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Author Topic: Tuning a Regulated PCP  (Read 51769 times))

Offline boothm

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Re: Tuning a Regulated PCP
« Reply #120 on: January 09, 2020, 09:35:59 PM »
Thanks Bob...that's simple enough--much appreciated.
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Offline outie

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Re: Tuning a Regulated PCP
« Reply #121 on: July 07, 2021, 03:23:37 PM »
Learned a lot reading just this thread alone.

I have been learning my Dreamline classic .177 these days. I have played with the reg pressure, TP wheel, and HST. My goal was to get lowest FPS (sub 10FPE) and lowest noise with consistency out of this rifle.

I recently tried to tune it with higher pressure and lower HST, which makes the gun tuned to the “downslope”. I was getting really low FPS (sub-600) and the noise was minimal (with pellet. dry fire is louder than lower setpoint). My understanding is that this makes the gun really quiet and very efficient, but shooting under setpoint will make FPS much higher. If you refill before reaching setpoint all the time, does it even matter? While the gun is tuned to the downslope, is it normal for the ES to be much higher? I thought Bob mentioned the FT shooters tune like this but wouldn’t their shot to shot consistency create an issue, besides the below setpoint FPF increases? Is it possible to tune to downslope yet with a minimal ES?

TIA.
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Offline rsterne

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Re: Tuning a Regulated PCP
« Reply #122 on: July 08, 2021, 01:18:20 AM »
You are correct, if you tune on the downslope and refill above the setpoint, you will never see the velocity rise.... The problem with doing that at a FT match is what would happen if they tested your gun JUST before you were going to refill it.... and as it dropped below the setpoint it went over 12 (or 20) FPE.... you'd be out, even if it was never your intent to shoot it that way.... The same could occur if you were in Canada, tuned to under 500 fps, but on the downslope and they checked the velocity and shot it down below the setpoint.... and it went above 500 fps.... Now you have a firearm....  ::)

The wild swings in ES when shooting well down on the downslope are due to the way the valve reacts to even TINY differences in hammer strike.... The valve is operating in partial valve lock, so a little bit of stickiness on the hammer and it locks more, and the velocity drops a lot.... If your hammer strike is PERFECT, every time, your ES should stay small.... I have an anecdotal report by one shooter who tuned well below the plateau, and managed to lower the ES by running a VERY strong valve spring.... He was running more pressure, and a light hammer strike, to get an efficient and quiet tune, but the ES was terrible.... He installed a heavier valve spring, and had to of course increase the hammer strike, and the ES dropped.... So he fitted a valve spring that was so stiff he could not even push it open with his thumb.... He had to crank up the hammer strike, of course, because a large portion of the hammer strike was now required to overpower the strong valve spring.... However, this meant that tiny hammer friction variations were lost in the much greater hammer strike, and the ES dropped to very low levels....

As I said, this is ancedotal, from one report, and I haven't tested it myself.... However, it makes sense that it could work to tune on the downslope, fit a very heavy valve spring, and then increase the hammer strike to compensate for the heavy valve spring.... It seems to me to be worth a try....  ;)

Bob

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Offline JungleShooter

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Re: Tuning a Regulated PCP
« Reply #123 on: July 08, 2021, 02:03:58 AM »
Bob,

thank you for that excellent post — really helpful for me to understand the intricacies of PCP tuning. 👍🏼

Matthias
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Offline outie

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Re: Tuning a Regulated PCP
« Reply #124 on: July 08, 2021, 04:04:15 AM »
Yes thank you Bob for all the insights and confirmations. I currently have my Dreamline tuned to the knee (sub 12FPE) and call it a day. The wild inconsistency between shots when tuned to downslope wasn’t worth it for the lower power (sub 10FPE). It was fun exploring all the tuning capabilities though!
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Offline EricFR

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Re: Tuning a Regulated PCP
« Reply #125 on: September 09, 2021, 03:50:18 PM »
Hello,
Thanks for this tutorial, it really helped me to adjust regulator pressure and hammer strike but I have one thing to say about it.
Like explained I have found the plateau (at a given reg pressure), the fps are even lower with the hammer spring fully compressed and then as I reduce preload, the speed increases until it decreases again. I did set the tension at those 3% less velocity than maximum on the downslope where the sweet spot is. But what happened is that with the regulator creep, the first shot is always lower in fps compared to the next ones that are good. What I did to reduce this issue is finally to set the HST to still be on the plateau but at the end of it where the fps just start to drop and with this, no more fps drop with the creep. With this I almost don't lose any more shots.

Now another question came to me. The plateau and sweet spot you're talking about is for a given pressure to achieve the wanted fps? Those aren't 3-5% less than the maximum the rifle is able to do all settings maxed out?
Thx
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Offline rsterne

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Re: Tuning a Regulated PCP
« Reply #126 on: September 09, 2021, 11:42:08 PM »
If your velocity in a regulated PCP drops as you add hammer spring preload past a certain point, the hammer is bouncing off the back of the valve.... That is basically a design fault, but if it is occurring then you should consider the plateau velocity to be the maximum velocity you can get at your setpoint pressure (before the hammer crashes into the valve).... Then tune to 3-5% below that velocity....

If you tune that way, and you are experiencing a lower velocity on the first shot, you must have a REALLY horrible regulator, as even 100 psi either way from the setpoint should not cause enough velocity change to even marginally affect the POI....

