• The AGN App is ready! Search "Airgun Nation" in your App store. To compliment this new tech we've assigned the "Threads" Feed & "Dark" Mode. To revert back click HERE.

Tuning Potential issues with large plenum to caliber/power ratio?

Jump to latest Follow Reply
UCChris

UCChris

Member
Mar 12, 2017
1,189
961
Chicago, IL, United States
Hey all, 

I'm setting up a bottled Prod bullpup for my wife and I am using the Prod tube with a 9ci bottle off the front. The bottle is 3k psi regulated down to 1350 psi, and the plenum is going to be somewhere around 50-55 cc. It's a .177 conversion that I'm going to be tuning to just under 18 fpe with 10.34 JSB. I'm wondering if I'm going to run into any issues using such a large plenum on a lower powered gun. I seem to remember a discussion about the possibility that the pressure drop in the plenum might be small enough to not trigger the regulator for a couple shots when using a large plenum on low power guns. I don't recall if the discussion was theoretical or someone had actually experienced the phenomenon. 

Any thoughts or advice? Anything is much appreciated.
 
Short answer...no, the phenomenon you speak of only occurs as a result of regulator creep.



Long answer. Any volume displacement will have to be replenished by the regulator by its nature, some say a larger pressure differential aids in pressure consistency...I say quite the opposite, as logic only applies in that, a smaller differential in pressure drop should result in a smaller differential of shot to shot consistency



I could bore you with the math but meh...




 
Upvote 0
Short answer...no, the phenomenon you speak of only occurs as a result of regulator creep.



Long answer. Any volume displacement will have to be replenished by the regulator by its nature, some say a larger pressure differential aids in pressure consistency...I say quite the opposite, as logic only applies in that, a smaller differential in pressure drop should result in a smaller differential of shot to shot consistency



I could bore you with the math but meh...



Click to expand...

I would love to see some math. Would you please share? 
 
Upvote 0
With any reasonable state of tune, it is not a concern.

Why? Well if the plenum volume is so large that taking a shot causes no meaningful drop in pressure, that means the next shot has the same pressure available. By “same” I don’t mean literally identical, but so close as to be immaterial from the perspective of the gun’s ability to self-regulate.

This all goes back to the ability of a conventional unregulated PCP to self-regulate exceedingly well for at least a couple hundred PSI on either side of the peak of the bell curve. So too will a regulated PCP when it is running at 95 - 97% of its maximum velocity.

It is certainly possible to make the gun sensitive to minor pressure variations by deliberately adjusting it poorly but given the fact you are sufficiently detail-oriented to be asking this question, I don’t think that’s really the scenario we are talking about.

tl;dr - Of the myriad of factors influencing the extreme spread, this one ranks near the bottom of the list.
 
Upvote 0
Let me share my experience: If a gun set at high reg pressure then shoot it at low power, large spread at entire shoot would be normal.

I suggest first decide the power you want then set reg that you could not exceed that power with that reg pressure and lastly turn hammer screw that by tightening more speed don't increase any more. (like mine while set at 120 bar max power i could get is 44j)

keep in mind that if you change the reg pressure you have to set valve spring too.

Also i don't think high plenum vol at low power could cause reg don't work properly.
 
Upvote 0
With any reasonable state of tune, it is not a concern.

Why? Well if the plenum volume is so large that taking a shot causes no meaningful drop in pressure, that means the next shot has the same pressure available. By “same” I don’t mean literally identical, but so close as to be immaterial from the perspective of the gun’s ability to self-regulate.

This all goes back to the ability of a conventional unregulated PCP to self-regulate exceedingly well for at least a couple hundred PSI on either side of the peak of the bell curve. So too will a regulated PCP when it is running at 95 - 97% of its maximum velocity.

It is certainly possible to make the gun sensitive to minor pressure variations by deliberately adjusting it poorly but given the fact you are sufficiently detail-oriented to be asking this question, I don’t think that’s really the scenario we are talking about.

tl;dr - Of the myriad of factors influencing the extreme spread, this one ranks near the bottom of the list.
Click to expand...

Thank you for chiming in here! It will be interesting what the knee ends up being with 50-55 cc, 1350 psi, and a barrel in the 15-18" range. Since I originally had this on a super small plenum, the valve and transfer port are both drilled out. I may end up needing to put a factory port in to choke it down. This one will be a fun one to tune, that's for sure!
 
Upvote 0
First the gun was unregulated so that means the entire tube is the “plenum” so I don’t see any issues.



second why 1350 PSI? The gun’s internals are made to go from 3000 psi down to around 1600 psi so you might need a lighter hammer to get the tune you want. I would recommend higher regulator pressure to not only match the original parts designed working pressure but also due to short barrel a lot quieter and more accurate. Noticed you mentioned 18 inch barrel? I would still bump it to within the factory working range of 1600 psi if not a little higher. Good to have hammer and regulator pressure balance. 



Lastly as most of these guns are designed the best consistency is produced not by regulator but by TP restriction, so get the smallest PRod TP and experiment around it. If needed drill the TP out very gradually. Maybe even the larger 1720 power TP. This is especially important given you are well below the working pressure range for the gun’s design. 



for reference my 1720 has the regulator set at 2000 PSI with PROD TP, very accurate but ES is in the low teens so the smallest PROD TP is tad too big for the 10 FPE in shooting at. But might work well for you since you are looking at 18 FPE. 




 
Upvote 0
First the gun was unregulated so that means the entire tube is the “plenum” so I don’t see any issues.



second why 1350 PSI? The gun’s internals are made to go from 3000 psi down to around 1600 psi so you might need a lighter hammer to get the tune you want. I would recommend higher regulator pressure to not only match the original parts designed working pressure but also due to short barrel a lot quieter and more accurate. Noticed you mentioned 18 inch barrel? I would still bump it to within the factory working range of 1600 psi if not a little higher. Good to have hammer and regulator pressure balance. 



Lastly as most of these guns are designed the best consistency is produced not by regulator but by TP restriction, so get the smallest PRod TP and experiment around it. If needed drill the TP out very gradually. Maybe even the larger 1720 power TP. This is especially important given you are well below the working pressure range for the gun’s design. 



for reference my 1720 has the regulator set at 2000 PSI with PROD TP, very accurate but ES is in the low teens so the smallest PROD TP is tad too big for the 10 FPE in shooting at. But might work well for you since you are looking at 18 FPE.
Click to expand...

1350 is what it is regged at right now, so that will be my starting point. It has a Ninja regulator, which have burst disks notorious for letting go around 1500-1600 psi. Though I seem to have found a 3k burst disk that fits, and my tank block uses four screws, so that will allow me to bump up if needed. That all being said, I do have a lightweight Delrin hammer.

Thank you for the info on the transfer port. Mine is drilled out to the max, so I'll very likely have to get a new one. 
 
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu