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General Category => Ask Bob => Topic started by: rsterne on November 16, 2016, 08:10:58 AM

Title: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:10:58 AM
Two years ago I wrote this thread on the GTA.... I keep getting so many people asking for information covered in it I thought I would add it here as a sticky.... I have copy pasted all the initial posts with the data and graphs, and then added a few (edited) comments where I answered a few important questions at the bottom.... I hope you find it useful....

There have been a lot of shooters who are disappointed in the initial results when first fitting a regulator to their PCP.... There are two primary reasons for this, one is that often there is not a large enough plenum (chamber) between the regulator and the valve, and the pressure at the valve seat drops during the shot to a much greater extent than in the unregulated version at the same pressure, leading to lower velocity than expected.... Assuming that is not the case, then the most common thing the shooter notices is that the shot count is much lower than he expected.... This almost always happens if the hammer strike is not reduced when the regulator is installed, and is a result of the gun having been previously tuned to peak at, say, 2200-2400 psi and now it is running on a much lower pressure, making it an air hog....

Fortunately, the solution is relatively simple, and in most cases all you need to do is reduce the preload on the hammer spring to retune the gun to the now lower pressure.... If you measure and plot the velocity while you back off the preload, and assuming the gun was previously tuned to peak at a higher pressure than what your setpoint is now, you will get a graph that looks something like this....

( (

You will notice that as the hammer spring preload is reduced, at first there is virtually no change in the velocity (the plateau), but the efficiency increases.... Between 4-5 turns out on the adjuster (in this case) the velocity starts to drop (the knee), and then past 5 turns out it declines nearly linearly with further decreases in hammer strike (the downslope).... What you are doing, of course, is you are changing what would be the peak of the bell-curve if the gun was still unregulated.... This second graph of what the velocity does in those three regions may help you understand what is happening.... Note this second graph is using a 1500 psi setpoint and does not correlate to the graph above, other than in concept....

( (

First of all you will notice that when the gun is tuned on the "plateau", which it likely would be with the original, unregulated, setup, there is a slight slope to the velocity as the tank pressure drops due to the output pressure of the regulator creeping.... This is common in most regulators, more severe in some designs than others, but is usually present, and the output may be 4-8% higher when the tank is full than when it is at the setpoint pressure.... Since the gun was tuned to peak at 2200-2400 psi, it is well down on the downslope of it's unregulated bell-curve, so small drops in pressure will (usually) cause the velocity to drop slightly.... Then when the tank pressure reaches the setpoint (in this case 1500 psi) the velocity starts to drop rapidly with decreasing tank pressure.... Remember, that since the efficiency is also low when the regulated gun is working up on the plateau, the shot count will suffer.... If the hammer strike is very high for the pressure, the chance of air-wasting hammer bounce is increased, and the gun becomes a real air-hog....

If the hammer strike is reduced to retune the gun to the "knee", what you have done is effectively tuned the gun as if it was unregulated with the peak of the velocity curve at, or very near to, the setpoint.... This means that from a full tank to about 100 psi below the setpoint, the gun is operating right in the "sweet spot", and the chance of hammer bounce is much reduced.... and the velocity variation is virtually nil (in fact usually due only to pellet variations).... You have given up only a few fps and may have doubled your shot count.... This is the way I tune ALL my regulated guns....

If you further reduce the hammer strike, you are now operating on the "downslope" (eg. at 6 turns out on the previous graph).... The velocity is significantly lower, and as the tank pressure drops, the velocity usually shows a slight rise until you hit the setpoint (due to regulator creep), at which point it then increases significantly before dropping off.... The peak of that bump in the velocity is where you have now tuned the gun to if it were unregulated, in this case 1200-1300 psi.... While the gun is shooting above the setpoint pressure, it will be VERY efficient, with virtually no chance of hammer bounce occurring, and this is a good tune for target shooting, or something like FT, with one exception.... that jump in velocity below the setpoint.... If you are competing in a class where the FPE is limited and have the gun tuned for, say, 19 FPE when it is above the setpoint and they check your velocity at the end of the course and the pressure is below the setpoint you could be over the allowable FPE level.... It could also cause you to start missing targets due the increase in velocity you weren't expecting....

I personally ALWAYS tune my regulated PCPs to the knee of the curve, ie I back off the hammer spring preload until the velocity just starts to drop.... Generally that extends my shot string 100-200 psi below the setpoint, giving me additional shots on top of the already efficient setup.... If the gun is shooting harder than I want, then I reduce the regulator setpoint a bit, reduce the hammer preload to get back to the (now lower) knee of the curve.... and end up with even more shots.... Yes, I could just back off the preload, but then I have to worry about that bump in the velocity curve below the setpoint, so instead I drop the regulator setpoint and retune to the new knee....

I am locking the thread for now until I get it copied here....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:12:33 AM
The second graph was not taken from actual data, I drew it to show the trends ONLY.... However, the first graph, on the QB, was from data, and I ended up running that gun at about 4.5 turns out, at about 845 fps.... so only down about 15 fps from the plateau, ie about 2%.... Other guns will be a bit lower to find an efficient point on the knee, it all becomes a matter of balancing shot count vs. velocity.... Here is a similar graph for my 2560....

( (

I tuned that gun for 950 fps, achieving this shot string at 1.01 FPE/CI.... about 4% below the plateau velocity....

( (

That represents an approximate range below the plateau for the knee.... 3% might be an average drop, certainly well worth it from a shot count and efficiency point of view.... With really powerful guns (which tend to use a lot of air) you may have to drop 5% below the plateau to get the shot count you want.... and the plenum size relative to the FPE may have to do with how far you have to drop as well.... Some guns won't provide a perfectly flat plateau, it will still have a slight slope to it, and some actually lose a bit of velocity with big increases in hammer spring preload, but finding the point where the plateau and the downslope transition, ie the knee, is the important part anyway..... Your ideal tune will be somewhere on that curved portion, exactly where is a matter of personal choice, balancing power vs. shot count....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:13:09 AM
Here is some actual data from a .177 cal Int'l FT rifle of 12 FPE that I built a couple of years ago, showing the velocity rise below the 1500 psi setpoint....

