Suggested operating pressure for quiet tune on hpa pistol?

[Long_Gun_Dallas]

Just trying to tweak this thing to be as quiet and efficient as I can.  Looking for suggestions on what pressure to reduce sound signature.  Currently at about 110bar on regulator.  Dialed in hammer travel to get ~550fps with 16gr in .22.   7.6" barrel.

Don't mind the large tube, just shooting some @27fpe while reg is out. 

[Hobbyman2007]

That’s quite the build you have there. I use 100 to 110 bar for both Artemis pistols. They are tuned to below 500fps and are about as quiet as I can get them , super light hammer spring settings .

[sactargetshooter]

I just finished tuning my Edgun Leshiy in the past few days, so I can share my experience.

Besides being quiet, there are some other considerations. For example, shot-string trend and first-shot consistency. If you want muzzle velocity (MV) to stay constant, and have consistent first-shot after letting the gun sit for a while, then you are looking at a more drawn-out tuning process.

There are many potential variables: hammer spring tension (HST), hammer mass, plenum volume, transfer port, regulator set point and barrel length. From the description of the OP, I am assuming both HST and regulator set point can be adjusted. That is the case for most PCP guns.

This is one way to dial to the lowest consistent (shot string and first-shot) configuration:

• dial HST to minimum
• set regulator to 120 bars
• fill gun to 120 bars, intentionally forcing the gun to operate out of regulation
• for each shot, record tank pressure and MV
  
• shoot until MV drops below a reasonable value

The MV (y-axis) to pressure (x-axis) plot should look like a inverted bowl. MV peaks at a certain pressure. That is the pressure that you want to center the regulator set point around. If the peak MV is too low, increase HST and repeat the experiment until the peak MV is reasonable along with muzzle report.

But wait, there is more!

A regulator does not actually fill the plenum with a constant pressure regardless of tank pressure. Tank pressure does change plenum pressure, slightly. If the OP has a regulator tester, it can be used to see the short term plenum pressure change in response to tank pressure change. My 12 FPE HUMA regulator has a 5 bar change (to plenum pressure) when tank pressure changes from 80 bars to 200 bars.

And then there is the long term plenum pressure change that occurs in a matter of minutes to hours. This affects the first-shot consistency after a gun has been unused for several hours. You can test this using a regulator tester if you have a reliable source of tank pressure over several hours. I use a hand-pump, so this option is not available to me. Nonetheless, keep in mind that plenum pressure can creep up over time. You can make an assumption of several bars of pressure. For example, in my case, the short term plenum pressure is 85 bars when tank pressure is 200 bars, so I can add an assumed 3 bars of pressure for long term creep.

This makes a total of 8 bars of plenum pressure variation.

Now get back to the pressure that gives you the maximum MV. Assuming the total plenum pressure variation is 8 bars, then you can set the minimum regulation pressure to 4 bars below the max MV pressure. This way, you have the leeway for consistent MV over the entire tank operating pressure, and you will also have consistent first-shot MV.

You can do all this without a regulator tester, but then you will end up taking the regulator out for adjustment and refilling the reservoir a few times. I have a hand pump, and the Leshiy has an extension tube, dumping, adjusting and refilling several times can be a good work out!

I am not sure whether I am explaining well enough here. Let me know if I am not making sense 

[nervoustrigger]

Minimizing the report is all about minimizing the residual pressure at the muzzle.  Strictly speaking, there is no optimum regulator pressure except to the extent that it is high enough to give you the velocity you want or at least can be content with.  Otherwise we just can just keep reducing the pressure ever lower to keep reducing the report.
 
The way to optimize for minimum residual pressure is to get the pellet accelerating as rapidly as possible and then quickly close the valve.  The valve will be closed before the pellet has moved very far, and the air will continue to expand behind it to accelerate it, but by the time the pellet leaves the muzzle, the pressure will have decayed appreciably…the result being a report that is minimized to the extent possible for the desired velocity to which it has been tuned.
 
Ingredients for the mixing bowl:
 
1.       Big porting – Up to full bore porting with an oblong port to avoid chambering issues.  Helps maximize the rate of acceleration early in the cycle of the valve opening.
2.       Lightweight hammer – Minimizes dwell time (less momentum to hold the valve open).  MDS nylon or PEEK.  100% plastic provided the trigger is a drop sear type.  For a direct sear (sear abrades the hammer), it will need a hardened steel contact surface, or just skeletonize the steel hammer to remove as much weight as you can.
3.       PEEK poppet – The material doesn’t compress as much as other common poppet materials.  That makes it easy to unseat.  A lightweight hammer smacking it at high velocity loses very little of that velocity, opening the valve to full lift very quickly.  The result of that is a sharper pressure rise than would otherwise be obtained, helping the pellet to accelerate very quickly in the early part of its travel down the barrel.
4.       Anti-bounce mechanism –  If needed, an SSG or similar device to prevent the valve from being knocked open more than once.  Note a lightweight hammer may not compress the hammer spring sufficiently on the rebound to pick up enough energy to knock the valve open a second time.  So you may not need it.
 
