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BRUCE · New Member

Hi guys I was wondering if anyone out there knows where I can get info on how to get more velocity out of my benjamin 392 .22 any help would be greatly appreciated......... Cool

AirGunEric · Site Admin

Look at the diagram (this diagram is for the original 392/397 but in the newer versions the concepts/basic layout is the same):

The 392 being a Co² gun- velocity/power improvements are all related to valving and gas flow. If you look at the diagram, part #31, called the "exhaust valve" actually has holes through that round 'cup' like part to allow gas to flow through to the rear of the pellet. Making these holes a bit larger (through drilling or sanding) will improve gas flow. The spring, part #33, controls how hard the exhaust valve is retained and 'pulled' back after the trigger is released, the valve head uncorked and gas starts flowing. Using a spring of similar size but not quite as strong will also allow for more gas flow (don't know of a specific spring you can use- so you may have to take it out and off to your local hardware store to see if they have something you can use).

Beyond that, only a custom-designed and sized valve would allow for greater amounts of Co² flow.

BRUCE · New Member

Thanks Eric, I'm goin to disassemble it and hit the part store as soon as I get off line. and get a variety pack. Any idea how much is to much ie; hole diameter and spring tension. Did a trigger job on it the other day and got it down to about 2-3 lbs way better than factory, (still a little creep, but not bad I would have gone a little farther but theres a little slop in the piston and I didn't want to over do it. I'll pick up my bro's crony tomorrow and see how it does. I'll get back to ya Cool

The outdoorsman · Veteran Member

http://www.crosman.com/site/listing/1053

isn't it a pneumatic?

AirGunEric · Site Admin

Aha! You appear to be correct- I guess the newer versions of both the 392 and 397 went to a pump. Either way, the valve modding idea is very much the same- although the parts layout may be slightly different.

With a pump gun, you can also strengthen the transfer port between the pump reservoir and the chamber behind the valve- oftentimes in newer guns this is a weak spot and doesn't allow for full or completely sealed airflow. Another thing is to look at weekly's modified 2289: http://www.airgunhome.com/agforum/viewtopic.php?t=573&start=15 but it has used custom parts he made himself. The concept he went with was increasing the amount of air pumped on each pump stroke- so less pumps are required to raise air pressure- the idea being more pressure means greater velocity. Of course- there is a point where trying to pump it up any higher will blow out the gun's seals and o-rings- I think weekly mentioned on his heavily modified 2289- that much more than 5 pumps and this will happen. So, for the absolute most power- one would do all the mods, but to start valve mods are probably the easiest and most productive with the least number of parts.

Is you gun an older Co² version, or the newer pump-up version, Bruce?

BRUCE · New Member

Hey Eric it's a pump...... Basically there's a valve assembly w/a discharge hole coming out the assembly into the chamber, is this what your talkin about drillin out? I'm tryin to post a pic but I can't figure it out yep, just looked at the link the outdoorsman put on there, thats the one........

BRUCE · New Member

see the hole on the right side of the pic on the valve assembly, is that the one yer talkin about? also should I mess with the hole goin in the chamber or leave it..........the hole looks to be about a #12...........

AirGunEric · Site Admin

The hole on the right side of the picture in the brass housing- that should be the transfer port hole- it would have been resting up against a hole in the reservoir- probably with an o-ring to seal them together. This is where the air moves from the reservoir to the valve and can leak or be too small to allow maximum flow. Many people drill both holes out, and then use plumbing-type plastic tubing that compresses when the two pieces are put together to seal things up better, with more flow.

As for the holes I was talking about drilling- it appears the 392 does not have them- on the brass head of that exhaust valve there could have been 3 or so holes to improve gas (air) flow- but obviously the 392 is not using them. Sometimes I wish I had one of every gun on my shelf to check these things first hand!

It appears that this will be a better source of info on your improvements- as most of the relevant stuff is already in there or linked:

http://www.airgunhome.com/agforum/viewtopic.php?t=40

The pictures that are on the link to the Crosman users site are very good- and should give you alot of good info. The 392 is obviously following the same design specs as 2289s and such now (Crosman tends to reduce the number of actual platforms more and more it seems- for example- they only have two different spring platforms they use, despite the fact they have about 8 different spring guns they sell).

Post up any questions here- if you ask on the original post on the Crosman forum- it won't "come up top" and no one will ever see them to be able to respond. Also let us know how you're making out on it.

BRUCE · New Member

that'll ought to work perfect, I'm just wondering what I should do with the pellet on the intake side (make it shorter or leave it)??????

AirGunEric · Site Admin

23ib0d0n · Moderator

Think he means check valve . . .

BRUCE · New Member

I guess thats what you'd call it, it's on the left in the above picture of the valve body thats taken apart. I'm working out of town right now so I won't be near a computer till next week but I'll be workin on it

weekly · Member

If you shorten that piece it will increase the valve volume. My 2289 valve has a shorter check valve than that but may be shorter overall. With the 14” barrel I would not want any more volume, it would just mean more pumping to reach the same presser and more noise from the high pressure air leaving the barrel after the pellet. Valve volume, pressure, and barrel volume (length) are all relevant.

What you do to the valve all depends on what you want and what you can do or pay for. There are guys that talk about pumping there guns 50 times for one shot. The other way to go is an air conserving gun that you pump many times initially than only a few pumps (around 4) between shots. I tried to get as much air in the valve per pump as possible.

If I were you id open the exhaust port also known as the transfer port. Put a heavier hammer spring in to increase the speed the valve opens and allow higher pressures to be dumped. A flat top piston and valve would also be a big improvement.

There has been some amazing things done with the 392 if you haven’t already you should check the green forum. Mods done to the 1322 will also be useful.

weekly · Member

To address something Eric mentioned earlier. The reason I cant pump my gun many times is the tube I used for the pump tube is slightly larger than the original Crossman tube. So the valve does not fit as tightly as I would like. I did at one time have a harder o-ring in it. A 90 duameter? Not sure if that is the right terminology. I was able to pump it up to much higher presser when that was installed. They are also very easy to damage when putting the valve in and out of the tube and I took a hunk out of the two I had. I will be getting more of the harder ones this week, I ordered 20 of them:) when I had the harder ones in before it would put Crossman pointed pellets through both sides of a coffee can at 50 meters 12 pumps.

AirGunEric · Site Admin

Close on the terminology weekly! It's "durometer", which is a measurement standard for rigidity and hardness in respect to plastic/rubber/urethane type parts.

Did you ever determine how many pumps it took using a 90 Durometer bushing/seal to blow it out?