GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Lathaniel on March 05, 2022, 02:05:44 AM
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I am working towards producing 60+ FPE on my .22 Leviathan using DIY/ low cost mods.thebudea is to make 60FPE accessible within a cheap platform, staying true to the cheap platform and not adding $100's to the price. Thus far I have got it just shy of 56 FPE with a total of R750= $50. I will share my learnings, the do's and don'ts and would appreciate other people's inputs.
The first thing I did was to correct the low POI . At full elevation on the scope, the POI was 5cm low at 30m. This was actually a quick and easy fix, though it took a lot of thought, trial and error to fix. The rear barrel shroud stabilizer was too tight, forcing the stabilizer to be misaligned with the barrel, shroud and stabilizer, forcing the barrel to face down.
Remove barrel shroud, using an Alan key loosen the grub screw, until it is in perfect alignment with the shroud, reattach the shroud, and your POI is fixed.
Next, I drilled the barrel and transfer ports to 5mm, my barrel port was 4mm, and the transfer port was already at 5mm. Remember to always deburr after drilling, I used a knitting needle and 800 grit sand paper. I will be honest; I was disappointed with the modest 3 FPE gain, still the gun was running 43 FPE at this stage.
This is where I spent some money, I bought an extended plenum. An extra 10cc of air, and with a small modification to the stock, I used a drill and file. I gained another 2 FPE. As an aside, to install the extended plenum, one needs to remove the air bleed valve. This should be simple; however, there is a hidden grub screw between the air and trigger blocks. Please remove this before, unscrewing the bleed screw cap.
A friend told me that it's possible to remove the valve return spring. I did thus, however, due to filling with a small Chinese compressor; I could not get the valve to seat. In the end, I took a very light spring off of an electric motor bush that happened to fit perfectly, and just like that, I was on 50 FPE.
If changing the valve spring had such an effect, that got me thinking of the hammer. I tried shimming it from the rear, as the parts diagram shows a spring washer in the 0.25. However, the sear would not engage at this point. So, back to the drawing board, In the end I took a 5mm machine screw, with a wide head, and filed it down to fit into the hammer, with the screw part, into the spring. 56 FPE, though my shot count was affected, I'm now only getting 15 shots per fill.
And this is where disaster struck. Checking ports, I noticed that the valve port is very small. After removing the valve stem, it is equivalent of a 3.5mm hole. To get this, convert the diameter of the valve hole and stem to area, then subtract the stem from the valve port area, and convert back to radius then to diameter. This is clearly a large bottle neck. I drilled the port to 6mm, which puts the valve port at just over 5mm, perfectly balanced. However, the valve would no longer seat unless I put the OEM valve spring back, which pushed my power back down to 50FPE. I am currently having a new valve, with the correct port sizes, machined, with an expected cost of R500= $25. Hopefully, with this installed, we will reach the 60FPE.
In the interim, I have started playing with the trigger. I read somewhere, possibly here, about a trigger mod to improve it. On that concept I stripped the trigger, and took the trigger tension screw out, and placed it in the rear hole, closest to the trigger, and placed the "blank " I removed into the trigger tension screw hole. It took some adjusting before the safety would work. But now, I believe the trigger is much lighter than stock. That said, I will need to wait for my new valve before I can properly test it.
I will keep this post updated on developments; if anyone has other ideas or opinions, I would appreciate it.
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Lathaniel, nice work and mods on your Avenger! I hope you reach your 60 FPE tune. I wonder if you get someone to fabricate a drop down block (will have to cut stock to attach) so you could run a larger carbon fiber reservoir to attain more shots and maybe increase your plenum volume more easily.
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Hello and thank you. There are people here running that block and tank, I had a chat to the one guy today, and he reckons he gets around 200 shots per fill, on his 500cc bottle, with it set to shoot 18gr JSB at 905 fps. That is incredible. I know Doux silencers here in SA did a large order for export to America recently.
It is definitely on my to-do list, but quite pricey at the moment, hopefully prices stabilize, or I manage to save a bit. But the Leviathan is incredibly slug hungry.
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I may have missed it, what weight projectile are you using?
Dave
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At the moment, I'm using the .218 23gr H&N HP slugs. I'm getting between 1040 and 1050 FPS. The thought is that when I've reached my target, I can use 27 or 30 gr slugs, and be between 950-1000 fps, which from what I understand, seems a good speed for long range accuracy using slugs
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Heavier slugs should yield you a few more fpe. You may be right at your target goal with heavier slugs.