( (

The velocity crept up from 800 fps at 3000 psi to 820 fps at 1500, at which point the air usage increased slightly, the velocity increased to peak at 843 fps at 1200 psi, and then dropped, getting back down to the 800 fps where it started at only 800 psi....  This rifle is a classic example of tuning a regulated PCP on the downslope....

Yes, the gun really did get 250 shots from 3000 psi down to 800 on a 13CI tank.... Each data point on the above graph is the average of 10 shots.... I ended up reducing the setpoint pressure and hammer spring preload to flatten the curve....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:13:59 AM
Here is some more data which you may find useful.... It shows what happens when you change pellet weight in a regulated PCP....

( (

As you can see, the position of the knee shifts to more preload as you increase pellet weight.... With 12 gr. Hobbies, it is at about 4 turns out, with 18 gr. Heavies it is at about 2 turns out, and with EunJins the knee cannot be reached even with the hammer spring at coil bind.... Doing testing such as this and graphing the results gives you a complete picture of exactly what is happening inside your regulated PCP.... It is worth doing just for the learning experience alone, at least once....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:16:13 AM
There is a relationship between regulator setpoint and the optimum hammer strike.... Hammer strike is a combination of hammer weight, travel, and average spring force.... Hammer travel and spring force affect both hammer energy (lift) and momentum (dwell).... Hammer weight only affects the momentum (dwell).... Generally, if you are regulating a PCP that originally operated at, say, 3000 psi.... and using a setpoint of, say 1500 psi.... you have to drastically reduce the total hammer strike or you will be wasting air.... That is the primary message of this thread.... You can reduce hammer strike by reducing any or all of hammer travel, weight, and spring force.... Motorhead has found that for many PCPs, when you fit a regulator, they work much better with a lighter hammer.... The downside is that if you make the hammer too light, you will have to then increase the travel, or spring force, to compensate.... This can become an issue in high-powered PCPs, as if may make the gun difficult to cock....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:17:17 AM
If you are stuck on the plateau and can't get down to the knee, you have two choices....

1. If you want more power, increase the setpoint....

2. If you have the power you want, and wish to increase your shot count.... reduce the hammer strike by lightening the hammer, shortening the throw, or installing a weaker or shorter spring....

or, of course you can just leave it, and enjoy what you have....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:18:32 AM
As you approach and fall through the setpoint:

On the plateau (thigh) the velocity falls quickly, possibly starting it's decline slightly above the setpoint if you have way too much hammer strike....
On the downslope (shin) the velocity rises noticeably as you pass the setpoint before falling after the pressure is some distance below the setpoint....
On the knee the velocity holds nearly constant, perhaps rising slighty, as you pass through the setpoint, before falling gently, then more rapidly....

When you are on the knee, the closer you are to the plateau the less chance of the velocity rising, and it will start to fall just below the setpoint.... If you are closer to the downslope, you will get a larger velocity rise, extending the usable shot count well below the setpoint, before the velocity drops too far to be usable.... When the rise is about 1% ES, that results in the highest shot usable shot count for that setpoint for most conditions....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 08:24:08 AM
Unregulated PCPs were very often tuned for extended strings for FT work by leaning on the hammer spring and choking up the transfer port.... You can certainly reduce the power of a regulated PCP very simply by installing a smaller transfer port, and if the gun is operating efficiently before, it will generally get even higher efficiency (in FPE/CI) when the power is reduced.... and it does get quieter in proportion to the FPE reduction.... It can be done externally with a adjusting screw, if that is incorporated into the design, such as my retracting bolt that can be restricted, or the velocity screw in an MROd valve port....

You can also reduce the power, and increase the efficiency dramatically, by reducing the hammer strike (preload) to use smaller sips of air, but if you go that route, you MUST stay above the setpoint, or you will experience a drastic velocity increase once the pressure drops below the regulated output.... Look back at the first post in this thread to see what I mean.... The great thing is this is instantly adjustable and reversible without any disassembly....

The velocity can also be reduced by dropping the setpoint pressure and readjusting to the knee of the now lower curve.... This gives more shots from the greater pressure range, and increased efficiency as well, although is more complex to do, and not as easily reversible as simply dialing back the hammer preload....

Generally, the very highest efficiency is obtained by using tiny sips of high pressure.... That means the valve is closing very early, and the air is still expanding as the pellet leaves the muzzle.... The shot will be very quiet as well.... The only drawback, as I said, is the velocity increase below the setpoint.... This likely makes using reduced preload the best method to detune temporarily, just watch the pressure gauge....


PS, I will unlock this topic so that you can ask questions and I will try and answer them to the best of my ability.... PLEASE STAY ON TOPIC or I will lock it to prevent clutter....
Title: Re: Tuning a Regulated PCP
Post by: Motorhead on November 16, 2016, 11:06:14 AM
Indeed all this data helps wrap ones mind around all the ways to get to the same place taking each part of system and marry it to another.  Think whats so frustrating for tuners is that there are so many ways to manipulate the working parts.  No real definitive Right or Wrong but like baking there is some talent required to get it right.
Certainly some ways work better than others, but sadly it is still an applied science to use what best fits the PCP your working with. * Which leaves us short of having a "Do This" to get there simple response for many HP to LP regulation conversions.

