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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Dual Tank Benchrest Rifle

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PikeP:

--- Quote from: rkr on March 07, 2023, 11:48:49 PM ---This kind of makes me wonder, with small calibers like .184 do we really benefit from balanced valves? Couldn't we just use 2mm valve stem and PEI poppet to get the long dwell with reasonable hammer strike while keeping it all simple and robust? After all we are now talking about small throat diameter valves and thus much reduced force keeping the valve closed.

--- End quote ---

It's what I do and personally recommend (and have for the longest) for small calibers. This valve otoh probably benefits from being balanced, where as I would have to force an oversized poppet on my small .234" .25 cal throat just to have it make sense to balance. I currently run peek with .02" sealing margins.


--- Quote from: rsterne on March 06, 2023, 11:10:08 PM --- The minimum clearance from piston the chamber will be 0.075", which is 1/4 of the valve throat diameter.... This means the valve can reach full flow without the poppet touching the piston.... That volume can be tripled if desired, to make the valve open easier....

Bob

--- End quote ---

I know .075" lift is where the valve reaches peek flow but wouldn't it need to lift significantly higher to produce max power ie: need more minimum distance between chamber and piston in your adjustment? I know it can be tripled but .075" seems really tiny if I am envisioning the space between between and chamber correctly, my guess is you would run somewhere in the middle?

rsterne:
The valve will absolutely open more than 0.075" when going for maximum power.... The lift and dwell are inevitably intertwined, for a given pressure, valve and hammer mass.... You don't NEED the extra lift past the 1/4 diameter where curtain area equals throat area, but you may end up with that to get the dwell you require.... On the other hand, every time I have physically measured the valve lift when a PCP is tuned for the knee of the curve, I have never found it to be greater than 1/4 the throat diameter.... If it is greater than that, the gun is more than likely an air hog, and tuned way up on the plateau for the pressure you are using.... That's my experience, anyways.... Having said that, I expect the best tuning volume to be in the middle of the range I have allowed for, and the poppet to never hit the piston....

Bob

mackeral5:
Bob, while it may be the less exotic of the two, I am looking forward to the .22 and seeing your results with the Monster Redesigned pellets.  I recently started shooting them out of a cleaned up Fortitude barrel, which i believe is around a 1:22 or so twist. It is sleeved up to 18mm with cf tubing, in a Brod.  While I have not tried them at slower speeds, they do not seem to mind running in the 990fps range.  They are not as difficult to get running fast as I anticipated---21.5" from barrel port to muzzle, around 2000psi reg.  Testing has shown many instances of 2-3 shots in basically the same ragged hole at 100 yards.  Unfortunately, more times than not 5+ shot groups open up to 1.5-2".  I hope some barrel/breech stability improvements (barrel band at tank block/Cobra reg) will help shrink those groups.  I have not tried head sizing/sorting/weighing...

PikeP:

--- Quote from: rsterne on March 14, 2023, 06:01:59 PM ---The valve will absolutely open more than 0.075" when going for maximum power.... The lift and dwell are inevitably intertwined, for a given pressure, valve and hammer mass.... You don't NEED the extra lift past the 1/4 diameter where curtain area equals throat area, but you may end up with that to get the dwell you require.... On the other hand, every time I have physically measured the valve lift when a PCP is tuned for the knee of the curve, I have never found it to be greater than 1/4 the throat diameter.... If it is greater than that, the gun is more than likely an air hog, and tuned way up on the plateau for the pressure you are using.... That's my experience, anyways.... Having said that, I expect the best tuning volume to be in the middle of the range I have allowed for, and the poppet to never hit the piston....

Bob

--- End quote ---

Makes sense, you need much less lift and dwell at 3600 psi than you would at 2400 psi for the same given power, and a lighter hammer needs more lift than a heavier one (even if marginal and often times negligible) due to the reduced momentum it contains fighting the valve closure.

Are you going to try out x-rings on this build or will you be staying with a traditional o-ring?

rsterne:
The goal is about 80-90 FPE for the .183 ca (3600 psi), but only 50 FPE for the .22 cal at 2200 psi.... I'm going to start with the same hammer for both, with more spring on the higher pressure....

I have a couple of 1/4" X-rings for the balance chamber piston, and will try those.... Scott (Motorhead) has good success with them....

Bob

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