Plenum Logic

[SpindleB]

I have had multiple discussions with various air gunners about plenum size. There is a belief that too large a plenum will cause turbulence at the barrel end when the pellet exits. Not having ever experienced that I have always been a bit skeptical.

I realized in the middle of the night that it made absolutely no sense.

What about unregulated guns with bottles or tubes that act as massive plenums? Shouldn't that be a problem there if it is a problem with plenums?

 

[BenjeneerJohn]

Hammer energy/valve dwell causes excess air. Plenum is just the air on tap when the valve is opened.

John

 

[SpindleB]

Hammer energy/valve dwell causes excess air. Plenum is just the air on tap when the valve is opened.

John

Yes, that is true especially with poppet valve guns. Not so true on direct feed guns, like AirForce. Yes, they have valve dwell but with some minor part changes you can actually push more air down the barrel than is required for the pellet to exit the barrel under acceleration.

In that case the regulated with plenum acts no different than the unregulated gun. When I did all my plenum testing last year I did: Baseline without regulator
Then regulated with no or minimal plenum
Regulated with plenum ie Huben Dream Valve 20cc
Regulated with various larger plenum

I did not at any point note a shift in POI or increase in group sizes. But I was looking for optimal plenum size so it has always been possible I missed something.

I'd share the data but some idjit formatted the USB with the data and then wrote a bunch of music to the drive.

 

[BenjeneerJohn]

Generally, Plenum is defined as the regulated volume between the valve and regulator. With that said, Unregulated systems don't have plenum.

I am not sure I follow your comment about Airforce. The hammer energy and valve dwell can still cause excess air.

I would not expect plenum volume to shift your POI or change your accuracy unless it greatly changes your velocity or the harmonics of the rifle.


John

 

[SpindleB]

Generally, Plenum is defined as the regulated volume between the valve and regulator. With that said, Unregulated systems don't have plenum.

I am not sure I follow your comment about Airforce. The hammer energy and valve dwell can still cause excess air.

I would not expect plenum volume to shift your POI or change your accuracy unless it greatly changes your velocity or the harmonics of the rifle.


John

What is an unregulated bottle if not a plenum?

As to the point about shift, that is exactly what my original point was about. Claims that too large a plenum causes turbulence upon exit. My point being too large a plenum would be the same as unregulated. Both have sufficient air to accelerate a pellet for the length of the barrel. Where a smaller plenum looses pressure and the pellet stops accelerating.

Poppet valve gun:
Air inlet port
Poppet valve size
Poppet spring
Hammer spring
Hammer weight
Turns in path
Transfer port another 90° turn
All affect air flow

Direct feed:
Valve port size
Valve spring
Hammer weight
Hammer spring
No turns, no transfer port, no inlet port
Valve directly feeds barrel
Only restriction in air path is internal port structure of valve.
High flow valve, heavy hammer and heavy spring provide enough air to accelerate a .25 pellet for the full length of a 24" barrel.

I do not know of a poppet valve gun that has that ability. They may exist but my experiments with various poppet valve guns there was little improvement beyond a few 10s of CC of plenum volume. Whereas the .25 24" Condor improved between 3-4x barrel volume (19.3cc).

Most .25 poppet valve guns I tested saw little improvement from 20cc to 50cc plenum

The only issue with a large plenum would be hysteresis of the plenum refill. If the plenum pressure fails to fall below the regulator open pressure and you do not have sufficient air to get full acceleration.

 

[Solo1]

What is an unregulated bottle if not a plenum?

As to the point about shift, that is exactly what my original point was about. Claims that too large a plenum causes turbulence upon exit. My point being too large a plenum would be the same as unregulated. Both have sufficient air to accelerate a pellet for the length of the barrel. Where a smaller plenum looses pressure and the pellet stops accelerating.

Poppet valve gun:
Air inlet port
Poppet valve size
Poppet spring
Hammer spring
Hammer weight
Turns in path
Transfer port another 90° turn
All affect air flow

Direct feed:
Valve port size
Valve spring
Hammer weight
Hammer spring
No turns, no transfer port, no inlet port
Valve directly feeds barrel
Only restriction in air path is internal port structure of valve.
High flow valve, heavy hammer and heavy spring provide enough air to accelerate a .25 pellet for the full length of a 24" barrel.