Great data Bob ... I'm sure it will help many for years to come who wish to take a bottomless trip down the rabbit hole of PCP tuning possibilities.  8)

Title: Re: Tuning a Regulated PCP
Post by: Shorty on November 16, 2016, 02:38:47 PM
Awesome work and post on "how to tune a regulated gun". As soon as the reg is in "it is what it is" "unless you pull it apart and start changing things again".

What some people may want to know is (and this is a question) "what" to expect if they add a regulator to their existing gun and what pressure,plenum sizing, or porting they should use to achieve that shot count or power level. So, they get it right the first time with what they are looking for in regards to power and shot count with their existing gun.

I know this may be a big explanation and starting another thread but, I think it may compliment the "how to tune" an existing regulated gun.

I know every gun is different along with barrel lengths which can change the efficiency but, do you have a method that others can follow to understand what they are going to get if they do X and Y to the gun before even adding the reg ?

Like Scott says, take the frustration out of regulating your gun.

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 16, 2016, 04:36:05 PM
It would be wonderful if I could write a simple thread on the "best" setup to use when you install a regulator, but as you say, not really part of this thread.... This thread is pertinent to how to get the most efficient operation from a PCP that already has one.... Some PCPs come regulated from the factory (and some of them are quite poorly tuned).... Others are created by installing a regulated bottle on a CO2 gun, or to convert an unregulated PCP, and you have more flexibility when creating the plenum, as it is a separate entity from the bottle, and changing the size of it doesn't affect the reservoir volume.... Still others are converted by installing an "in-tube" regulator in an existing reservoir, so you have an additional compromise to make, because increasing the plenum reduces the primary reservoir, creating yet another variable / balancing act....

The one constant is that for any given combination of barrel, ports, and projectile, the velocity should primarily be set by the regulator setpoint.... If you try and get too much velocity for the pressure, you will end up with an air hog.... If you dial the velocity way down, you will have to deal with a "bump-up" in velocity below the setpoint.... Only by tuning in the vicinity of the "knee" of the curve will you get the best combination of velocity, efficiency, and shot count.... THAT is what this thread is all about....

Title: Re: Tuning a Regulated PCP
Post by: charmin99 on November 19, 2017, 10:05:53 AM
Hi Bob,

I read this whole post with great interest and learnt a lot.

I'm in the UK and as you prob know have max 12ft/lb by law. I have just fitted an Altros regulator to my Air Arms S510 Sportet Ultimate, as you say the ft/sec has dropped slightly from 786 to 775 but to have 40 level shots is great, there is about 4ft/sec variation before it hits the downward knee, the velocity starts to drop what seems pretty linear with the drop in chamber pressure.
It does seem to be a bit of an air hog, reg set at 135bar, 200bar fill, I used to get about 35 shots across the power curve that were useable for FT, so as you said I was hoping for a few more regulated.
Is it worth reducing the hammer spring down when just being able to use 12ft/lb, I have no idea how to find a suitable spring and there is no adjustment apart from removing the seating collar on the end of the spring, about 2mm, and fit a very thin washer instead or find a weaker spring somehow.
The transfer port adjustment is optimised for max power which is the 775, it's a .177 as well.
Any thoughts....
Many thanks

Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 19, 2017, 10:26:35 AM
Without the ability to try different hammer spring preload settings, it is pretty tough to know where you are on the power curve.... If you are up on the plateau, then reducing preload will decrease air use without a loss in velocity.... If you are down on the downslope, then any reduction in preload will lose velocity....

Title: Re: Tuning a Regulated PCP
Post by: bnowlin on November 19, 2017, 12:39:24 PM
Ok let's say you need a lighter hammer from Travis' MDS hammer do you mill out some of the center brass if it is thick as min is and make sure there is enough room to have plenty room to screw the cocking screw in without the end of the crew not touching the spring. or use a weaker spring?
Title: Re: Tuning a Regulated PCP
Post by: charmin99 on November 19, 2017, 01:53:01 PM

OK, understood, so is there any other way to find where I am on the plateau or down the knee apart from playing with various spring preload, what if I was to re-set the regulator set-point higher gradually and if needed, lower gradually, measure the output power,
Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 19, 2017, 04:35:06 PM
As you lower the pressure the plateau velocity will lower as well.... Only if you are below the knee of the curve will the velocity increase with reduced pressure.... If that happens then you can assume you are somewhere on the downslope.... ie more hammer strike would increase velocity....

Title: Re: Tuning a Regulated PCP
Post by: charmin99 on November 23, 2017, 03:55:26 PM
Hello Bob,

Last question..

OK, so I have found the knee by trying various preload and dwell settings on the hammer and spring changing (costly..) and some calculation, also opened up the transfer port to optimise available setpoint pressure.

I have plotted it out and as your example it is pretty flat with a 2ft/sec variation with an amazing flat shot string of 80 then quickly falling off below at what would be the setpoint pressure.
Set point 125 bar, cylinder fill 205 bar.

The original un-regulated setup gave quite a power curve with only 38 useable shots and chronoed at 11.6 Ft/lb - 786 Ft/sec with JSB Exact 15.44

Now I have 11.0 Ft/lb 765 Ft/sec with same pellets but 80 string shots flat as a pancake...

The question, as the variation is now very good and more likely due to chrono variation I am quite happy with the result as I shoot HFT I want repeatability, however if I wanted to increase the velocity up, to say 11.6Ft/lb (785Ft/sec) and re-adjust the setpoint up, now the balance between the firing valve, striker and transfer port are set, if I just adjust the setpoint up marginally to the 11.6Ft/lb I guess I will loose a few flat shot, loose  some of the efficiency and sacrifice that for a bit higher power, is that adjustment to the setpoint now going to be a linear adjustment or close to it or would I have to start all again, which I couldnt face to be honest.

Or as you say, "enjoy what you've got.."



