Maximizing power for a minimal noise level

[jackssmirkingrevenge]

When I first joined this forum I thought I knew most of what there was to know about pneumatics but it was quickly apparent that I had merely scratched the surface.  I've been poring over Bob's series of technical articles that look at the various parameters involved and the effects of tweaking them and think I have come to the right conclusions but I defer to the more knowledgeable to check my working.

I'm looking to tune the most basic of PCPs to give the most power it can while being quiet enough for backyard plinking without disturbing the neighbors, and without the assistance of any sort of muzzle device.

In order to meet the noise requirements, it has to have the lowest possible pressure at the muzzle, so an SSG of some description is going to be necessary to open the valve only as much as is needed.  I would however like to squeeze as much FPE as possible, so my reasoning would be that you want barrel pressure to jump as fast as possible at the breech end and taper off rapidly as the pellet is about to leave the muzzle.

In that regard, would it make sense to maximize airflow in terms of valves and transfer ports, then increase efficiency and reduce noise by just tapping that valve more lightly?  To me it seems that in general a high flow valve that opens for a short interval is going to give better performance than a low flow valve that opens for a long interval, for the same volume of air used.  I would be interested to hear some thoughts on the matter. 

[lloyd-ss]

Jack, That is the cool thing about these airguns: the deeper you dig, the more complex you find out they are. Or you can just shoot them and forget about it, LOL.
You know that you have asked a very complex question, correct? Muzzle pressure, caliber, plenum volume, and velocity, are a few of the factors that affect the sound. Think of how loud a popped balloon is. Maybe a couple of PSI, but lots of volume and maybe a very large caliber?? And probably too loud to be backyard friendly.
But you can easily control the muzzle pressure, and you are correct, a valve that has a quick square wave (ideal world) function would probably be best.
Here are a few internal ballistics graphs (calculated, but probably similar to real-world) to ponder. Barrel length is the horizontal axis. Pressure (blue)  and velocity (green) are the right vertical axis. FPE is the red left vertical axis.

The cusp in the blue line is when the valve closes. The third chart shows the effects of a small plenum. You can also surmise how keeping the valve open overly long only wastes air and makes noise.
I hope this helps.
Lloyd

Visualize some what-ifs in your head.

[jackssmirkingrevenge]

Thank you Lloyd, appreciate the response.

Muzzle pressure, caliber, plenum volume, and velocity, are a few of the factors that affect the sound.

You're right, in this case I focused on pressure at the muzzle as it seems to be the most tangible parameter, as well as being a reflection of efficiency which is inversely proportional to sound levels if one understands muzzle noise to correspond to wasted energy.

Your first two charts more or less confirm what I had in my head, that higher pressure at the breech with a valve that re-seals faster has the potential to give both higher FPE and lower pressure at the muzzle.

With a small plenum it would seem that the pressure tapers off significantly faster, therefore I would surmise that one is best off with a large plenum and focus on having the hammer as tunable as possible in order to control opening time.

Is one better off with a stiffer valve spring in order to help it to re-seal faster, or will the corresponding need for a greater force to knock the valve open negate any benefit?

[rsterne]

Jack, you are correct that for a given FPE, using a small sip of high pressure is more efficient, and quieter, than a larger gulp of low pressure.... Basically the shorter the valve dwell, the closer to the breech the pellet is when the valve closes.... and the more room for the air to expand as the pellet heads towards the exit.... Therefore, larger ports and a shorter dwell, should be more efficient, and quieter, for a given FPE at a given pressure.... because you are releasing more HPA in a shorter time, giving it longer to expand (and producing a lower muzzle pressure)….

Smaller plenums cause the pressure to drop quicker, either reducing the FPE, or requiring a higher pressure to get the same power.... While the effect can be calculated and measured even for unregulated PCPs (compared to an infinite reservoir volume)…. it becomes very important when the plenum is less than about 1 cc per FPE.... because that is when the pressure drop during the valve open stage becomes really significant.... Here is a graph showing a gun like an MRod, and what happens to the FPE as you reduce the plenum volume....



The red dot is 1 cc per FPE....

Bob

[jackssmirkingrevenge]

Thanks Bob!  Good to know I'm on the right track

I have a used 0.22" Maximus on the way that's going to be the basis for these ideas to be fleshed out.

I understand that adding a third screw to the valve block will allow higher pressures to be safely used which would correspond to higher FPE potential.

[jackssmirkingrevenge]

With the goal of having the valve open quickly for a short interval, would it make sense to go for a lighter hammer than the stock, or can the same effect be achieved with a weaker spring?