I do not know of a poppet valve gun that has that ability. They may exist but my experiments with various poppet valve guns there was little improvement beyond a few 10s of CC of plenum volume. Whereas the .25 24" Condor improved between 3-4x barrel volume (19.3cc).

Most .25 poppet valve guns I tested saw little improvement from 20cc to 50cc plenum

The only issue with a large plenum would be hysteresis of the plenum refill. If the plenum pressure fails to fall below the regulator open pressure and you do not have sufficient air to get full acceleration.

But the plenum maintains its regulated shot after shot where an unregulated gun looses some pressure shot after shot. If the plenum was large enough to allow excess air to push past end of barrel it would do so till you dropped below reg pressure.
@Franklink please join this as you probably understand this better than myself

 

[Solo1]

Additionally an unregulated gun isn’t always filled to max pressure so as to take advantage of the sweet spot of the velocity curve. I can’t speak much about different valves as i just don’t know.

 

[SpindleB]

But the plenum maintains its regulated shot after shot where an unregulated gun looses some pressure shot after shot. If the plenum was large enough to allow excess air to push past end of barrel it would do so till you dropped below reg pressure.
@Franklink please join this as you probably understand this better than myself

Yes, but the effect of a large plenum and a bottle are exactly the same for a given pressure.
120cc plenum, .25 24" barrel 19.3cc
220cc bottle, .25 24" barrel 19.3cc
If both are at 2500psi what is the effective difference for that shot?
Since plenum effectiveness with a direct feed gun does not increase beyond 4x barrel volume.
Both will accelerate the pellet for the length of the barrel and the FPS at exit will be virtually the same.

Plenum = bottle above 80cc at the same pressure.

 

[Franklink]

I'll take a stab at it...

The excess turbulence (too much air) minimizing accuracy/precision isn't due to only the plenum size, but happens when a large plenum and over-porting are combined.

I've played with lots of unregulated guns, and in some ways even prefer them. Depending on design, yes, in an unregulated gun, the air tube/tank can somewhat be considered plenum volume. The most exaggerated of this would be a USFT. The air volume of the massive airtube on mine comes out to something like 650-700ccs of air. So, considering all of that air being available to the valve with each hammer strike....yeah, at first glance it seems like there'd be a too-much-airflow/turbulence issue. But there's not. And that's due to a couple aspects of the gun. First is that the air passageway from the tube to the pellet is small, both in the valve and in the swing breach. Couple that with the low working pressure of the platform, and not much air actually squeaks it's way through with each hammer strike. It's a blip of air, not a BLAAAAASSSSSSTTTTTT of air. Which results in the USFT being one of the most storied (accuracy/precision) field target guns.

By the same token as a USFT, when we swap out the top hats/ring locs of an unregulated Airforce gun to an appropriately sized one for what we're wanting to do, we get improved accuracy. Shoot a .22/18.13 with the largest bored ring loc to see first hand what we're talking about with the excess air causing an accuracy problem. The top hat/ring loc is essentially the transfer port. And in an unregulated gun, strangling the transfer port (if the air passageways are too large for what we're wanting to do) is how we usually find accuracy and efficiency.

This all seems to be alluding to what I've been finding with the Blackwolf. That platform has MASSIVE valve porting and a very large-for-class plenum. It was designed to push LOTS of air to shoot large bore and/or heavy-for-caliber projectiles. That's what Daystate thought we Americans wanted, so that's what they made. And they are probably right for lots of Americans. BUT, combine that high energy potential of the design, with a barrel that is also generously ported, and try to shoot pellets at 1/2 or 1/3 the projectile weight appropriate for all that air...and you're going to see poor accuracy. Simply too much air with each tap of the valve stem.

 

[Pumacarl]

I'll take a stab at it...

The excess turbulence (too much air) minimizing accuracy/precision isn't due to the only the plenum size, but happens when a large plenum and over-porting are combined.