Dave
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Thank you. As soon as I get my valve back, I will try the heavier slugs, much appreciated
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So instead of just waiting for the new valve, I decided to work on the trigger. In tests, without air in the gun, unfortunately it seems to have a single action, with a much crisper and cleaner break.
For those that are interested I will detail the process below.
1) Remove the trigger, a pin easily pushed out of the breech block, top and closest to the muzzle.
2) Remove the trigger tension grub screw.
3) Remove the "blank" grub screw, from the rear of the trigger, closest to the trigger.
4) Insert the trigger tension grub screw into the hole left by the blank grub screw.
5) inside the block, under the trigger, us the safety lock grub screw. You can find it easily by engaging and disengaging safety with the trigger out. Loosen this a little bit, if you don't, when you adjust the new screw to get a lighter break, there will be a point, with very little gain, where the safety does not engage.
6) Optional add another screw to where you removed the trigger tension screw and replace the spring. If you don't do this, the trigger may seem a bit loose when not cooked.
7) Reassemble trigger
8) Adjust the trigger to your liking, using the rear 2 screws.
9) Test the safety when you are happy, if it does not engage, either back the safety screw out, or make your trigger a bit stiffer.
10) Do the bump test, in a safe manner, make sure that the adjustment is safe before being happy with it.
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OK, so getting a new valve seems to be an exercise in futility. I can't seem to get delrin, peek or even acetal to make a new poppet here in SA. I was busy importing some delrin, but a week later I was told they don't ship here. I looked at after market valves, but sadly, while I believe they would be worth every cent it is purely unaffordable for us in sunny south africa.So I made a metal poppet, I will attach a photo. Sadly this resulted in a decrease in muzzle velocity, 23gr slugs now only pushing 1000 - 1010, meaning a decrease of fpe, so sitting around 51 fpe now. Not to mention the incredibly annoying slow leak I now have.
After accepting that, that is what it is, I chanced upon the old poppet, took the vernier and measured it, the back is wide enough to seal, so I drilled a new hole, put the valve stem in the back, polished the new sealing face, and have filled the old hole, with epidermix to prevent leaks, now it's a waiting game, but I have full confidence it will work
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Hope it works for you!
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5) inside the block, under the trigger, us the safety lock grub screw. You can find it easily by engaging and disengaging safety with the trigger out. Loosen this a little bit, if you don't, when you adjust the new screw to get a lighter break, there will be a point, with very little gain, where the safety does not engage.
If you've adjusted the screw to where the safety will no longer engage, you can replace that safety grub screw with an M3x10mm screw. That will give you a little more room to make the trigger lighter and still let the safety engage.
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Found something that my little brain can't understand. Gamo Pro magnum 15.43gr is shooting at 940 fps, whereas jsb jumbo monster 25.39 gr is shooting at 980 fps, how is this possible. Thanks in advance
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Found something that my little brain can't understand. Gamo Pro magnum 15.43gr is shooting at 940 fps, whereas jsb jumbo monster 25.39 gr is shooting at 980 fps, how is this possible. Thanks in advance
what chrony?
are you using the stock 3.5mm valve port?
as you drilled barrel/transfer port if you use the stock valve porting you waste more air i believe, you'd want the opposite so air gets strangled and creates more pressure behind the pellet, otherwise there's higher dwell which favours heavier pellets
i'd balance the porting then play with valve stem length and bstaley or FFH/SSG
you can use a hardened steel drill bit as valve stem to test things
fpe is already higher than the .25 for your gun so congrats on that
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Hello, thanks for your reply. I have done quite a bit if work so far, opened barrel and transfer ports to 5mm, valve to 6mm, with a reworked poppet ( i need to make a new valve stem, the one i have is ~ 3mm shorter than standard), ultra light valve return spring, and weighted the hammer and shimmed the spring a bit. Also put a plenum extender in.
I'm using the chrony F1, and my shot strings are very consistent ~ 2fps exterme spread. I was just very confused to see the opposite of what I expected, but the dwell time makes sense. Im aiming to shoot 30gr slugs when I can find them again, pellets seem in short supply at the moment. So your theory means I should gain more than I expected.
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Lathaniel,
It is possible that you have mentioned your Avenger's pressure regulator setting in another thread. Else, that is very much part of what determines maximum power for a PCP.
Hammer strike energy needs to increase with plenum pressure, else increasing pressure will limit how far the valve can open, and the valve dwell.