Title: Re: Tuning a Regulated PCP
Post by: rsterne on November 23, 2017, 04:56:09 PM
So is the 765 fps the fastest you can get at your current setpoint, regardless of hammer preload?.... If that is the case, then yes, you need to increase the setpoint slightly to get closer to 12 FPE.... The increase in FPE should, in theory, be proportional to pressure, so if you are at 125 bar now, then 130 bar should give 11.44 FPE and 135 bar about 11.88 FPE.... Yes, you will lose some shots, both because the efficiency will be slightly less (more power) and also you will have less pressure drop from full to setpoint.... Currently you have 205-125 = 80 bar of air usable, with 135 bar you would only have 205-135 = 70 bar.... At best, your shot count should drop to 80 x 70 / 80 = 70 shots.... but probably more like 65 shots at 11.9 FPE, maybe a bit less....

or, as you say, be happy with what you have now....

Title: Re: Tuning a Regulated PCP
Post by: charmin99 on November 23, 2017, 05:37:49 PM
Thanks Bob,

Just wanted to check my thinking was correct and yes the 765 fps is the fastest.

I will have a think about it after shooting off a few more pellets (200 so far for just for this..) and see how it behaves, scope and everything set up for current setting now and as you well know this is an iterative job and personally I find you've had enough fiddling after a while... haha.


I can well live with 60 ish shots but as I said the flatness is key for me, just that over here in the UK, wind and rain on a Sunday morning shooting you can do with that last few fps...

If I do it I will let you know how it went, but can I just say many thanks, I have learnt an awful lot, yours is probably the best and most concise, relevant info I have found to date.

Many thanks

Title: Re: Tuning a Regulated PCP
Post by: Josparky on April 12, 2018, 03:09:17 PM
Bob thank for such a very informative post. I have read through it many times and thought I understood what to do but have run in to a problem. I seem to be stuck on the plateau I think trying to get to the knee so all shots are flat. I have a gen 2 Maruader with a huma regulator I just installed. I set the gun I thought before I put the regulator in but apparently something is still wrong. I have the Hammer spring set to 4 turns in the Transfer Port screw out 4 turns and locked down and the Hammer Strike the full shortest I could set it from the book to 12 Turns in. Here is what I get.

Just installed a Huma Regulator in a stock gen 2 Marauder 177. Have the TP screw set to 4 turns out, the HS 4 turns in and the HT 12 turns in here is the string I got out of it below. It seems to drop off slowly all the way to 1450 where I have the regulator set at. What can I do to get it to stay more the same. Note I will be keeping all the stock parts. Just wanted to add the regulator. It was at 1450 at the 90th shot. Any help would be appreciated. Thanks

Ok guys here is what I got on 3rd string after regulator started settling in a little and I made some adjustments. I run the HT in 12 turns that is when I started saving air and the HS in 4. It was at 1450 at shot 90 which is where I set the regulator at. Seems to be dropping slightly throughout the shot string. Is there a way to make it stay the same throughout the shot string???

Created: 04-11-2018 04:20:22 PM
Description: testing reg 3 4 spring 12 throw
Notes 1:
Notes 2:
Distance to Chrono (FT): 0.00
Ballistic Coefficient: 1.000
Bullet Weight (gr): 0.000
Altitude (FT): 0.0
Temp: 70 °F
BP: 30.03 inHG
93 848 0.00 0.00
92 859 0.00 0.00
91 862 0.00 0.00
90 861 0.00 0.00
89 858 0.00 0.00
88 856 0.00 0.00
87 861 0.00 0.00
86 859 0.00 0.00
85 866 0.00 0.00
84 861 0.00 0.00
83 868 0.00 0.00
82 ERROR 10
81 871 0.00 0.00
80 872 0.00 0.00
79 862 0.00 0.00
78 858 0.00 0.00
77 863 0.00 0.00
76 865 0.00 0.00
75 856 0.00 0.00
74 867 0.00 0.00
73 859 0.00 0.00
72 863 0.00 0.00
71 867 0.00 0.00
70 865 0.00 0.00
69 865 0.00 0.00
68 863 0.00 0.00
67 871 0.00 0.00
66 876 0.00 0.00
65 873 0.00 0.00
64 872 0.00 0.00
63 870 0.00 0.00
62 863 0.00 0.00
61 872 0.00 0.00
60 867 0.00 0.00
59 869 0.00 0.00
58 868 0.00 0.00
57 866 0.00 0.00
56 870 0.00 0.00
55 868 0.00 0.00
54 863 0.00 0.00
53 870 0.00 0.00
52 869 0.00 0.00
51 872 0.00 0.00
50 870 0.00 0.00
49 872 0.00 0.00
48 872 0.00 0.00
47 868 0.00 0.00
46 869 0.00 0.00
45 872 0.00 0.00
44 876 0.00 0.00
43 872 0.00 0.00
42 880 0.00 0.00
41 878 0.00 0.00
40 878 0.00 0.00
39 875 0.00 0.00
38 881 0.00 0.00
37 878 0.00 0.00
36 880 0.00 0.00
35 879 0.00 0.00
34 878 0.00 0.00
33 872 0.00 0.00
32 884 0.00 0.00
31 880 0.00 0.00
30 880 0.00 0.00
29 886 0.00 0.00
28 873 0.00 0.00
27 881 0.00 0.00
26 885 0.00 0.00
25 884 0.00 0.00
24 886 0.00 0.00
23 887 0.00 0.00
22 889 0.00 0.00
21 877 0.00 0.00
20 885 0.00 0.00
19 887 0.00 0.00
18 881 0.00 0.00
17 893 0.00 0.00
16 885 0.00 0.00
15 887 0.00 0.00
14 887 0.00 0.00
13 888 0.00 0.00
12 890 0.00 0.00
11 884 0.00 0.00
10 883 0.00 0.00
9 884 0.00 0.00
8 886 0.00 0.00
7 890 0.00 0.00
6 890 0.00 0.00
5 892 0.00 0.00
4 888 0.00 0.00
3 880 0.00 0.00
2 889 0.00 0.00
1 884 0.00 0.00
Average: 874.01
StdDev: 10.24
Min: 848
Max: 893
Spread: 45
True MV: 874.01
Shots/sec: 0.03
Group Size (IN): 0.00