I've played with lots of unregulated guns, and in some ways even prefer them. Depending on design, yes, in an unregulated gun, the air tube/tank can somewhat be considered plenum volume. The most exaggerated of this would be a USFT. The air volume of the massive airtube on mine comes out to something like 650-700ccs of air. So, considering all of that air being available to the valve with each hammer strike....yeah, at first glance it seems like there'd be a too-much-airflow/turbulence issue. But there's not. And that's due to a couple aspects of the gun. First is that the air passageway from the tube to the pellet is small, both in the valve and in the swing breach. Couple that with the low working pressure of the platform, and not much air actually squeaks it's way through with each hammer strike. It's a blip of air, not a BLAAAAASSSSSSTTTTTT of air. Which results in the USFT being one of the most storied (accuracy/precision) field target guns.

By the same token as a USFT, when we swap out the top hats/ring locs of an unregulated Airforce gun to an appropriately sized one for what we're wanting to do, we get improved accuracy. Shoot a .22/18.13 with the largest bored ring loc to see first hand what we're talking about with the excess air causing an accuracy problem. The top hat/ring loc is essentially the transfer port. And in an unregulated gun, strangling the transfer port (if the air passageways are too large for what we're wanting to do) is how we usually find accuracy and efficiency.

This all seems to be alluding to what I've been finding with the Blackwolf. That platform has MASSIVE valve porting and a very large-for-class plenum. It was designed to push LOTS of air to shoot large bore and/or heavy-for-caliber projectiles. That's what Daystate thought we Americans wanted, so that's what they made. And they are probably right for lots of Americans. BUT, combine that high energy potential of the design, with a barrel that is also generously ported, and try to shoot pellets at 1/2 or 1/3 the projectile weight appropriate for all that air...and you're going to see poor accuracy. Simply too much air with each tap of the valve stem.

I'm thankful that you posted. I don't have the energy for long explanations anymore. Good job!

...and I would throw in that plenums can be the wrong size to not only the porting but also to the regulator recovery time with some units.

 

[SpindleB]

What was greater accuracy?
Can you put a number to it?
1/3" became 1/4"

At 25 and 45 yards with the same gun
No reg/plenum
Reg no plenum
Reg small plenum
Reg large plenum

All I saw was the first and last were virtually the same FPS. No notable accuracy differences in any of the 4. But I was not in any way looking for competition level accuracy. My good days when things are not twitchy 45 yards 1/2" is good. My bad days barns at 25 yards are safe. I don't shoot on my bad days.

But my point stands, a large plenum and a bottle are effectively the same thing. And yes bottles decrease in pressure but the valve limits the flow and as a result for a significant period of time plenum=bottle.

 

[SpindleB]

I'm thankful that you posted. I don't have the energy for long explanations anymore. Good job!

...and I would throw in that plenums can be the wrong size to not only the porting but also to the regulator recovery time with some units.

Neither of which have anything to do with my original point.

I have only ever heard that too large a plenum caused accuracy issues. Never have I heard that an unregulated gun has the same problem.

 

[Franklink]

Neither of which have anything to do with my original point.

I have only ever heard that too large a plenum caused accuracy issues. Never have I heard that an unregulated gun has the same problem.

Because unregulated guns are either designed with an appropriately sized transfer port, or they're modified to such. The constriction of the transfer port mitigates the plenum/bottle being too large by only letting a limited amount of air make it to the back of the projectile, thereby doing away with the excess air flow, and accuracy-ruining turbulence.

Again, to really see this effect, run an unregulated Airforce gun with the full bore ring loc and something at about half the weight of max for that caliber

 

[Danman]

I have had multiple discussions with various air gunners about plenum size. There is a belief that too large a plenum will cause turbulence at the barrel end when the pellet exits. Not having ever experienced that I have always been a bit skeptical.

I realized in the middle of the night that it made absolutely no sense.

What about unregulated guns with bottles or tubes that act as massive plenums? Shouldn't that be a problem there if it is a problem with plenums?

I think this post is why pcp tuning is so important to get the best for accuracy, shot count, power, and a happy sounding pcp. You can usually tune a pcp 2+ different ways to get the same pellet speed. One tune could result in poor accuracy and one might be better with the same ammo.