Now, it is possible that your regulator came from the factory with a sufficiently high pressure setting for max power. I say max power, because you want to reach 60 FPE, but have reached 50 so far.
Perhaps setting the reg pressure up is so obvious that you started there. Else, what others have done might be useful:
https://youtu.be/2D_Zml_2wd4 (https://youtu.be/2D_Zml_2wd4)
There have been a lot of Avenger tuning threads on GTA, and perhaps you have seen them. Anyway, finding them via google is easier than using the GTA search engine:
www.google.com/search?q=site%3Agatewaytoairguns.org+.22+avenger+power+tune (http://www.google.com/search?q=site%3Agatewaytoairguns.org+.22+avenger+power+tune)
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Thanks for the reply, I've got the reg set to 2700 psi, and the hammer spring all the way in, shimmed and hammer slightly weighted. Still shy of the 60. Before i started with the mids, 40 fpe was the best i could achieve, so 50 odd isnt bad, just a bit shy of where i want it. I have recently extended the valve length, which should assist a bit, however, the poppet isn't sitting flush now, so need to re face it ( hopefully get to that this weekend). Thank you for the links, I will read through them and see if there is anything I'm missing.
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2700 PSI sounds high enough to be useful. Much more than that, and your shot count per fill may not be enough. Depending on your fill pressure. If you have a 4500 PSI source, then less of a concern.
In a much broader context, here is a thread about
achieving the maximum theoretical velocity from a PCP:
https://www.gatewaytoairguns.org/GTA/index.php?topic=94054.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=94054.0)
Perhaps not relevant to the Avenger directly, and ultimately at the expense of efficiency. The point is to identify and eliminate the typical bottlenecks.
One aspect you have already seen is that heavier projectiles are more efficient. I don't know how heavy you could go, and if that would help much in achieving 60 FPE. At some point the projectile may not be going fast enough for your purpose.
950 FPS is usually fast enough for power, efficiency and a flat trajectory. If you are shooting pellets, much faster than 950 FPS tends to not group as well at longer distances; and the pellets slow down so fast, the exercise is not very practical. If you want to shoot over 1000 FPS, slugs are a much better bet. Just keep them sub sonic. 1050 FPS or thereabouts.
Speaking of purpose; other than breaking 60 FPE, what is your purpose - if that is not a rude question?
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All of that said, Lathaniel, at 50 FPE you may already have exceeded what most .22 Avenger tuners have achieved, by a wide margin.
If you can't find heavier pellets, then perhaps some "low friction" slugs - if such an animal exists (cast, with narrow middle and rear drivebands). I think you need at least a 30 grain projectile to achieve 60 FPE, considering the limited expansion volume of a .22 with a 22.75" barrel length.
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Delrin is the sold as acetal in SA.
Plastem in Johannesburg has Acetal and Peek: https://plastem.co.za/plastic-technical-data/ (https://plastem.co.za/plastic-technical-data/)
Contact: https://plastem.co.za/contact-plastem/ (https://plastem.co.za/contact-plastem/)
Engineering plastic supplier in Durban: http://www.gartech.co.za/products.html#pom (http://www.gartech.co.za/products.html#pom)
DURBAN HEAD OFFICE
2 Teakfield Road, Springfield Park
Durban, South Africa
Tel: (+27) 31 579 9300
Email:admin@dems.co.za
Website: www.dems.co.za (http://www.dems.co.za)
More SA suppliers, here: www.google.com/search?q=acetal+for+sale+south+africa (http://www.google.com/search?q=acetal+for+sale+south+africa)
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Thanks for all that useful information.
I have recently gotten 30gr javelin slugs, and was getting around 910 fps with them. However, I let other people shoot, and it was double loaded, which I managed to shoot out, however, since then my muzzle velocity dropped to 825, so I have stripped and made a new valve stem, however it isn't sealing properly on the poppet, so I need to re face that, and hopefully I will be on. I will look at those suppliers you suggested, better to make a whole new poppet than fix the old one. I want to use it for pest control as well as long range shooting, 100m and up. There is a 300m gong shoot with air rifles happening soon, not far from where I live. So would be great to be able to do that. Are you also from South Africa?
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I wonder if the double load shot dislodged (partly extruded) an o-ring in your transfer port. Or blew a breech seal O-ring; accounting for the reduced muzzle velocity due to a partial leak. Such a leak may be fixed by taking apart and reassembly; unless the O-ring was damaged in process.
