Why does a bigger plenum allow the same results with lower reg pressure?

[Glem.Chally]

Oldspook, thanks for your useful response.

Ok, I can see that a larger plenum, combined with a short barrel, is not the answer for more velocity. Then, this would be my second question: with the same short barrel, if I add a larger plenum and keep the fps the same, would the shot count increase? If so, any estimate of how many more shots could be expected, relative to the original tune/configuration?

@cilami I tend to disagree with the answer you received above. I don’t do this from theory but from experience. Here’s why I KNOW, not THINK that a plenum in a shorter barreled gun will increase power and/or efficiency (shot count)

On my .22 Cricket mini Carbine, with a 15.3 inch barrel, I was shooting the JSB 18.1 at 880 FPS and getting three magazines (42 shots) per 250b fill. I wasn’t satisfied with that and had a Cricket power plenum lying around that I had gotten for my .30 Vulcan Copperhead project. (I only ended up using one of the two that I bought). So I installed the plenum in my Cricket mini. No other changes. The gun then shot the 18.1s at 905 to 910 FPS. So I backed off the HST to shoot at 880 FPS where it is super accurate and now get 4+ magazines per 250b fill, or over 54 shots plus a few more before speed drops off. So that’s my experience with plenums and short barreled rifles

That is interesting! Thanks for posting your experience. Unfortunately I don't have the knowledge to understand why that "discrepancy" between the theory and your results with the Cricket. To add the larger plenum to my Taipan Mutant shortie will require to buy a unf adapter plus a new LDC plus the plenum itself...$200+. Not willing to invest that amount of money to experiment...

Its not an experiment if its what most people would expect to happen. (More power, ability to lower reg pressure, higher shot count) a combination of the three or can tune to get 2 or 1 of those options.

The question would be , what result most people would expect to happen? So far it is a tie 1-1 (that's assuming no valve tuning etc.. Just adding the plenum)

Why are you kneecaping it by not tuning it for the change in parts? Even with that i bet the majority would assume it would shoot faster at the same reg pressure and/or suddenly be using too much hammer spring. 

Read the title of the thread its not some crazy experiment.

 

[JungleShooter]

Sooo, if larger plenums are so beneficial... ➔ WHY on earth do airgun manufacturers make such dinky plenums?!



I mean, adding a couple of inches shouldn't cost the manufacturer the world.

But it costs me $100 @ Huma and $70 from Lee for the Skyhawk.

Now, if the manufacturers (like FX) were selling us the extended plenums, I'd understand the scheme.... — But they usually do not sell larger plenums.



Just wondering....

Matthias

 

[cilami]

Glem.Chally wrote:

Why are you kneecaping it by not tuning it for the change in parts? Even with that i bet the majority would assume it would shoot faster at the same reg pressure and/or suddenly be using too much hammer spring. 

Read the title of the thread its not some crazy experiment.



I agree this is not an experiment. Plenty of posts and videos proving and explaining it. Again, where things are not so clear to me is when you are dealing with a 14" barrel. My Mutant is a great gun, shooting great and, although I have some experience modifying a Marauder pistol and a Crosman 2240, I am not inclined to alter my almost "perfect" Mutant, more than some plug and play addition.. Maybe when I get my next airgun, the Mutant will become my new guinea pig.

 

[cilami]

Will a bigger plenum allow the same results with a lower regulator pressure in a shorter barrel, like 350 mm? I would like to know if it would be worth to add a plenum to my Taipan Mutant short.

Any of you calculus guys know if there is an exact way to figure out the exact plenum size for a given barrel. I would assume you would have to find the volume of the barrel then that would be the size plenum you need?



Where is Bob Sterne? 

 

[Pale_Rider]

Will a bigger plenum allow the same results with a lower regulator pressure in a shorter barrel, like 350 mm? I would like to know if it would be worth to add a plenum to my Taipan Mutant short.

Any of you calculus guys know if there is an exact way to figure out the exact plenum size for a given barrel. I would assume you would have to find the volume of the barrel then that would be the size plenum you need?



Where is Bob Sterne? 

 

[tor47]

Bob Sterne has a good article about this https://hardairmagazine.com/ham-columns/introduction-to-the-pcp-regulator-dont-ignore-the-plenum/

According to him 1/2cc to 1cc of plenum per fpe, seems to be a nice rule of thumb. More than 1cc per fpe seems to gain litle, while 1/2cc per fpe seems to be the minimum recommended.