Bob
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Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline EricFR

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Re: Tuning a Regulated PCP
« Reply #127 on: September 10, 2021, 03:22:12 AM »
Thanks Bob,

I've whitnessed it on FX Wildcat, same on 22 and 25 cal. Looks like how you say, the valve is bouncing on the spring on the other side of its travel. It happens especially with a lower reg pressure in the 125-130b range. I'm able to reduce this pressure because I've installed the Huma plenum XL but its in combination with the FX AMP regulator which works way better for me than the Huma that was delivered with it (it is a second hand WC). I've already opened the Huma but saw nothing special and for the moment I don't have planed to change the o-rings on it since the AMP is sufficient. Strangely with both regs I've always a plenum with a pressure of 10-15b lower than the tank after its off-reg and the gauges are correct.
Concerning the creep. The first shot is recorded at about 915fps and all the other shots are around 930 with +/- 6 fps change. Setting the HST a bit more in the plateau brought the 1st shot at the same level as the others.
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Offline rsterne

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Re: Tuning a Regulated PCP
« Reply #128 on: September 10, 2021, 12:18:56 PM »
Makes perfect sense.... Closer to the plateau uses a bit more air, but reduces the shot to shot variation, in most regulated PCPs.... You may be experiencing some sort of hammer, or valve, stiction after the gun sits for a while.... If it is occurring only on the first shot after a refill, that could be the regulator, and may be very hard to eliminate.... If you shoot below the setpoint, the regulator is open when you start your refill, and the plenum side may be getting a bit of extra pressure before the regulator closes, hence the low velocity on the first shot.... Stopping your shots before the setpoint should eliminate it, if that is the problem....

Bob
« Last Edit: September 10, 2021, 12:23:00 PM by rsterne »
  • Coalmont, BC, Canada
🇺🇦 Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦

Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline rsterne

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Re: Tuning a Regulated PCP
« Reply #129 on: March 21, 2022, 07:23:34 PM »
I thought I should probably add the latest version of the chart showing the effects of various ways to tune a regulated PCP....



You will notice that the grey line (tuning on the downslope) shows the shot-to-shot variation in velocity often experienced with this type of tune.... Any small variations in hammer strike are amplified because the valve is barely opening.... With this type of tune, the gun is using tiny "sips" of air at higher than the required pressure.... It is very efficient, but prone to this phenomenon....

Bob

  • Coalmont, BC, Canada
🇺🇦 Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦

Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline PikeP

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Re: Tuning a Regulated PCP
« Reply #130 on: March 21, 2022, 08:20:41 PM »
I thought I should probably add the latest version of the chart showing the effects of various ways to tune a regulated PCP....



You will notice that the grey line (tuning on the downslope) shows the shot-to-shot variation in velocity often experienced with this type of tune.... Any small variations in hammer strike are amplified because the valve is barely opening.... With this type of tune, the gun is using tiny "sips" of air at higher than the required pressure.... It is very efficient, but prone to this phenomenon....

Bob

I counter the shot-to-shot variance in lower power tunes by utilizing a valve lift limiter built into my hammer that can be tuned externally with an allen wrench. Works well for tunes at 3-5% and 10% below the plateau, although you still get the rise in fps once you're off the regulator with lower power tunes, its worth those extra shots while on regulator if the lower power suffices.

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Offline JungleShooter

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Re: Tuning a Regulated PCP
« Reply #131 on: March 21, 2022, 11:33:34 PM »
And this grey colored tune is also very quiet, right?

Matthias
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Offline rsterne

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Re: Tuning a Regulated PCP
« Reply #132 on: March 22, 2022, 12:57:01 AM »
Yes, it is quiet.... Matt is obviously using excess hammer strike, and then tuning the lift/dwell by limiting the travel of the valve stem to avoid the shot-to-shot variation.... I have not done that, but it should work fine.... I do not understand why you should get the velocity rise below the setpoint if you are limiting the lift, but if Matt says it happens, I would believe him.... I just don't know why it would occur.... Unregulated PCPs that have travel limiters, like the Korean guns, have dropping velocity at lower power tunes as the pressure drops....

Bob
  • Coalmont, BC, Canada
🇺🇦 Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦

Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline nervoustrigger

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Re: Tuning a Regulated PCP
« Reply #133 on: March 22, 2022, 01:46:42 AM »
About the soft report of being adjusted well under the knee...I'll never forget a few years back when a buddy asked me to come over to his suburban yard one Saturday morning and take out some gray squirrels.  He told me the shots would be a max of 30-35 yards so the night before I took a regulated .22 cal at 30fpe and began dialing back the hammer spring tension to see how it would do at about half power and switch to the lightweight JSB RS (13.4gr) to keep the trajectory fairly flat.

At the time I had no idea what to expect but I remember pulling the trigger and hearing sort of a pip of the hammer gently tapping the valve stem.  I thought, oh I went too far and looked down at the chronograph expecting to see 300fps.  No, it was 700fps.  The velocity was jumping around but it was keeping all the pellets inside a dime at 25 yards so that's where I left it. 

Well that was just the ticket.  We had 3 down within minutes of arriving, the report so soft they didn't perceive a threat.  It went on to do 6 for 6 that day.  So yeah for a scenario like that or for something like basement target practice, it makes for a very pleasant shooting experience.

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