I was born in the Transvaal. Grew up mostly in the Western Cape. Have been in the USA over 26 years.
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I could not get the valve to seat. In the end, I took a very light spring off of an electric motor bush that happened to fit perfectly
Perhaps this is also obvious: You should cock the hammer when you fill the air tank from empty. Else the hammer may rest on the valve stem (in some PCPs), and prevent the valve from seating. So, it will leak continuously.
The fact that yours fills with a light spring may have solved your problem, and make no difference, should you remove the spring. Providing you can get the valve to seat to start filling. Gravity may then have something to do with it. Position the air rifle such that gravity closes the valve...
Ek weet; ek praat te veel :)
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Have you been back to sunny SA since you left? Place had changed hectic over the last few years, but still an awesome place to live. I was born in natal, spent most of my childhood years in the Eastern transvaal, and a few of my adult in the southern Cape, now back in natal.
Ja, the blown o rings was my first suspicion, so I stripped, oiled and reassembled, but I figured while it's apart I may aswell do the valve stem. On the lighter spring, strangely, I found if u put a slightly stiffer spring in( from a chainsaw throttle) I actually increased mv. I'm going to order some acetal tomorrow and make a new poppet, and that will allow me to open the valve throat a bit more. I'm hoping that gets me where I want to be. I must be honest, I'm having a blast working on this pcp, first time ever doing that.
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Well, I don't own an Avenger, so it took me a while to realize that you are seeking and getting .25 caliber power from your .22. In other words, you might be able to teach tuners about tuning the .22, more than they can teach you :)
Your valve spring experience is interesting. I wonder if without a spring it is not slamming full open so hard that it bounces shut prematurely. Whereas, with a soft spring, there is perhaps less bounce. Strange things can happen. For example (speculation), the spring is not so much controlling the valve as adding friction by applying a slight side load; should the spring not be perfectly coaxial with the spring.
Anyway, I try to have a working theory; and never dismiss results. Even if I don't understand something because it seems to violate theory, rather than the result being deemed wrong, the theory is incomplete.
If I don't own an Avenger, why am I posting in this thread? Because I wanted to try and help a fellow from SA. And, I do try to be helpful in general, even if those biting their tongues might not thinks so.
What went through my head was that because the FX Impact M3 is reaching 100 FPE in .22, 60 FPE with the shorter Avenger barrel does not seem unreasonable. Of course, the devil is in the details.
Ek het elke drie jaar SA besoek. Laste keer was 2019. My 90 jaar oud ouers woon nog daar, en ek praat elke week met hulle. As dit nie was dat die plek nou meer kaoties is nie, het ek miskien in SA kom aftree.
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I always enjoy reading your posts Subscriber. Whether you have an Avenger or not, the dynamics of getting/keeping air to the tail of the projectile are generally quite similar among conventional valved guns.
Dave
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Thanks Dave,
I am glad to see that a successful hot-rodder such as yourself is looking at this thread.
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I really appreciate your help, i have been flying blind. Trying to theorise how to do things and why things don't work as well as one thinks they should. I agree with the valve spring, my thinking is that the spring absorbs some of thw force with the back pressure, resulting in less of an impact on the back, and resultant bounce. Now thanks to your links, I am going to make a shorter poppet, which might alleviate that issue, but like you say some things are contraintuituve, which I'm slowly starting to learn.
As u weer in suid afrika is, most u aseblief vir my n boodskap stour, dark kan ons by mekaar uit kom, en nog n bietjie leer. Wear by u ours.
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Have you had any issues with your regulator with the mods?
I ask because it seems mine drops 200-400 psi on the regulator after each shot and if you shoot too quickly it affects accuracy and extreme spread.
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The reg seems OK, so far, after the mods i do use more air, im getting ~ 20 shots per fill now. I have noticed that shooting too quick, does have an effect, I have just slowed down and all is good so far.
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im getting ~ 20 shots per fill now.
What fill pressure are you using, Lathaniel?
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I'm going up to the full 310 bar pressure. Sometimes, slightly over or slightly under, hard to see those silly gauges
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Not sure if it’s been discussed yet but running a smaller od poppet stem (you need to put in a bushing in the stem guide)
reduces the closing force as well as offering more flow area in the throat. This coupled with the heavier hammer will increase dwell time.