[rsterne]

Too generic a question.... In some guns you may need a heavy hammer and strong spring, in others you could use a light hammer and light spring.... However, lightening the hammer changes the relationship between hammer energy and momentum.... For a given spring force and hammer travel, you get the same hammer energy, but less momentum, with a light hammer.... Cracking the valve off the seat uses up energy and reduces the hammer velocity, hence also its momentum.... Lift depends on residual hammer energy, whereas dwell depends on residual momentum.... so a lighter hammer (same spring and travel) should leave the lift unchanged but reduce the dwell.... Providing you have enough lift times dwell (the area under the flow curve) to provide the FPE you want, and providing the ports are large enough to allow the required air to move in the dwell you end up with.... then a lighter hammer MAY produce more efficiency....

For a given FPE, using a lighter hammer will generally require a bit stronger hammer spring.... If you are having difficulty getting to the plateau (or at least to the FPE you want), without the gun getting hard to cock.... then try a heavier hammer.... If cocking is a breeze, then you should be able to use a lighter hammer....

Bob

[jackssmirkingrevenge]

Thanks Bob, looks like there's going to be some trial and error in my immediate future.  I will start with the standard weight and go from there.

[Dairyboy]

Problem with the Maximus is getting it backyard friendly without any LDC/Shroud of sorts you will likely need to be sub 12fpe. Regardless of how efficient it is it will have a pop to it with nothing.

[lloyd-ss]

Bob, Those 2 paragraphs in reply # 6 are loaded with information like a water balloon that is ready to explode. As many times as we've discussed this, I still have to read it a couple of times before I shake my head "yes".  Well said.

Michael, the hammer weight and spring, the return spring, the valve seat & valve stem diameters, reservoir pressure, all heavily influence the shape of the air pulse. Plus, there are about that many more influencing factors. Yes, a lot of it is trial and error, but as you can see Bob has quantified and documented that trial and error.
And each tuner probably has a few favorite winning combinations .

[jackssmirkingrevenge]

Problem with the Maximus is getting it backyard friendly without any LDC/Shroud of sorts you will likely need to be sub 12fpe. Regardless of how efficient it is it will have a pop to it with nothing.

That might turn out to be the case but I'm willing to see how far I can go only by tweaking the internal ballistics.

Yes, a lot of it is trial and error, but as you can see Bob has quantified and documented that trial and error.
And each tuner probably has a few favorite winning combinations .

Yes, definitely standing on the shoulders of giants here.

The plan is to max out flow from valve to probe, as well as plenum volume, then build an adjustable SSG for tuning purposes.

[jackssmirkingrevenge]

Picked up the Maximus today and was pleasantly surprised, it's no Daystate but definitely on point for the price!

I had 14.3g Crosman pellets to hand so a shot string through the chrony was in order:



What surprised me was the noise, it started as "this isn't half as bad as I was expected" and progressed to "maybe hearing protection is a good idea" about 60% of the way through.  The logical explanation is that as pressure in the chamber goes down, resistance to the hammer decreases and valve dwell time increases.

That gives me a starting point, now to tear it apart and start changing things.  I'll also need to get some heavier pellets.

[lloyd-ss]

Jack, I can see that you are going into this with plenty of enthusiasm and are planning on learning by doing. Excellent!
A couple of things about the Maximus limit its tuneability, but I think you already know that and have some thoughts about what to do. One missing link is an adjustable hammer spring, but you already mentioned an SSG, so you have a plan there. Another issue (for some folks, anyway) is the non-adjustability of the hammer travel, i.e., no adjustable striker. The P-rod has an adjustable striker and should be a drop-in fit. Also, the t-port is fixed in size, but you can modify that as needed. The reason I mention this is because many years ago, when Crosman introduced the Marauder rifle, it was one of the first moderately priced, readily available "nice" air rifles that came with all those tuning features built-in. Plus the excellent trigger. There were enough adjustments on the gun that you could easily mess it up enough to make it shoot poorly, or unsafely.

But Crosman was working with Hans and Ray Apelles to turn the Marauder into an exceptional rifle (for the price) for the masses. They came up with a very detailed tuning procedure that could be followed by a serious M-Rod owner, to achieve whatever results they were willing to invest the time in.

There work is still valid today and is worth a thorough read for a serious tuner. Even though your goal isn't exactly like any of the many that they talk about, the theory and procedure should be very helpful, and adaptable. 

Here is a link to a long article on another forum, and about half way thru the thread, somebody has posted a link to the "A-Team Tuning Guide." Take a look.
https://www.airgunnation.com/topic/tuning-a-marauder-what-to-adjust-and-where-to-start/

[rsterne]

Jack, you have discovered one of the curious things about unregulated PCPs, that is proof positive of the better efficiency of small sips of higher pressure.... My Discovery, the first PCP I had (and very similar to the Maximus) was exactly the same.... not too loud for the first few shot, then gradually louder and by the end of the string you needed hearing protection indoors, for sure.... especially if you shot it down to 1000 psi....