 

[Pale_Rider]

Bob Sterne has a good article about this https://hardairmagazine.com/ham-columns/introduction-to-the-pcp-regulator-dont-ignore-the-plenum/

According to him 1/2cc to 1cc of plenum per fpe, seems to be a nice rule of thumb. More than 1cc per fpe seems to gain litle, while 1/2cc per fpe seems to be the minimum recommended.

Great find, great read. Thanks

 

[nervoustrig]

A quick comment on the 0.5 – 1.0 cc per fpe guidance…

The 1.0cc/fpe figure tends to apply most readily to setups like bottle conversions and add-on plenums.

For in-tube regulators, there is a tradeoff to consider. Devoting that much volume for the plenum eats up valuable reservoir volume which reduces the potential shot count. In that case, something closer to 0.5cc per fpe tends to balance out better.

 

[Glem.Chally]

Sooo, if larger plenums are so beneficial... ➔ WHY on earth do airgun manufacturers make such dinky plenums?!

I mean, adding a couple of inches shouldn't cost the manufacturer the world.

But it costs me $100 @ Huma and $70 from Lee for the Skyhawk.

Now, if the manufacturers (like FX) were selling us the extended plenums, I'd understand the scheme....

— But they usually do not sell larger plenums.



Just wondering....

Matthias

Probably an evolution of airguns, moving forward i bet more will be part of the design.

 

[cilami]

A quick comment on the 0.5 – 1.0 cc per fpe guidance…

The 1.0cc/fpe figure tends to apply most readily to setups like bottle conversions and add-on plenums.

For in-tube regulators, there is a tradeoff to consider. Devoting that much volume for the plenum eats up valuable reservoir volume which reduces the potential shot count. In that case, something closer to 0.5cc per fpe tends to balance out better.

So, besides the controversial barrel length, this is another argument against adding a larger plenum to the Mutant shortie, especially if the goal is increase the shot count, not the power, correct?

 

[Pale_Rider]

In this thread by @motorhead (https://www.gatewaytoairguns.org/GTA/index.php?topic=112498.msg1097397#msg1097397), he says the oem plenum spacer was .700”. That was for a standard (in 2016). Yours being a shorty might have a smaller plenum. Do you know how long the plenum spacer is? You should get an increase of 40 fps adding a plenum that is .225” longer(according to Motörhead’s experimenting). Then if you backed off the HS or lowered reg pressure to get the original fps you would get your extra shots. I’m curious what would give more shots; backing off the HS or lowering the reg?

 

[Centercut]

I’m curious what would give more shots; backing off the HS or lowering the reg?

It depends. Usually to get the gun in tune with the plenum you would need to lower the reg pressure and possibly back off the hammer. But you would be shooting at the same FPE and get significantly improved shot count (efficiency). The plenum also increases efficiency, since the plenum volume is larger, the plenum doesn’t lower in pressure as much, so the higher after shot pressure functions to shut the firing valve quicker. Win win with the power plenum. When I added a power plenum to my .30 Vulcan2, the efficiency increased by 50% after I properly tuned the gun.

 

[Pale_Rider]

I’m curious what would give more shots; backing off the HS or lowering the reg?

It depends. Usually to get the gun in tune with the plenum you would need to lower the reg pressure and possibly back off the hammer. But you would be shooting at the same FPE and get significantly improved shot count (efficiency). The plenum also increases efficiency, since the plenum volume is larger, the plenum doesn’t lower in pressure as much, so the higher after shot pressure functions to shut the firing valve quicker. Win win with the power plenum. When I added a power plenum to my .30 Vulcan2, the efficiency increased by 50%.

That’s an incredible increase. 
Besides losing reservoir space in a tube gun, have you experienced any negative effects from adding too much plenum?

 

[Centercut]

That’s an incredible increase. 
Besides losing reservoir space in a tube gun, have you experienced any negative effects from adding too much plenum?

I didn’t lose any reservoir space since I used an external plenum. It does make your air tube longer, maybe 1 3/4 inches. My overall plenum size is comparable to the newer FX Impact power plenum, and overall efficiency is also about the same. 

 

[MushroomPsyche]

So, besides the controversial barrel length, this is another argument against adding a larger plenum to the Mutant shortie, especially if the goal is increase the shot count, not the power, correct?

In general this is true but the plenums being sold for the Veteran (which I assume to be the same for the Mutant) actually adds the length of the plenum to the air cylinder instead of just replacing the spacer for a longer one to move the regulator further into the tube.. 

 

[cilami]

 Centercut wrote: The plenum also increases efficiency, since the plenum volume is larger, the plenum doesn’t lower in pressure as much, so the higher after shot pressure functions to shut the firing valve quicker. Win win with the power plenum. When I added a power plenum to my .30 Vulcan2, the efficiency increased by 50%.

!