, it took 80 pumps to fill it from zero, which is why I cheated and installed a Foster for initial fills.... 

There have been lots of 392's built over the years, some very powerful, some air conserving (Retained Air Pumpers / RAPs / ACPs)…. I'm not sure where this project will lead yet, but I am leaning towards a 2-shot RAP at the highest power level reasonable.... I'm pretty sure it will end up with a 2-piece valve (maybe with gauge)…. We'll see where I end up.... 

Bob

[superchikn]

When I was a kid I did not have an airgun of my own but when i got my hands on friends 760s and 880s I always pumped them to the max recommended number and often quite a few extra pumps.  Now I realize that I had valve locked some of them.
I'm definitely a power freak.     But I don't want to have to do a ton of pumping for a much more powerful single shot.
I do really like the idea of an air conserving pumper.  They make the most sense to me with the availability of springers and PCPs etc.
I have a few 13XXs an 880 and now a 397.  I've been tuning triggers, and whatever else but have not touched the powerplants yet.
Looking forward to what you come up with Bob, and following a fresh journey into pumpers.
Ray

[Psipumper]

Hmmm, for the life of me, other than for bragging rights i cannot figure out why I would want to put so many pumps into a gun for a single shot.  Unless I needed to.

You are absolutely right to question that kind of craziness Ray.  It's a tremendous amount of effort just to get somewhat close to PCP power levels.  The only reason I ever did it was to verify Timmy Mac's claim that it can be done.  If you're looking for that kind of hand-pumped power (actually more), it's much easier to work a hand pump and fill a PCP.

I don’t think it is crazy at all. You just do it. If you have a 40 fpe 392 , you don’t mind pumping it.  The gun is versatile and can be set up to make good power at 8 pumps or less.