Dave
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What I did, was reduce the poppet valve stem to ~2mm, within 1cm from the poppet, the remaining 20mm is still 3mm od, would I need to look at a bushing still? I did that small piece, hoping not to affect the sealing in the valve guide, while increasing air flow in the valve throat. I will be honest, the valve guide is something that has bothered me, because as I see it, the stem moves relatively easily through it, meaning ( in my mind atleast) it dosent seal very well, and the hammer chamber is sealed not at all, how much air do you think we lose through the valve guide?
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I am not familiar with the parts, but if one looks at the potential leak path at the valve stem, it depends on the circumference, the radial clearance, and the overlap length between stem and guide. The greater the overlap, the less leak, because the annular channel is in effect a very long narrow pipe, with a huge flow loss (pressure drop) across it.
A 3 mm diameter valve stem has a circumference of 9.43 mm. If the radial gap is 0.02 mm, then the flow area is 0.1886 square millimeter. Compared to 19.64 square millimeter for your 5 mm diameter TP flow area. So, the leak is 1/104 of the TP area, assuming the radial clearance is correct. If the radial clearance was 0.01 mm, then the valve stem leak area would be 1/208 of the TP area.
The missing piece of the equation is that flow area is not the only important consideration. The "edge effect" of the valve stem leak is huge, where stagnant boundary layer air clings to both the surfaces of the valve stem and guide. Compared to just the ID of the TP (down its length). Here is where the length of the valve stem overlapping the guide helps reduce the leak.
The crude sketch below attempts to illustrate how much more efficient a large flow area is, compared to a narrow one; due to the stagnant boundary layer effect. The read line is flow velocity of the air across the section. The narrow gap acts as if it even narrower due to the boundary layer effect. It has much less percentage effect on the TP.
That is not to suggest that one should ignore the valve stem leak. I think the smaller the radial gap the better, up to the point that there is too much drag on the valve, and it does not open and close reliably. I don't know if any PCP's use and O-ring seals on the valve stem, but somehow I get the impression that some leak at the stem while the valve is open, is considered normal. It occurs only for a very short time. Not long enough to matter very much.
The hammer chamber must be vented, or air compressing in front of the hammer will slow it down during "flight".
Perhaps someone with more specific knowledge and informed opinion will chime in.
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So if I understand correctly, the valve guide leak is negligible. And can be further negated by having a longer valve stem, which(up to a point) will also increase dwell time. This could also explain why having a stiffer valve spring produced more power than the flimsy one. Not so much preventing bounce, as maintaining a longer portion of the valve stem in the valve guide?
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It is not just the stem length, but the length where stem is shrouded by guide length. So, the guide needs to have sufficient length too, for the leak to be minimized. How long to approach zero leak? Probably impractically long.
I would try to halve valve stem to guide clearance before trying to double the stem to guide overlap length. I am at the disadvantage of not knowing what the packaging limits are.
A very long stem perhaps just opens the valve further, and takes longer to close, thus releasing more air. An overly long stem my bend and ring when struck by the hammer; even if just elastically.
I think there may be more factors involved with the valve spring increasing power, over all else being identical, with the spring removed. It was a small change. The question is, does it still hold if you removed the spring? I would like to understand this, but it seems less important than if it added more power.
I think there is a flow limit reached after a certain valve lift is reached. More lift just increases valve dwell. Also, the further the valve opens, the harder it will slam shut. The flow is then limited by the flow area through the valve throat, less stem area. Some of the links I posted above almost certainly go into detail about this.
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Even coked, and using gravity to help assist with a seal, I never achieved a seal without a spring. I suspect that my little pump fills too slowly to create enough pressure on the rear of the poppet, to make a proper seal. Another person I know, removed the spring, and managed to get a seal, he was, however filling from a tank, so very quick pressure, to create enough force to seal? Or perhaps things don't align 100% in my assembly, so it needs that bit of pressure from the valve spring to close it properly. While waiting for that acetal, I am going to work my frankenstein poppet face and see if anything changes, with my new valve stem. Everything is done by hand, i have a drill and grinder, so things take some time and don't always align as I'd like them to off the bat, but perciverance usually sees me through.
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OK. You can't fill without the valve stem spring. It is weak and can't do any harm; while appearing to help with more power. That may change after a few 100 shots to bed the valve better.
I am guessing that a valve with slightly larger head and stem diameters, using appropriate lift should boost your power a bit more. As long as the valve is not too heavy for your hammer. Certainly, the hammer will need to work harder to open a larger diameter valve against the same air pressure due to the larger area.
