Power plenum and low power shots.

Caliber 22 said:

Theory - Power plenum is a negative for low powered shooting.
What is your experiences?

Some time ago I installed a power plenum on my pcp for the obvious purpose of shooting heavy pellets and slugs at higher speeds. After using it for a while I moved away from heavy pellets and especially slugs and use mostly 18gr pellets.

I then wanted to shoot light 14gr pellets at lower speeds but did not lower the regulator pressure. - Yes I know I should but it was set perfectly for 18gr which is my main pellets and I did not want to change it. - What I experienced is that it would have a big extreme spread in speed. My theory is that once the regulator let air through to the power plenum, the air consumption per shot is to low for the plenum pressure to drop enough for the regulator to open after each shot. It takes a 5 shot cycle for the regulator to open again. Because of this the pressure and thus speed goes up and down and give vertical stringing. Once the regulator open, the speed is lower because of lower hammer, ,remember the regulator is set for 18gr at higher speed, and speed increases as the plenum pressure drop. Once the regulator open again the speed drop again. Speed confirmed by chronograph.

That is why one should have a smaller plenum and adjust the regulator for the lower power if that is what your goal is, like for pesting in a barn or similar situation.

What do you think about this theory?

Click to expand...

That's interesting, did you adjust the hammer to shoot the lighter pellets? It didn't read as if you had done so.

If so, doesn't it take a few shots for the new hammer setting and speed to settle in?

I did encounter something like that a while when I had my impact going from shooting 30gr slugs to shooting 18gr pellets, I never thought to attribute it to the plenum.

All you have to do is lower your regulator pressure.

Or choke the porting.

You'd lack consistency no different with a smaller plenum if you went from shooting 18 gr at its ideal settings and dropped your pellet weight to 14 gr and also dropped velocity to well below the knee. Your issue more-so is the guns tune than the power plenum...

-Matt

Yes, if taking a very small sip of air from a large plenum, the pressure drop may be insufficient to open the regulator’s valve seat a meaningful amount. However if the effect is a wide ES, it is an indication of a system imbalance…something that can very likely be remedied or improved substantially by adjusting the hammer strike and/or by changing the regulator setpoint.

Side bar…consider that a well-tuned unregulated PCP will hold a stable velocity over a wide pressure range. What is an unregulated PCP? It is a regulated one with a gigantic plenum whose regulator doesn’t work very well / doesn’t cycle after each shot. Yet we have many examples that hold a stable velocity over a pressure range of 500psi (35 bar) or so. Suffice it to say, no functioning regulator has a hysteresis of 500psi so a huge plenum is not the problem in and of itself.

With that said, it won’t always be possible to bring the system into good balance by simply adjusting the hammer spring tension. Especially where the goal is to re-tune to a vastly different energy level. For example the hammer itself may be way too heavy or way too light to produce a good result. Or the regulator setpoint may be too too high or too low, or porting may be way too large or way too small. These sorts of things can make it difficult to get a good blend of lift and dwell at a power level very different from the range to which it was originally designed. But since HST is generally one of the easiest things to manipulate, it’s a good thing to explore first. In your particular scenario where you’re presumably dropping the power a considerable amount, you’ll almost certainly want to reduce the regulator’s setpoint as Matt described.

If your regulator is not cycling after you shoot, the more likely culprit is that the regulator is experiencing creep beyond its baseline setpoint, and you have to shoot it below that for it to cycle.

Smaller plenums that experience creep need less shots than larger plenums that experience creep to restore the gun to baseline set point when tuned to incredibly low powers in a poor state. When tuned at a lower power, and you also lower the regulator pressure to an ideal setting for the power so more air is used during the shot cycle, you restore balance, similarly to a smaller plenum or reducing plenum volume would do.

-Matt

I have a couple guns with plenums around 35cc but that is not very large these days. A 60cc plenum is not uncommon which is about the size of the entire air storage in my Prod. I saw an assertion that the right size for the plenum is about 1cc for each fpe of the tune. So a 60cc plenum would be good for a 60 fpe gun. That seems reasonable to me. One of my guns with a 35 cc plenum is a P35-25 that came with about a 10cc plenum. When I increased it to 35cc it added significant velocity to the JSB 34 grain pellets it likes. I think it could use even more plenum volume.

But trying to decrease velocity exclusively by using the hammer spring doesn't work well for me. I think that is basically what is happening to the OP. I get low velocity on the first shot sometimes when I try to reduce velocity by more than about 5% using the hammer spring. I wouldn't be surprised to see the ES or standard deviation affected too. I do not have enough experience with big plenums to know if they make things worse but I do see a little bit low velocity on my Caiman X which is my other gun with a 35cc plenum and I have it's hammer spring a little more than 5% below the peak velocity.

JimD said:

I have a couple guns with plenums around 35cc but that is not very large these days. A 60cc plenum is not uncommon which is about the size of the entire air storage in my Prod. I saw an assertion that the right size for the plenum is about 1cc for each fpe of the tune. So a 60cc plenum would be good for a 60 fpe gun. That seems reasonable to me. One of my guns with a 35 cc plenum is a P35-25 that came with about a 10cc plenum. When I increased it to 35cc it added significant velocity to the JSB 34 grain pellets it likes. I think it could use even more plenum volume.