Now I tried turning the Hammer Spring out to reduce the spring tension and it is lowering the velocity but still drops over the total string what can I do to get it as flat as possible all I need to get is 60 good shots so I don't care about loosing some shots I just want it on the knee as you show in the drawing. Thanks for any help you can provide.
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 12, 2018, 05:52:42 PM
I never use hammer travel to tune, so pretty hard for me to comment on a tune where you have reduced the travel by 12 turns (about half the available travel).... The problem with the way the MRod is designed is that as you decrease the hammer travel, you increase the hammer spring preload, so you have two adjustments fighting each other....

What happens if you use the hammer travel you have now, and start with the preload at maximum, shoot one shot, reduce the preload a turn, shoot another shot, etc.etc..... That is how you determine where the knee is.... If you are stuck at maximum velocity all the way to minimum preload, then you need a lighter or shorter hammer spring....

Title: Re: Tuning a Regulated PCP
Post by: Josparky on April 12, 2018, 06:02:23 PM
Thanks for the reply if I increase the hammer spring now it raises the fps if I lower the hammer spring it decreases the fps.
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 12, 2018, 06:13:48 PM
Then you are not on the plateau, you are either on the downslope or the knee.... What is the maximum velocity you can achieve at the current 1450 psi setpoint?.... What velocity would be your ideal goal?....

Title: Re: Tuning a Regulated PCP
Post by: Josparky on April 12, 2018, 06:41:30 PM
My ideal goal is to stay around 875 to 880 throughout the whole string right now when it drops off the regulator it starts dropping as you can see from the string above. So if my studying is right it should be in the pleatue if I spelled that right lol. But when I start dropping the hammer spring it starts dropping the fps. Now I can lower the hammer throw and get that fps back up but only a few turns. I will loose some shots but that's ok with me if that's what I need to do. I did before dark  let the hammer spring out one turn so it is at 3 in now and I turned the hammer throw out one turn also now at 11 to get the fps back up to around 880 and it seemed consistent still but I only had time to take a few shots after that didn't get a whole string in but the shots where around 875 to 885. I know the regulator needs to settle in from what I am told it may take around 300 shots for that. I have run around 150 to 200 through it so far.
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 12, 2018, 08:19:05 PM
Josparky, don't take this personally, it is intended to help ALL those who continually ask me the same questions....

If you don't know what the maximum velocity is, you don't know where you are in relation to the plateau.... This is the most common problem people have when trying to tune a PCP, they don't take the time and the few pellets to find out where they are at.... THE FIRST STEP IS TO DETERMINE THE MAXIMUM (PLATEAU) VELOCITY, WHERE THE KNEE IS, AND WHERE THE DOWNSLOPE STARTS.... I am getting frustrated with people asking for help, and not taking this simple first step.... PLEASE, EVERYONE, DO THIS FIRST, I CAN'T HELP YOU IF YOU WON'T HELP YOURSELVES....

1.  Crank the preload to the maximum, shoot a shot, record it....
2.  Reduce the preload a turn, and repeat....
3.  Continue until the velocity is falling a lot
(like 100 fps per turn), or it is way below what you would ever want....
4.  Draw a simple graph like this, you don't need to use a spreadsheet, graph paper will work just fine.... (two pellet weights are shown)....

( (

5. Determine at what preload the plateau starts (-3 T for the 34 gr., -4 T for the 25 gr. above).... you will never need more preload than that....
6. Determine what the velocity is on that plateau (this is the maximum at your setpoint pressure, 1060 for the 25 gr., 970 for the 34 gr.)....
7. The downslope is where the velocity is dropping rapidly, maybe 100 fps per turn (850 @ -5T for the 34 gr., 900 @ -5.8 T for the 25 gr.)....
8. The knee of the curve is the part between the plateau and the downslope, where the line is the most curved.... THAT is the only area you want to tune within....
9. If the velocity on the knee is higher than your goal, reduce the setpoint pressure....
10. If the velocity on the knee is lower than your goal, increase the setpoint pressure....

The best tune, IMO, is usually between 3-5% below the plateau velocity.... and NEVER more than 10% below it....
You can tune lower than that, but the velocity will INCREASE below the setpoint, and often the ES is large, with a high shot-to-shot variation, because the valve is operating in partial valve lock.... You need to reduce the setpoint pressure to get back on the knee of the (now lower) curve.... If the plateau is lower than your required velocity, you need to raise the setpoint and start over.... Each time you change the setpoint, do another curve, DON'T GUESS....

Using the above curves, I tuned for the 34 gr. at 940 fps at -4 turns.... That is 3% below the plateau.... I could have tuned it for more shots at 900 fps (-4.5 turns), which is 7% below the plateau, but much lower than that I would reduce the setpoint.... This 1900 psi setpoint is ALMOST TOO HIGH for the 25 gr. pellets.... To drop the velocity to 950 fps, where I would want it, I would be operating nearly on the downslope, and about 10% below the plateau.... The gun would be very efficient tuned like that.... but the ES would be increasing, and once the pressure dropped below the setpoint, the velocity would bump up more than a percent, likely 2-3%.... I would obtain a better tune by reducing the setpoint to about 1700 psi to drop the plateau velocity to 1000 fps or just below, and then tune on the knee, 3-5% below that (now lower) plateau, to get my desired 950 fps....