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So, I decided to take lunch today, and go home to play with my avenger. Good news I've got the poppet seating and the rifle filling now. I have attached a photo of the valve assembly, too. What you say makes sense, something I sort of thought of, but dismissed. When I originally opened the valve throat, the poppet got stuck in it. So I reversed the poppet, using the back as the new face. However, after drilling a new hole in the poppet, the pressure blew the valve stem out, so i put a screw in the original valve stem hole, which solved that problem, but did increase the weight of the poppet. That's why I've been trying to find delrin, is to make a new poppet. And get away from the extra weight and frankenstein look. I have 2 more questions if you'd be so kind, subscriber. 1) is peek much better than delrin? I've ordered the acetal, and it's cheap R27 a meter, where peek is R380 a meter, with acetal being so cheap, I don't mind wasting that money and ordering peek too, if it's worth it, what are your feelings?
2)When I receive the material, i was thinking of making the poppet with a conical shape on the back, easier to cut through the pressurised air, so means less hammer to open the valve, or am I barking up the wrong tree?
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Peek is almost 50% stronger than the stronger form of acetal. Peek is almost like metal in its stability and machinability. Note that Peek's impact strength is not as good as acetal's, so if you valve slams shut hard, it may suffer brittle failure, depending on the design.
https://www.mcmaster.com/peek/wear-and-chemical-resistant-peek-rods-and-discs/ (https://www.mcmaster.com/peek/wear-and-chemical-resistant-peek-rods-and-discs/)
Tensile Strength: 14,000-16,000 psi (Excellent)
Impact Strength: 0.6-0.95 ft.-lbs./in. (Poor)
Stronger acetal:
https://www.mcmaster.com/acetal/wear-resistant-easy-to-machine-delrin-acetal-resin-rods-and-discs/ (https://www.mcmaster.com/acetal/wear-resistant-easy-to-machine-delrin-acetal-resin-rods-and-discs/)
Tensile Strength: 10,000-11,000 psi (Good)
Impact Strength: 1-1.5 ft.-lbs./in (Poor)
Weaker acetal (slippery):
https://www.mcmaster.com/acetal/slippery-wear-resistant-easy-to-machine-acetal-copolymer/ (https://www.mcmaster.com/acetal/slippery-wear-resistant-easy-to-machine-acetal-copolymer/)
Tensile Strength:
Blue: 7,600 psi (Good)
Red: 5,900 psi (Good)
Both the peek and first acetal are about the same hardness, so a tough call. The second acetal is much weaker. So, it depends which one you can get. I think plenty of PCP valve makers on this forum use Delrin / acetal because it is 1/3 of the price of peek. Not sure which grade they use. Probably what they can get...
Creating a shape on the back of a the valve to smooth air flow is a good idea, providing it does not choke the flow. Think of car engine intake valve head shape.
Pressure differential across the valve, and front and rear areas for air pressure to act on affect force to open valve. When the valve is open, the closing force follows similar principles, except the pressure on the TP side of the valve is high initially. Then drops once the pellet accelerates down the barrel.
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What I did, was reduce the poppet valve stem to ~2mm, within 1cm from the poppet, the remaining 20mm is still 3mm od, would I need to look at a bushing still? I did that small piece, hoping not to affect the sealing in the valve guide, while increasing air flow in the valve throat. I will be honest, the valve guide is something that has bothered me, because as I see it, the stem moves relatively easily through it, meaning ( in my mind atleast) it dosent seal very well, and the hammer chamber is sealed not at all, how much air do you think we lose through the valve guide?
I don’t think the leak is an issue.
My suggestion was for obtaining more dwell time. Consider the stems piston effect DURING the shot cycle. It creates a closing force quite similar to a strong poppet spring. A smaller stem has less closing force.
Dave
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OK, I think I get it, the tighter the fit between the valve stem and the valve guide, the more energy it takes to close the valve again, negating the effect of the valve spring, increasing dwell time? I didn't think of that, but it makes sense, in a weird way, lighter spring increases dwell time from within the valve, tighter fit valve stem from outside. And my thinner piece, near the poppet, helps with airflow, negating the impact of a thicker od valve stem? Obviously it can't be too tight as that way we lose momentum from the hammer strike. Am I right?