Bob

[jackssmirkingrevenge]

There work is still valid today and is worth a thorough read for a serious tuner. Even though your goal isn't exactly like any of the many that they talk about, the theory and procedure should be very helpful, and adaptable.

Thank you for the resources!  I appreciate that the Marauder is more tunable out of the box but since I'm reasonably handy with a lathe and mill, I elected to go for the Maximus and manufacture/modify my own parts if necessary.

My Discovery, the first PCP I had (and very similar to the Maximus) was exactly the same...

I had found this thread while doing some research, hard to believe you've only been at this for a decade.  I suppose that when you're inquisitive and meticulous by nature, the result is the same no matter what you apply yourself to.

The Maximus is now in pieces and I've been studying the parts.

The first thing I noticed is that the previous owner seems to have had a go at "tuning" himself:



Compared to this image of a standard valve, the transfer port seems to have been somewhat ineptly drilled out, and the valve stem diameter has been reduced.



My original plan was to re-make most components but after taking some measurements it seems that modifying the existing ones should do. 11/64" porting at the barrel sounds about right going by the 75% of caliber rule of thumb, and working backwards from there towards the valve the only work that needs to be done is properly drilling the transfer port and making a new transfer port sleeve.  I'm thinking of making the latter in Delrin since a synthetic material should give a better seal.

I will also drill out the gauge block to 11/64", off-center to avoid weakening the component.

Is there any benefit to turning down the end of the pellet probe?

One missing link is an adjustable hammer spring, but you already mentioned an SSG, so you have a plan there. Another issue (for some folks, anyway) is the non-adjustability of the hammer travel, i.e., no adjustable striker.

Something to keep in mind, thanks.  I will start with the SSG and go from there.

[lloyd-ss]

Re: bolt probe. You should only need to turn the probe down if you are looking for medium to high power and the cross sectional  area calculations show that the bolt probe is causing a choke point in the flow. (No reference to "choked flow" intended.)

Very good that you are handy with lathe and mill. That makes the possibilities almost limitless.

I don't know if you have bought spare parts from Crosman, but they are extremely reasonable in price (cheaper than you can make them even if you are only charging yourself $1/hr, LOL) and they charge a flat shipping fee of maybe $7 per order.

[SILENT SQUIRREL]

Hello gentlemen
Despite being a novice, I've find the logic & the details comprehensible
Thanks to each of you for writing clearly.
My question is one of terminology
It seems to me an SSG is  also referred to as an FFL (Hamer free flight adjuster)
Is that correct?

Thanks
Ed

[jackssmirkingrevenge]

It seems to me an SSG is  also referred to as an FFL (Hamer free flight adjuster)

https://hardairmagazine.com/ham-columns/conserving-air-in-pcps-hammer-bounce-and-the-ssg/

My understanding is that the SSG actively arrests the spring while a "Free Flight Hammer" uses a spring shorter than the hammer travel, what Bob refers to as the "Short Stiff Spring (SSS)" in his article.

[SILENT SQUIRREL]

It seems to me an SSG is  also referred to as an FFL (Hamer free flight adjuster)

https://hardairmagazine.com/ham-columns/conserving-air-in-pcps-hammer-bounce-and-the-ssg/

My understanding is that the SSG actively arrests the spring while a "Free Flight Hammer" uses a spring shorter than the hammer travel, what Bob refers to as the "Short Stiff Spring (SSS)" in his article.

Thank you,  jackssmirkingrevenge
Upon rereading, and following the link provided, it appears as if you are correct
There's a lot to be said for learning by asking questions

[jackssmirkingrevenge]

Some progress, made the following modifications:

- gauge block drilled out off-center to 7/32"
- valve block porting drilled out to 11/64"
- barrel porting drilled out to 11/64"
- new Delrin transfer port bushing with 11/64" ID

I then put another shot string to the chrony, with the same 14.3 grain Crosman Premier Domed Ultra Magnum pellets and going from 2000 psi to 1250 psi:



I got the same number of shots for the same pressure drop, assuming the rifle's pressure gauge is accurate.  Initially unimpressed by the velocity readings but as you can see, muzzle energy climbed steadily to almost 30 ft lbs.

There was an increase in perceived muzzle noise as the pressure decreased, but interestingly it was nowhere near the levels I experienced with the first string and I felt no compulsion to reach for my hearing protection.

One thing that is bothering me is the hammer.  There is no pretension in the spring at all, in fact it is loose between the hammer and the rear endcap and there is about 0.20" of hammer travel between the end of the hammer touching the valve stem and the rear of the hammer touching the spring.  Essentially it's a Short Stiff Spring setup.  Is this normal for the Maximus out of the box?  I have no memory of it being like that when I disassembled it but I'm pretty sure I put it back together correctly and no parts are missing.  The rear endcap has a threaded hole but it's empty and there were no screws in there when I received it.  It also feels different to cock but that might be because I did not bother attaching the stock for this second string.