First qb79 mod - more volume!

[Millipede]

qb79 Valve volume modification. Basicly cut of the whole valve and kept the threaded part to screw on for the oring to stay tight

Before:


After:




I put it together and realized i needed a way to tighten it so i cut a couple notches in the aluminum piece to tighten it once its inserted in the tube. Also decided i didnt like how the spring didnt have a spot to rest so i drilled out the adapter with a 23/64ths bit to fit a valve spring from a 392 valve.

that adds a bit more volume also



haven't tested it yet because im out of co2 for now.

[stalwart]

Great pics!

Will follow this closely, I'm loving my 79.

[Raizer]

Good idea using the tank block to hold the spring!

A friend suggested this idea to me, it would let you adjust the pressure of the valve spring too

[Millipede]

i dont think there's enough room on the stem for that. maybe if you just used a c-clip

[stalwart]

Maybe a T shape, with access to the grub screw between the coils?

[Raizer]

As long as the fastener and compressed spring length is shorter than the stop block it should be ok?

[stalwart]

I don't see why not. I'd make the retainer a spring guide as well... as long as possible w/o interference.

I planned to cut my 2078 down to about 15g capacity. It will live on a tether, but I plan to unleash it for occasional precision plinking.

Wonder how much volume the spring takes?

[rsterne]

I first suggested this idea a few months ago on the Canadian Airgun Forum, but I've been waiting for my QB79 and parts to arrive before I dove into it.... congrats on the parallel thinking, you beat me to the reality of doing it.... *grin*.... I plan on doing mine slightly differently, making the front part of the valve into only a retaining ring for the O-ring, either gluing or loctiting it in place and then just stretching the O-ring over into the groove and then sliding the valve down into the tube the way it's done on virtually all other PCPs.... There is no reason to have to tighten up the front portion and compress the O-ring and squeeze it outwards, providing the O-ring groove is the correct dimensions in the first place....

I plan on using a spring guide mounted on the tank block, but it will be a toss-up between guiding the valve spring and losing the volume of the guide.... Even getting all the volume you can, it's barely enough for a 30 FPE gun, and in reality there is a benefit from creating twice the volume you can get this way, even for a gun of that power.... However, it will be a HUGE improvement over the stock QB79 volume.... Like I said, congrats on the parallel thinking....

Bob

[Millipede]

the way it's done on virtually all other PCPs.... There is no reason to have to tighten up the front portion and compress the O-ring and squeeze it outwards, providing the O-ring groove is the correct dimensions in the first place....


Bob

I know right?  If it weren't for the screw on part the valve would be as simple as a valve could possibly get.  Here's a concept i thought of while working on my valve:



two orings to seal the tube and the exhaust port and then make the throat as short as possible, maybe widen it a bit too, Feel free to make me one bob   

[stalwart]

I first suggested this idea a few months ago on the Canadian Airgun Forum, but I've been waiting for my QB79 and parts to arrive before I dove into it.... congrats on the parallel thinking, you beat me to the reality of doing it.... *grin*.... I plan on doing mine slightly differently, making the front part of the valve into only a retaining ring for the O-ring, either gluing or loctiting it in place and then just stretching the O-ring over into the groove and then sliding the valve down into the tube the way it's done on virtually all other PCPs.... There is no reason to have to tighten up the front portion and compress the O-ring and squeeze it outwards, providing the O-ring groove is the correct dimensions in the first place....

I plan on using a spring guide mounted on the tank block, but it will be a toss-up between guiding the valve spring and losing the volume of the guide.... Even getting all the volume you can, it's barely enough for a 30 FPE gun, and in reality there is a benefit from creating twice the volume you can get this way, even for a gun of that power.... However, it will be a HUGE improvement over the stock QB79 volume.... Like I said, congrats on the parallel thinking....

Bob

If I cut down a 78 tube on a mostly tethered gun that will eventually (hopefully) see HPA up to .30 cal, what would be the max volume I would ever need for this area?

[rsterne]

The optimum is about 1 cc per FPE you want the gun to shoot between the regulator output and the seat of the valve.... You can get away with half that, but your regulator pressure will have to go up about 100 psi more.... If the plenum is less than 1/2cc per FPE, the regulator pressure has to get higher and higher to compensate because it's the AVERAGE pressure at the valve seat you are interested in.... If the volume is too small, that average pressure drops too much during the shot as the regulator response time is too slow to maintain the pressure....

Bob

[stalwart]

Thanks, Bob. That's easy to remember!

[wilsonj1018]

I made a lot of power in mine when my reg took a poo lol the little delrin ball seal failed had about 1600 psi straight into the gun scared the bejesus out of my when the burst disk blew the next fill but it did make 39 fpe with 14.3s  when making a 79 shoot hot u need one thing volume. with a bit of volume these guns will make great amounts of power. now if only I could get a pcp tube and an adaptor for with tank threads on one end and the other to mount inside a stainless tube and secure it well. would end up costing way to much though.....im looking forward to seeing the end result of all this I might have to open ole ugly back up one more time lol
Josh

[rsterne]

In case some of you have missed it....   http://www.gatewaytoairguns.org/GTA/index.php?topic=21681

Bob