I hope this helps!

[Long_Gun_Dallas]

All excellent replies.  I'll address them in more detail when I am at home later.

I will say that the hammer spring retainer has been back-bored to allow the use of the factory spring, but still allow the hammer to rest with no spring tension on it.  The hammer has an adjustable striker, so I can make the gun shoot 100fps if I want to, just adjusting the striker.  I shot it at 150 bar, and was barely able to open the valve with the back-bored spacer. (just a hair shy of 500fps)  With a factory spring retainer, it was shooting over 800fps, and was extremely loud (when I had the regulator out earlier).

So, there is a lot of adjustment there.

I understand higher pressure allows for shorter dwell, and more immediate acceleration of the projectile.  The smaller volume of hpa lends to quieter report.  I was just curious if someone had tuned something of similar barrel length/caliber with this in mind....

@targetshooter, everything you said made sense.  I'll see if I can find that peak later 🙂

@nervous Thanks for the tips!  Do you have any suggestions on how to get a peek poppet made?  I have a spare one for this gun, but I'm certain it is acetyl.

What you said about the ports makes sense.  I'll see about touching mine up.  I originally wanted to tune the gun to not use a regulator, so, I never touched them.  I actually got the regulator to reduce sound signature.  It is pretty quiet for having a tiny integrated mod.  Just searching for that little bit of improvement I left on the table 🙂

Reg. allowed me to pull out resting spring tension on hammer, eleminating bounce for good. 

[nervoustrigger]

Regarding a PEEK poppet, you can make one yourself with hand tools if you have the patience.  Several of us here on the forum have done it, though it will be a quicker job on a lathe.  PEEK is utterly unforgiving of any surface imperfections so the trick is to make a simple sanding block to flatten the face.

Here's a DIY guide I drafted a while back on how to make a poppet: 

With a lathe, turn a piece to the desired diameter.  Center drill it to roughly 2/3 depth to accept the stem.  For example, for a poppet that's 0.5" long, I would drill it about 0.3" deep.  The drill size should be just a hair under (maybe 0.003" - 0.005") that of the stem to give it an interference fit.  Then use a vise or clamp to press in the stem.

If you don't have access to a lathe, you can do it with a drill press or a handheld drill.  Cut a piece of the plastic rod stock to length and center drill it to the proper depth and press in a piece of hardened rod.  Then chuck up the rod in your drill or drill press and get it spinning, and use a burr in a Dremel to reduce the plastic rod down to the diameter you want.  Then you can smooth the surface with a file and/or sandpaper. 

Now carefully face off the sealing surface as best you can.  Don't concern yourself with getting it perfect at this point.  The idea is just to remove any high spots.  Don't use a file for this step because if you attempt to hold a file against the surface, it will just ride up and down over the high spots and low spots.  Instead, use a burr or grinding wheel in a Dremel and slowly bring it toward the surface until it just kisses the high spots as the poppet spins around.  Eventually you'll get to a point where it's taking a bite for the full rotation of the poppet.  You can stop when that happens.

Now finish flattening it with a piece of sandpaper held against a flat backer.  Here's a flattening jig I made with a scrap of wood. 

It's nothing more than a block of wood with a hole just big enough for the stem to pass through.  Use a drill press to make the hole if you can so it will be square to the surface, but drilling by hand is okay too.  You will just end  up with a slightly tapered poppet face and that will seal just fine.

Then I used a hollow punch to knock out a bunch of holes in a piece of 220 grit sandpaper.  From there, it's just a matter of putting the stem through the sandpaper and the wood block, and grabbing the stem from the back side with a drill and spinning it against the sandpaper to smooth it out.  If you color the face of the poppet with a marker, you can monitor for the low spots as you go.  It will be obvious when you've done enough because the marker will be completely removed.

Lastly, lap the poppet to the valve to ensure they mate fully.  This is a necessary step even if you have a professionally made poppet.  Again, color the surfaces with a marker and spin them together with a mild abrasive like J-B bore compound or a metal polish.  When both surfaces develop a neatly polished ring where they meet, you're done.