The greatest stability in shot-to-shot velocity, and the lowest ES.... plus the minimum effect from any regulator creep.... will occur when you tune to the knee of the curve.... It is like shooting an unregulated gun right near the top of the bell-curve, where the ES is the smallest, and slight differences in hammer strike or pressure make almost no difference to the velocity.... The closer you are to the plateau velocity, the more power but the lower the efficiency and shot count.... The closer you are to the downslope, the less power but the higher the efficiency and shot count BUT watch out for increase ES and the velocity INCREASING below the setpoint....

Title: Re: Tuning a Regulated PCP
Post by: Josparky on April 12, 2018, 08:47:19 PM
Thanks Bob again for your help and reply. I take nothing personal in this life anymore my friend. We are not all as smart as some of you and we do really appreciate the time you take to help us. I will take the time to do exactly what you said this last description I am sure will help others to understand what they need to do as well. This does add some lite to how to get to where we need to be. I can only think of one more thing to help me and others out so you are not bothered with us as much. Can you add to that last post a simple explanation of if we never get to the plateau or vise versa cant get off the plateau which way we would need to move the setting pressure on the regulator to find it. I hope you understand what I am asking. Then I think most could figure this out a lot easier without having to ask anymore questions. I hope I have made since with my last question. If it is already answered in here please forgive my ignorance in not seeing it. AGAIN I REALLY REALLY REALLY DO APPRECIATE YOUR WILLINGNESS TO SHARE YOUR KNOWLEDGE WITH THE COMMUNITY NOT EVERYONE IS WILLING TO DO THAT.
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 12, 2018, 09:41:26 PM
Since in almost all cases when a regulator is installed in a PCP, you are reducing the pressure at the valve, there should never be a situation where you can't get to the plateau, assuming you had any reasonable bell-curve in the unregulated version.... More likely, you may not be able to tune the preload LOW enough to get below the plateau.... However, here is that information....

If you are stuck on the plateau and no matter what you do you can't reduce the velocity, you need to either shorten the hammer spring or replace it with a weaker one, and/or fit a lighter hammer.... Alternately, if you want more velocity, you can increase the setpoint pressure....

If you are on the downslope and cannot reach the plateau (and hence are not sure where the knee is), you need a stronger hammer spring, or a spacer to increase the preload (if the spring isn't at coil bind), or you can try a heavier hammer.... Alternately, if you have too much velocity, you can decrease the setpoint pressure....

Title: Re: Tuning a Regulated PCP
Post by: Motorhead on April 13, 2018, 08:21:36 AM
I had strongly suggested a lighter hammer and higher set point in another thread where the same inquire was made ....
sadly attempting to stay with 100% oem parts with a regulator added has been proven time & time again to fall way short of ideal or able to do what a gun can that has been altered to best utilize regulation modifications.
Title: Re: Tuning a Regulated PCP
Post by: kayakr on April 26, 2018, 05:52:49 PM
Awesome thread guys. I'm reading the summary of the thread as "After installing a regulator, max out your hammer energy (high spring tension) and then back it off until you just start losing shot FPS (3-5%) which means you're at the point where you're supplying just enough and not burping extra air.  If backing off all the way isn't reducing FPS, then you need a lighter hammer or spring."

So what should one select as a lighter hammer?  I see Motorhead said for Marauder to select one at 50% of stock and JSP is selling one at 25% of stock weight. Probably the hammer weight needs to be lower if you're going for maximum shot count with a lower shot energy ft / lbs, right?

Once you have reached the edge of the plateau and or have the light weight hammer, then additional hammer debounce systems such as SSG or TSS become unnecessary, correct?

Say one wants to detune a bit, how should the regulator pressure setpoint be determined?  The thread seems to indicate you want to primarily control FPS with regulator setpoint, run with ports open and then just tweak the hammer energy until you get the best sized plug of air for that regulated energy.  Is there an approximate guideline such as1800 PSI for full power and 1500 PSI for lower energy (say 12 ft/lbs in .177)?   Can one determine this by setting up their gun with vent port full open and then shoot the reservor down until the desired FPS is reached and then read the tank pressure? 

Huma suggested guidelines, but I suppose these would be based on the assumption the user wants a high FPS and also tuned to the size of their plenum. ? Huma FAQ - "Typical rifle in .177 it is around 125 bar and for .22 caliber it is around 130-135 bar. Average set pressure for .25 is 140-145 bar."

Also, any pros and cons to share on the various regulators?   The reviews seem to be a bit scanty. I've heard of Huma:, Audrius:, Lane: ,  The Altaros seems to have some negative about not venting to atmospheric pressure but I don't understand what that means. Without other information, Audrius looks like the best value at the moment, and maybe Huma and Lane being best sellers.

Thanks for the help. Lost 4 hours sleep over the last couple days trying to get this approximately figured out.
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 26, 2018, 08:03:21 PM
The summary in your first paragraph is a good one....

I tend to find that a 50% reduction in hammer weight is enough to make a significant change in the shot characteristics.... eg. 100%, 50%, 25%..... Small changes do little.... Hammer strike will need to be reduced to reduce the FPE, and hammer weight is only one way to achieve that....

Use of a system to reduce or eliminate hammer bounce is always a good idea.... Just being on the knee of the curve (3-10% below the plateau) will help, but depending on many variables, an SSS, SSG, or TSS may produce additional shots by increasing efficiency....

If you are starting with an unregulated gun, then shooting the pressure down until you get the velocity you want, and then using slightly more than that for your setpoint, is generally the way to start your tuning process....

I am not thrilled with in-tube regulators, and have never used any of the commercial ones, so you are asking the wrong guy for advice on those.... Most of my regulated PCPs are of the higher-power varieties, with a bottle and external regulator.... That allows me to use 1 cc of plenum per FPE without losing primary reservoir capacity....