Back to the Blackwolf.....it comes with the largest plenum of any Daystate yet (52cc), and they have an add-on plenum chamber(70cc) if you so desire. Soren Drost added that to his .177 slug shooting B.W. and has insane accuracy (thats 122cc of air for a .177!). That extra air capacity allowed him to lower the reg and not loose any power vs without the plenum extension. I was very surprised it would even benefit the small caliber but the proof is in the pudding. Barrel length comes into play, as does transfer port size, hammer weight, valving, spring tension, etc, etc. If I ever get the 28in slug barrel in my Brk Ghost i WILL be adding a plenum extension in the gauge port to see if it will benefit anything. It may, it may not....it all depends on the most restictive part in the system.
You said you lost your info, but do you recall the ammo weight/speeds you were getting with your 4 combos?

 

[Solo1]

I think this post is why pcp tuning is so important to get the best for accuracy, shot count, power, and a happy sounding pcp. You can usually tune a pcp 2+ different ways to get the same pellet speed. One tune could result in poor accuracy and one might be better with the same ammo.

Back to the Blackwolf.....it comes with the largest plenum of any Daystate yet (52cc), and they have an add-on plenum chamber(70cc) if you so desire. Soren Drost added that to his .177 slug shooting B.W. and has insane accuracy (thats 122cc of air for a .177!). That extra air capacity allowed him to lower the reg and not loose any power vs without the plenum extension. I was very surprised it would even benefit the small caliber but the proof is in the pudding. Barrel length comes into play, as does transfer port size, hammer weight, valving, spring tension, etc, etc. If I ever get the 28in slug barrel in my Brk Ghost i WILL be adding a plenum extension in the gauge port to see if it will benefit anything. It may, it may not....it all depends on the most restictive part in the system.
You said you lost your info, but do you recall the ammo weight/speeds you were getting with your 4 combos?

The most restrictive part i believe is the transfer port in the barrel. I was on another forum today talking to someone who measured the tp on a sub moa black wolf barrel and it is larger than my barrel and the bw brk barrel tp is larger yet! Being able to get 75 fpe with the ghost and a smaller tp to me means it could be more powerful with a larger port

 

[Danman]

The most restrictive part i believe is the transfer port in the barrel. I was on another forum today talking to someone who measured the tp on a sub moa black wolf barrel and it is larger than my barrel and the bw brk barrel tp is larger yet! Being able to get 75 fpe with the ghost and a smaller tp to me means it could be more powerful with a larger port

That possibly coupled with the 38(?)cc plenum....the stock B.W. Transfer Port is the fancy new brass (fx styled) one with a L.W. barrel attached to it. Definitely allows for easier tuneability vs drilling out your barrel port....then there's no going back. The air (in the plenum) is coming out regardless once you pull the trigger but a smaller T.P. should slow the flow down and the pellet is moving/gone before all the stored air is used as its propellant.
As-is i think the Ghost has excellent usable power, but....I am curious

 

[SpindleB]

I think this post is why pcp tuning is so important to get the best for accuracy, shot count, power, and a happy sounding pcp. You can usually tune a pcp 2+ different ways to get the same pellet speed. One tune could result in poor accuracy and one might be better with the same ammo.

Back to the Blackwolf.....it comes with the largest plenum of any Daystate yet (52cc), and they have an add-on plenum chamber(70cc) if you so desire. Soren Drost added that to his .177 slug shooting B.W. and has insane accuracy (thats 122cc of air for a .177!). That extra air capacity allowed him to lower the reg and not loose any power vs without the plenum extension. I was very surprised it would even benefit the small caliber but the proof is in the pudding. Barrel length comes into play, as does transfer port size, hammer weight, valving, spring tension, etc, etc. If I ever get the 28in slug barrel in my Brk Ghost i WILL be adding a plenum extension in the gauge port to see if it will benefit anything. It may, it may not....it all depends on the most restictive part in the system.
You said you lost your info, but do you recall the ammo weight/speeds you were getting with your 4 combos?

Speed would vary a lot between the setups.

Started out using H&N Baracuda 30.86gr but switched to JTS 25.39gr.

Currently the .25 Condor is shooting JTS 25.39gr at 1,000 FPS with just 1700psi.

Same plenum setup on the .22 18" Talon is 850FPS @ 1700psi with H&N Baracuda Match 21.14gr

That extra 6" on the barrel makes a very big difference