That's what I love about tinkering, and talking to other people,.so many different ways of approaching a problem, and each one add to the final result
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So between Dave and Subscriber, I think I will stay with delrin for the material, make a slightly thicker valve stem, and make the rear of the poppet conical, to assist cutting through the air on opening. This should, decrease the force needed to open the valve, and increase dwell time, because air the doesn't have as much surface area to act on when closing?
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Maybe this will help
Dave
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Thanks for that, so I think I did understand what you said, thicker OD = more closing force required = increased dwell = in theory more muzzle energy, obviously barrel length plays a part here
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Larger od stem = more closing for = less dwell = lower velocity
Smaller od stem (with a bushing) = less closing force = more dwell = higher velocity
Yes, depending if barrel length can make use of longer dwell.
Dave
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This is where my small brain shows, apologies for not grasping this concept. Lower closing force = more dwell. I would have thought it opposite, the more energy it takes to close, the longer it stays open. Thank yoy for the explanation, as subscriber says, practice trumps theory ( especially my theory, haha) will do that when I get my material, assuming I can make the bushing, that is.
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It’s just a tuning option to consider when trying to make power. Maybe not needed here.
Dave
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Every option I have is greatly appreciated, thank you
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As they say, the devil is in the details.
As for a mental model, the poppet valve air flow is more like that of an engine exhaust valve than the intake valve; even though the two valves look very similar.
When one considers the style of fill probe shown below, there is no lock and no tendency for the air pressure to "spit out" the probe. That is because the thrust forces produced at the two O-rings cancel each other. If you understand how that works, then the effect of valve stem diameter on closing force is easier to understand.
Obviously a poppet valve won't work, if its head and stem were the same diameter, but the thrust from the stem is like one side of this style of fill probe. Uneven for the valve; even for the probe.
Note Foster fitting on the left of the probe, with only O-rings where it goes into the gun on the right - no lock:
(https://www.pyramydair.com/images/acc/PY-A-9036_Seneca-Aspen-Fill-Probe_1561991483.jpg)
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I've got the frankenstein poppet sealing reliably now, but shooting over the chrony, still sees me 80-100 fps below where I was. I've stirupped and rechecked( didnt see any obvious damage ir misalignmen though)/ lubed seals again, hopefully that works. I did notice a much larger spread ~ 20 fps vs ~ 2 i was getting.
Failing that I have ordered a new set of o ri gs from captain o ring, insanely expensive here in SA, but I can't think of anything else it could be. I took the valve guide out, and like a dumbass didn't take a photo, but this valve guide does have a little o ring inside of it.
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Transfer port alignment to the receiver or barrel may have shifted in assembly.
Valve friction may be up. That may come down with use.
To much grease may interfere with part movement. Including in exaggerating the potential air cushion or vacuum that may slow down the hammer.
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That's one thing they got right, there are divots on the tip of the barrel, with grub screws to prevent misalignment.
The valve is now heavier, thanks to the "fix" so might be that. I'm leaning towards the hammer though, how long at a guess does it take for the hammer spring to become weakened? Although one would think that would be a gradual process, that said I haven't shot it over a chrony for awhile, ~ 2000 shots. It's only when it's accuracy went for a ball, I checked the speeds.
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I think new springs take a set early on, then perform the same for a very long time.
Spring air rifle piston springs are more highly stressed. As could be your hammer spring, if you were running it near coil bind. Then it may have lost some uncompressed length. That should be easy to measure.
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I think it is close, I have it set till just before the sear will no longer engage when cooking it. I would them need to compare length to a new spring, to see where we are then. Do you think it could be part of the problem?
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Spring set could be robbing you of hammer strike on the valve.
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I did try reducing spring tension, and that resulted in lower MV, which is why I'm thinking it could be a spring issue?
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That seems to be a signal. Try the reverse.
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Subscriber, I am leaning toward your idea.of misaligned ports, I initially didn't think it possible, due to the divots in the top of the barrel essential forcing alignment when tightening the barrel screws, however after reassembling, I noticed the magazine no longer fit, so I loosened the screws and moved the barrel back. All in all, I moved it back 2.5 to 3mm. Which tells me the divots are not forcing the barrel into proper alignment, so apologies for dismissing your theory, I think you were sopt on. Now for the tricky part, if the divots are not forcing proper alignment, how do I make sure the ports are aligned, as they are not visible once replacing the barrel, any ideas?