Title: Re: Tuning a Regulated PCP
Post by: mcoulter on April 28, 2018, 07:44:08 AM
Since in almost all cases when a regulator is installed in a PCP, you are reducing the pressure at the valve, there should never be a situation where you can't get to the plateau, assuming you had any reasonable bell-curve in the unregulated version.... More likely, you may not be able to tune the preload LOW enough to get below the plateau.... However, here is that information....

If you are stuck on the plateau and no matter what you do you can't reduce the velocity, you need to either shorten the hammer spring or replace it with a weaker one, and/or fit a lighter hammer.... Alternately, if you want more velocity, you can increase the setpoint pressure....

If you are on the downslope and cannot reach the plateau (and hence are not sure where the knee is), you need a stronger hammer spring, or a spacer to increase the preload (if the spring isn't at coil bind), or you can try a heavier hammer.... Alternately, if you have too much velocity, you can decrease the setpoint pressure....


Hi Bob,

I have done some testing on my FX Royale which came regulated from the factory.  I have replaced the regulator with a HUMA.  (FX replaced the original regulator once for me but I've not had very good luck with them.)  I shot four pellets over the chrony and took the average for each at six hammer spring settings (zero to six turns out) and got this data.  I never see the plateau.  Does this indicate that I should be thinking about doing what you describe above in the blue text?

Pellets = JSB 18
Temp = 55f
Regulator = 125 bar (I'm suspect of this and wonder if it's actually much higher)
Chrony distance = 14 feet

Google Sheet Link -


Any advice would be appreciated!
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 28, 2018, 08:27:16 AM
You are getting close to the plateau at your maximum preload, you can tell by the flattening of the curve on the left end.... If you extrapolate the curve, you can imagine it going horizontal at about 960-970 fps.... One more increase in preload (if you weren't already at coil bind) would probably make little if any difference to the velocity.... which for practical purposes is the plateau....

OK, so let's assume the plateau is 970 fps (I really don't think it would be any higher) with your current setpoint.... You want to tune the gun to 3-5% below that, which would be 920-940 fps.... If the plateau is actually 960 fps, then you would tune at 910-930 fps.... Therefore there is no need to increase the setpoint, unless you want to achieve a higher velocity than about 940 fps.... If you want a bit less, you could tune down 7% or so below the plateau (~890-900 fps) and get more shots and higher efficiency, but you are likely to start seeing a slight increase in velocity below the setpoint.... If you tune 10% below the plateau (ie about 860-870 fps) you will get lots of shots, but the chances are your ES will be quite large (high shot-to-shot variation) and once the pressure drops below the setpoint you will likely see quite an increase in velocity.... This is because the gun would be acting like an unregulated gun, with a bell-curve, below the setpoint....

Bottom line is, if you are happy anywhere between 890-940 fps, you probably don't need to change the setpoint pressure.... Just shoot a string at each preload setting, keeping track of the number of shots and the pressure drop, so that you can calculate the efficiency (FPE/CI) at each setting.... Also, pay attention to what happens to the velocity at the end of the string, when the pressure drops below the setpoint, and how many usable shots you can get below the setpoint, before the velocity goes too high or too low.... That will give you all the information you need to make an informed decision to balance against your desired velocity.... I find the "best" tune is where you can shoot a few shots below the setpoint pressure, with the velocity staying constant or rising not more than 1% before falling.... and terminate the string when the velocity drops 1% below your average.... That is usually 100-300 psi below the setpoint.... This gives the best balance between power, efficiency and shot count at that pressure....

Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 28, 2018, 09:46:41 AM
With time comes increased clarity and understanding (ie I'm always learning).... so I thought I would revisit this subject to add a chart that might help you understand what happens as you tune a regulated PCP with progressively less velocity than the plateau.... Here is an updated version of the chart in the first post of this thread....

( (

The 3%, 5% and 10% below Plateau velocities are approximate only, but will give you an idea of what happens as you reduce the hammer strike, and hence the velocity, on a regulated PCP.... It will show you exactly why I like to tune 3-5% below that plateau.... because (at about 5% down) you get a few more usable shots below the setpoint before the velocity drops off.... or at the very least (at about 3% down) you save a lot of air compared to tuning up on the plateau, with very little loss of power.... The chart also shows what happens if you tune about 10% (or more) below the plateau.... Note that below the setpoint the velocity increases significantly.... and while operating in that realm gives you extremely good efficiency, for some applications (eg. Field Target, or in some countries where there is an FPE or velocity limit) that bump could put you over the limit....

Every gun is a bit different, and yours might produce a curve like the "5%" one above only 3% below the plateau, or it might be 7% below.... but to date those would be about the outside limits I have found to produce a tune that has a usable shot string (ie within a ~1% ES) extending slightly below the setpoint, which is the tune I prefer.... I hope this chart allows you to visualize what I have tried to confer in this thread....

Title: Re: Tuning a Regulated PCP
Post by: kayakr on April 28, 2018, 05:29:49 PM
I picked up a used .177 mrod which I would prefer to run at 800FPS and get more shots versus 1000 and fewer. I would think about giving it a regulator and a light weight hammer, but I think i just discovered the hammer is an unknown, because the tension is already backed out all the way, shooting as if tuned for high pressure with the backed out hammer and I can't find the inner 1/8 in hex adjustment screw inside.  So maybe I should just be happy with the 50ish shots I get as is between 2750 (about as high as I can pump) and 1800. I'm not sure how much more efficiency I would get without being able to pump it higher. I might have to get the light weight hammer as well as a bunch of components because of the unknowns with this hammer.

Any opinions about the efficiency being seen here?  Pellet is Crossman premier heavy 10.3 grains, 15 ft/lbs

(shot string graph attached)
Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 28, 2018, 06:13:43 PM
Respectfully, this thread is about tuning regulated PCPs in general, not an unregulated MRod.... I am far from an expert on the latter, so I would suggest you ask your question in the PCP Section....