But trying to decrease velocity exclusively by using the hammer spring doesn't work well for me. I think that is basically what is happening to the OP. I get low velocity on the first shot sometimes when I try to reduce velocity by more than about 5% using the hammer spring. I wouldn't be surprised to see the ES or standard deviation affected too. I do not have enough experience with big plenums to know if they make things worse but I do see a little bit low velocity on my Caiman X which is my other gun with a 35cc plenum and I have it's hammer spring a little more than 5% below the peak velocity.

Click to expand...

Going over 5% below your plateau is asking for what you're describing. Regulated PCP's really perform their best at around 2-3% below plateau...the further from that you go the more variation in velocity you will see. There are a few ways to manage/mitigate that but ultimately its just the nature of regulated pcp's, unless they digitize them and make each shot 100% the same pressure output, basically 'relieving' any creep which in many cases would be a permanent leak...we're doomed to be better off tuning 2-3% from plateau.

-Matt

Quick run down how creep / regulator setting plays into this so intimately, more importantly residual hammer energy after cracking the valve, which also applies to balanced valves and stiction.

At 2,000 psi, a .22 caliber airgun conventionally may have 190bs of force holding it shut. A balanced valve may take that down to 114 lbs when balanced around 60%. A pilot valve such as the one in my gun has 24 lbs on the pilot which is the only concern for the below.

If you experience even just 5% creep, you go from 190 lbs to 199 lbs holding it shut conventionally, 114 to119.5lbs balanced, and from 24 to 25 lbs with the pilot valve in my gun.

The hammer/spring then has to overcome the extra 9, 5, or 1 lb of force holding the valve shut. Balanced valves also have to overcome stiction regardless of how well made/tuned they are due to the nature of dynamic o-rings with a large pressure delta on either side, conventional and pilot valves do not, so balanced valves add another variable into the equation that is not favorable for extreme spread.

When tuned closer to plateau, you're imparting more hammer energy, roughly at least 15-20% more. Your plateau at 2000 psi may be 900 fps, but at 2100 psi its 920 fps. So if you tune to 98% which makes for about 882 FPS, and after sitting, your hammer has to overcome 2100 psi which with more energy going into cracking the valve then creating lift dwell, you'll have reduced power, but alas with 20 more fps on top for plateau, it shouldn't be greatly reduced, since your hammer energy is then about 4% away from plateau, and still within reason, very likely your fps variability remains within reason as well, because cracking the valve consumed a bit of the hammers energy, the remaining energy to create lift is enough to make comparable power at 2100 psi as it was at 2000 psi because the exchange in slightly reduced lift with less remaining hammer energy is compensated by the extra potential 100 psi flowing passed the poppet.

Now if you go and tune 6% from plateau, that means you're around 846 fps, your reg creeps to 2100 psi, your slightly reduced hammer energy means that there is less left over energy to create enough lift at the 2100 psi mark, thus throwing the reduced lift and extra air release out of balance, to the point the extra air released can't compensate for the reduced lift and you experience a wider spread/ES.

-Matt

Caliber 22 said:

I did mentioned that and the reason why I did not. What I can add here is that the regulator is in the air tube and a big hassle to adjust.

My rifle cannot do it.

Before I installed the power plenum it was not an issue, I did use it like that then and the consistency was good with the lower weight pellets without adjusting the regulator but just the hammer.

Click to expand...

Before you installed the power plenum you couldn't make the same power you're at now, so you can either sacrifice gain in power by lowering your regulator setting to the same power you had without a power plenum, which will restore harmony for the lower power tune, or increase (and lower expectations on) your lower power tune until its back in harmony.

Larger plenums simply allow you to run less pressure to achieve the same or more power, you can compensate an increase of plenum volume with a decrease in plenum pressure.

-Matt

I sort of agree with your theory OP. Realistically, if your performance is not able to be achieved, the only 2 options would be to lower the reg or remove the plenum. But only need to do one. I’d keep the plenum and lower the reg. This enables you to enjoy both pellet weights optimally by adjusting hammer spring and also enjoy high shot counts.

Caliber 22 said:

This rifle's regulator as it is now can sit for two days without creep. With 18gr pellets the first shot +- 10 - 15fps is the same as the last before it was last time before storage.

Click to expand...

Really in no scenario outside of creep should your regulator not cycle, only cycle too slow, or erratically from shot to shot resulting in various pressures.

It may take a long while to refresh a very small sip of air back to baseline, so try allowing more time between the shot cycle if you're rapid firing across a chronograph.

-Matt

Caliber 22 said:

Like I said before, I don't want to lower the reg, it is perfect for 18gr pellets. To lower the internal regulator I must disassemble the rifle to get to the regulator inside the tube. Also, I don't want to shoot 14gr as the permanent choice of pellet, that is what I use 18gr for.

Click to expand...

I understand, you would most likely have to disassemble the rifle to remove the plenum. Figured it was same work. Otherwise it seems either you accept the current results or ditch the 14.3 and go 15.9g which would be closer to the 18g in performance.