Thank you
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Lathaniel,
Traditionally, if one looks down the barrel retention screw holes in the breech, the dimples in the barrel should be centered in the screw holes. This page has advice on using the magazine for positioning the barrel:
https://www.airgunseurope.com/blogs/blogs/air-venturi-avenger-umarex-origin-barrel-removal/ (https://www.airgunseurope.com/blogs/blogs/air-venturi-avenger-umarex-origin-barrel-removal/)
Of course, mag fit is important, but TP alignment fore/aft and in rotation supersedes that. Or at least, they should not fight each other.
To get an idea of what the parts look like I watched this video: https://youtu.be/yHhMJxPzwq4?t=2968 (https://youtu.be/yHhMJxPzwq4?t=2968)
What I would do is to use the rear face of the barrel, as measurement reference to the center of the barrel TP. Then, also measure the rear of the breech block to the center of its TP. In reassembly, use those measurements to confirm that the centers of the TPs are real close.
For rotation I would find, or create a reference on the barrel that confirms it is "clocked" correctly. Again, the screw dimples should be 180 degrees from the TP. If they are not, you will need to allow for that. You might even attach a mark to the top of the barrel that won't be scrubbed off by inserting into the breech block. It could be a dot of white-out (Tippex correction fluid). Or a white patch with a black ink mark on it.
Such marks could be on the top of the barrel just ahead of the breech block when installed. Or on the rear face of the barrel breech. The white-out blob may interfere with mag installation and feel, so consider that.
Short answer; use features that exist; or create your own to act as TP alignment indicators. Creating your own marks is more important, if the barrel screw dimples are in conflict with the actual TP to TP position.
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I am surprised how difficult it is to find info on TP alignment when reinstalling the Avenger barrel. Then again, that suggests that not many people struggle with it, to end up making internet posts on the subject. That it should be easy when applying general PCP principles would seem to be good news.
When researching a particular question I often find interesting material that is generally related. Here is a post by Bob Sterne at the Hard Air Magazine about the effect of TP diameter on PCP power: https://hardairmagazine.com/ham-columns/the-effect-of-port-sizes-on-pcp-airgun-power/
The above might help ground expectations. I would read it in conjunction with the fact that the FX Impact M3 is easily reaching 96 FPE in .22 caliber, using 800 mm barrels. The latter alone, suggests you should be able to reach 60 FPE with your .22 Avenger. Of course, as always, the devil is in the details. For instance; including plenum volume, and potential loading probe airflow interference.
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As always an incredibly in depth answer, your knowledge must surely be legendary, I will try your suggestion for getting proper alignment, I'm now certain it's out, 2mm in a 5mm hole is a huge restriction, once again thank you.
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There members on this forum that know vastly more than I. I invited one expert to weigh in on this thread two weeks ago via PM. He declined because he has no experience with Avengers.
I also have no experience with the Avenger, but a lack of direct knowledge does not stop me. That is because I am willing to try and gather available information and knowledge, where it may be found. I am able navigate that information by applying basic scientific and engineering principles. Where there are holes in the information, I am willing to speculate. I don't have a reputation to uphold, so I am not concerned about looking silly if, my speculation turns out to be nonsense.
I don't have the desire or ability to know everything. But, I am driven to understand as much as possible. I have learned just enough from the experts to help me develop a useful level of understanding.
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Hello subscriber
Spot on, to be honest I also know very little about the avenger, and even less about PCP's in general. But I am willing to learn, and love the challenge of that. I don't have an engineering or scientific background, so I approach a problem from a purely logical point of view, and often this is where I miss opportunities to understand and achieve my aim. That is where forums such as this, and the people here, who have added their thoughts are of immeasurable value. Its partly why I started this thread, I'm certain there are others like me, working in the dark on a rifle that has very little information regarding modding, partly to learn myself and partly to have the information available for the next person.
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The only absolute when working on PCPs is degassing them before taking them apart. As you have clearly gone beyond that important detail, I would say you are pioneering the 60 FPE level for the .22 Avenger. Creating reference material; even if you manage "only" 50 something FPE.
It is possible and even likely that you will improve some parts to the point of needing to replace them :) . Many on this forum have had that experience.
Understanding what makes more power and what makes more efficiency might factor in to the trade-offs that are likely in the pursuit of a lot more power. There are boatloads of reference materials on those topics on this forum. On the other hand, if you want to clout pests with only a few mag-fulls of shots, perhaps efficiency does not matter that much.
I mention efficiency, because opening the flow path after the valve helps boosts peak achievable power, but it tends to reduce the shot count; even when you are shooting at moderate power. This assumes the valve is not a bottleneck.