Title: Re: Tuning a Regulated PCP
Post by: mcoulter on April 30, 2018, 04:13:56 AM
Bob, thank you very much for your insight and comments on this process.  Your posts above brings considerable clarity to this process! 

The overall velocities that I am was seeing did not seem right to me.  (Too high.)  So I took the regulator out again.  I found this...  Somehow in handling / installing the regulator, I am also inadvertently actuating the pressure adjustment.  I had installed it at 130 bar...


I turned it back to 120 BAR and shot a full-string but this indicates that i'm at least a full turn (counter-clockwise) past the scale. 

I accidental overfilled by 10 bar so this string is from 230 bar to 148 bar.  My normal starting fill is 220 bar.  Velocities seemed to drop around 179 bar which was about 50 shots:


For reference here is the target that I was shooting during the above string.  53 yards, 12-shot groups.  The winds were gusty and variable so it's hard to judge which velocity worked best, but it *felt* like 910 fps were best (bottom left group).


My next step is to remove the regulator again and screw the adjuster in one-full turn and repeat the above tests  :-)

Title: Re: Tuning a Regulated PCP
Post by: rsterne on April 30, 2018, 08:07:14 AM
The shot string indicates that the regulator it not regulating, IMO.... It looks like the most stable operation (least shot to shot variation) is occurring at around 900 fps.... If you know what the pressure was at that velocity, that is where to set your regulator, or slightly above that pressure....

Title: Re: Tuning a Regulated PCP
Post by: mcoulter on May 01, 2018, 05:04:27 PM
Yes, fortunately I did keep track of the pressure as shown by the gauge on the rifle.  Here are some points:

Shot #56 = 171 BAR = 912 fps
Shot #67 = 161 BAR = 899 fps
Shot #78 = 151 BAR = 887 fps

The second to last string here is the best group and includes most 900-ish fps velocities.  This was in the neighborhood of 160 BAR.


Oh, I should also mention that I have reached out to HUMA (admitting my mistakes here...) and have asked if they can advise on what the typical number turn in/out should be on the regulator or if there is a typical length to get the reg back in the normal  90 to 160 bar range.  Again the marker was on 120 bar but was more likely delivering 160 bar due to my inadvertent "adjustment".

5/9/2018 - Update

Please see this post while I work through some adjustment "issues" with my HUMA regulator...  I'll likely post back here again when I feel like I've stopped chasing my tail  ;-)'s-in-the-house/msg20588/
Title: Re: Tuning a Regulated PCP
Post by: O2bmark on May 08, 2020, 08:57:26 AM
1st question:  I have a Rapid Air Weapons HM1000x .25 rifle which at present shoots 33.95 JSB diablo MKII pellets at about 882fps.  I haven't any satisfactory way to determine exact regular pressure but by extrapolation (gauge on gun when it comes off reg, comparison to my tank gauge etc.) it seems to be about 145BAR.  I was under the impression that they came from the factory set up for 25gr pellet so I thought maybe gun not optimal for 34gr I prefer. Well I cranked up the hammer tension till velocity hit maximum (897 fps).  I wouldn't mind more shots but when I came off 3% to about 870 fps the gun was more erratic until I came back to 882.  I guess the real question is: do some guns just need to be near plateau?
2nd question:  How are you guys making the graphs?  Is there a program you use? 
Title: Re: Tuning a Regulated PCP
Post by: mcoulter on May 08, 2020, 12:23:02 PM
I'll take a shot at the graphing question...  If you have Microsoft Excel this is the best one out there (IMHO):

It does take some time to learn! Be sure to follow his tutorial that's linked from the same page above.

If you don't have Microsoft Excel, then you may want to try Google Sheets which is an online spreadsheet.  You will need a Google account to do this.   It is very simple in comparison to Keith's Excel sheet.

Here's a sample google spreadsheet:

Title: Re: Tuning a Regulated PCP
Post by: O2bmark on May 08, 2020, 07:29:58 PM
Thank you for the link.  I see graphs all over the forums and wondered if there was a standard method everyone was using (except me).
Title: Re: Tuning a Regulated PCP
Post by: 761xl on May 09, 2020, 04:41:05 AM
dang ! im getting a feeling this tuning stuff is as hard as doing chinese arithmetic while drinking good whiskey,and trying to make good whiskey ,without water... if mr. sterne doesn't make housecalls i might as well trade off my imperfect pcps :D much respect to those who truly understand all facets of this , especially mr. sterne. excellent reading gentlemen[ and ladies , just in case].
this place is awesome !
Title: Re: Tuning a Regulated PCP
Post by: rsterne on May 09, 2020, 03:02:11 PM

The closer you are to the plateau, the more air you will use, but generally the smaller the ES (shot to shot velocity swings)…. Some guns can easily tolerate 5% below the plateau without much change in velocity, while others need to be closer to the plateau.... Further away makes the gun more sensitive to hammer strike variations.... Having said that, if your shot to shot swings are less than 1%, just shoot it and enjoy, you won't see smaller groups by getting fussier....

Title: Re: Tuning a Regulated PCP
Post by: O2bmark on May 10, 2020, 12:03:57 PM
Thank you for the reply Bob.  As the weather got warmer and I'm sort of stuck at home I started playing with my PCP airguns.  The airwolf .22 battery was permanently dead which I believe is the original so I can't complain and if I had a spot welder I'd make a new one (7AAA).  If I get really bored I may try to make a spot welder.  So, I broke out the RAW .25 and my Labradar to see what I could see.  Since my original post I did weigh out 2 tins of pellets was surprised at the spread in weight (33.3-34.8gr).  Head size is much more uniform than weight.  Now if I could only make the gun more quiet (presently 2 chambers 1 solid baffle).  Suddenly I want a 3D printer....