Tuning Effect of Plenum Size?

[ivan888]

I am glad to read that other people have so much knowledge and more even are willing to share to each of us who like to scratch their head thinking why and how things happen.

Like many before me i have also put some testing into plenum size about a year ago and am happy to report that indeed increasing plenum alone touching nothing else made the valve cycle shorter (when prior tuned at knee) but alowed for higher energy potential (if no other bottleneck is present).

I didn't yet analyse the general rule but one thing that i observed in my testing was that by increasing working pressure the plenum capacity should also increase for a benefit in potential energy. I cannot however say by how much or come by a general rule myself because there are only 3 volume arrangements i had at the time.

The tests were made in a FX maverick with different barrel arrangements and hammer was adjusted in each case for max energy respectively at 2 different working pressures.
P stands for the hammer spring position and had enough travel to accommodate all barrels plateau levels.

Maverick energy @120bar
120bar .177 700mm
Zan 20gr plenum 32cc 910 p15 49j
Zan 20gr plenum 89cc 970 p16 56j
Zan 20gr plenum 146cc 970 p16 56j

120bar .22 800mm
HN 30gr plenum 89cc 980 p16 86j
HN 30gr plenum 146cc 980 p16 86j

120bar .25 600mm
Zan 41gr plenum 89cc 856 p17 90j
Zan 41gr plenum 146cc 870 p17 93j

120bar .30 700mm
Zan 68gr plenum 89cc 823 p17 138j
Zan 68gr plenum 146cc 845 p18 146j

Maverick energy @135bar
135bar .177 700mm
Zan 20gr plenum 89cc 1000 p15 60j
Zan 20gr plenum 146cc 1007 p15 61j

135bar .22 800mm
Zan 40gr plenum 89cc 905 p19 98j
Zan 40gr plenum 146cc 930 p19 104j

135bar .25 600mm
Zan 41gr plenum 89cc 900 p19 100j
Zan 41gr plenum 146cc 907 p18 101j

135bar .30 700mm
Zan 68gr plenum 89cc 845 p19 146j
Zan 68gr plenum 146cc 868 p20 154j

Sorry if for most barrel arrangements there is a test with the smallest 32cc plenum test missing but at the time I was aiming to discover what was the smallest vilume i needed for each barrel.

Hello. Can you share more information about 135bar .22 800mm -Zan 40gr plenum 146cc 930 p19 104j. What is working pressure. What hammer did you use? What a spring. Other actions you have taken?

 

[Trucker3573]

I have actually noticed #3 in my early maverick compact when set to low fpe for shooting in the front yard. I was punching paper at 50yards and it was extremely inconsistent, then I noticed I didn’t hear the plenum fill after every shot. I started turning up the hammer until it I heard it every shot and it became more consistent.

Yes I have experienced this at times in both my maverick and m3. Mostly when the gun sits for some time and the reg pressure goes up 3-5 bar. The first shot doesn’t trigger a plenum refill so the second shot is much lower velocity due to the lower plenum pressure.

 

[Ignoto]

Hello. Can you share more information about 135bar .22 800mm -Zan 40gr plenum 146cc 930 p19 104j. What is working pressure. What hammer did you use? What a spring. Other actions you have taken?

The pressure is has implied 135bar for the stated barel caliber and lenght with the mentioned projectile and is data i have collected purely so that we can have a guideline of when more plenum is actually beneficial and how much.
Fps value was achieved with the hammer adjusted for its plateau aka max power for given pressure.

What you look for is probably a tune for your PCP wich would not be the same as mine even if it had the same parts and same pressure.
but if you are curious you can consult my testing setup here: https://www.airgunnation.com/threads/fx-maverick-full-beans.1176711/

 

[Ignoto]

The biggest advantage of a large Plenum is when tuning for high fpe. Example my FX crown has a rather small plenum so when you want to shoot slugs it soon hits max.... And its also getting airhungry.. The promblem is you set the reg. To like 150 bar. With a small plenum and high fps you hit the valve hard with the hammer.... the problem is the plenum pressure drops a lot. And by the low plenum pressure there is reduced helping force from the remaining air pressure in the plenum to close the valve again. This means the valve is open too long time.... this means waste of air and further pressure drop, and dirty air behind the projectile.... the double size Plenum means the remaining air pressure will only drop less than half for the same slug speed.... But this differece is less notible with air riflees with balanced valves ....

I wouldn´t call it air hungry as long as you dont go too close to plateau levels like you mentioned, but you definitely get fewer shots per fill with higher reg pressures.
My experience tells me that the higher the regulator and\or the plenum= more efficient the air usage will be.
As for the rest of your statement i completely agree, have a Crown mk1 and it is the most forgiving and tuneable pcp i used as far as extreme spread is a concern (not my thing though unless shooting at ELR)

I'm not sure what your data says, but it seems to me that with the exception of the 68gr .30, you always had too much plenum with the 146cc size. Wouldn't we need the shot string numbers to see the fluctuations to see if that were so?

As for extreme spreads mentioned before i would say it depends like @nervoustrig stated

''3. For exceedingly large plenums, there is a potential that firing will consume so little air as to not produce a sufficient pressure drop to cause the regulator to cycle open. The extreme spread may be negatively affected as a result.''

it will negatively affect extreme spread by augmenting the amount of hammer strike variance between shots and if the regulator does not cycle between every shot.
I can say it is possible to shoot 67gr slugs .25 @890 and at the same time 13gr .177 @920 and have 10~20fps max extreme spread at lower energy with 146cc at 140bar as long has you have a lubed and polished clock. (I stopped using wildcat\maverick hammer wheels for this reason) if you want to optimise the rifle you must be prepared to try out different setups and like what you are doing..