PCP tuning question

[ER00z]

I've always viewed the hammer stroke (limiting) adjustment as a last step in fine tuning. Others may feel differently, but I figure get velocity/fpe close to what's desired with the hammer spring and/or transfer port diameter (if applicable), then playing with the amount of hammer travel to fine tune.

I've only owned one pcp with the hammer stroke adjustment (Marauder), and honestly never tinkered with it. On a regulated gun, I'm not sure if it's a redundant feature or not, but offers another option in tuning. If easily accessible, it could be a quick way to change the velocity for various situations.
 
Just thinking if my Marauder was regulated, the easiest settings to access would be the hammer spring & stroke adjustments. Limiting the stroke to a short "blip" of the valve could be handy for when lower power is wanted, or allowing full force for high power shots...

[JimD]

The smaller inner allen head adjustment on my Prod controls how far out the piece that contacts the valve is extended from the hammer.  Before I did a B-Staley tune I turned it sometimes but it seemed to have no effect.  But with a B-Staley, a small fraction of a turn makes a much larger impact on my velocity than does an equivalent turn of the hammer spring.  So I use the smaller screw, hammer throw, first to get in the ballpark and then I use the hammer spring to make smaller adjustments and try and tune for accuracy.  The useful positions seem to be from about 1 turn out to about 2 turns out.  But 1/10th of a turn will make a difference in this range (least might actually be around 1.25 turn). 

[nervoustrigger]

In the simplest terms, a hammer stroke adjustment provides a way to alter the balance of lift and dwell.  I would say that is a feature more useful on an unregulated PCP than a regulated one, provided they both have a means to adjust the hammer spring tension.  The reason for that is an unregulated PCP relies on the balance of lift and dwell to optimize its self-regulating characteristics…the feature that allows it to produce a pretty consistent velocity over a wide range of pressure.  Meanwhile a regulated PCP operates at a (more or less) constant pressure…simply tweaking on the hammer spring tension for the velocity knee will produce some fixed amount of lift and dwell that will yield a stable velocity.
 
Granted like horsepower and torque, lift and dwell are interconnected.  Increase one and the other goes with it.  But to the extent we can manipulate them with a simple adjustment, longer stroke produces more dwell (relates to hammer momentum) and more hammer spring tension produces more lift (relates to hammer energy).
 
And by the way, it could just be my interpretation but based on the presence of the term “limiter” in the discussion, I get the sense there is some blending of hammer stroke adjustment and a valve limiter which is a distinctly different topic.

[subscriber]

Thanks Jason

[nervoustrigger]

claims he removes the valve springs in (some of?) his pcp guns entirely. He relies just on the valve pressure to keep the (valve stem) poppet closed. Claims you only need the spring to get the pressure up during filling, but he uses a compressor (or bottle) to fill his guns. Going that route, he's getting away with very little hammer spring tension.

I'm not advising this...

A regulated PCP will generally work fine without a valve spring.  However it plays an important role in the self-regulating characteristics of a conventional PCP where the operating pressure covers a wide range. 

Regulators aren’t perfect so there is an argument for always using a valve spring.  Plus of course the filling from zero pressure with a hand pump that was already mentioned.

Except in the extreme case where you just can’t get enough dwell (heavy slugs in long barrels), my advice is to keep it.

[rsterne]

Here is a graph of the typical relationship between hammer strike and velocity and efficiency in a regulated PCP....



You can see the velocity plateau, which starts about 2 turns out from maximum preload.... At about 4 turns out and less preload, the velocity drops in a more or less linear fashion as you reduce preload.... Between 2 and 4 turns out is the "knee" of the curve, and provides the best balance between power and efficiency....

Hammer energy is what creates lift, whereas hammer momentum is what creates dwell.... Increasing preload or hammer travel both increase the energy.... Increasing the hammer weight (spring and travel being constant) does not change the energy, but does increase the momentum.... Therefore, hammer mass changes the relationship between lift and dwell.... Since the amount of air delivered by the valve is somewhat proportional to the area under the lift to dwell curve, a heavier hammer will give a bit more power than a light one, because the valve is staying open longer.... Generally if you lighten the hammer, you will need a bit more travel or spring power to make up in lift what you lost in dwell....

In a regulated PCP, you really only need enough valve spring to seal the valve during filling.... More than that will require a corresponding increase in hammer energy, making the gun harder to cock (at a given pressure).... However, fitting a stiffer valve spring, and increasing the hammer strike, can reduce the shot-to-shot variation in velocity caused by an inconsistent hammer strike.... This is particularly noticeable when tuning on the downslope, where hammer strike is very light and the gun is operating on tiny sips of air.... When the lift and dwell are very small, any tiny variation in hammer strike will result in a large ES....

Bob

[ER00z]

Rsterne and Nervoustrigger, thank you very for your insights. A big thanks to everyone else also. There is a lot to take in when tuning a pcp, and I've gotten lucky a few times 

I just filled/pressurized a new Origin, which has a piston in the air tube (pressurized on the back side). Its a little different from filling any other pcp I've encountered. After a pressure hold test (and the rain stops) I'm looking to do a simple chrony/spring adjustment (if necessary) and get shooting. Keeping it simple with this one 

[ER00z]

Something interesting I encountered, but not directly related to PCP tuning... Air leaks!!! Lol

I figured it might be worth mentioning for those newer to pcp's reading through this thread, as it seems to happen with all pcp's eventually, especially when tuning/modifying/etc...

Rsterne, among others have had some great threads on tuning and various modifications which I found overwhelming as a newbie (still have to read over things a time or two... or four, Lol). The one thing I found when getting things back together after alterations is I've ended up with a slow air leak in a few instances. Being extra careful not to damage o-rings and using pure silicone grease (divers grease), silicone oil and Teflon tape (on threads) usually has kept leaks at bay, or to a minimum.

A slow pressure leak I'm currently in the process of sorting out is on a spare air cylinder for a Hatsan. Since it was a spare/unregulated cylinder for "full power" use, it's been on the back burner for a while. Using balloons I was able to track the leak to whare the valve threads into the cylinder (not a poppet leak, checked that first), but a water dunk test would work just as well. A while ago (a year or two) I replaced the o-ring that seals the valve, and used divers grease on the o-ring. The oring that was used came as a spare with the gun, which was manufactured in 2013 (old-but unused o-ring). When I checked it this morning, it seems the silicone grease "dried out" some and the oring took a set. I figure before I change it for a fresh one, I'd lube it with silicone oil and reassemble/pressurise to see if that would solve the issue (a.k.a. being lazy    ). Best bet would be to replace it normally, but I'm curious if silicone oil would help "revive" the o-ring/seal the leak.
(So far so good)

The nice thing about have multiple cylinders (and one being regulated), is I can fill to "X" Bar/psi on the un-regulated cylinder, swap it with the regulated cylinder and shoot over the chrony to get a snapshot of what current settings would do at various pressures. Might not be exact for adjusting the regulator, but will get me in the ballpark (I hope, Lol).

I really appreciate all the information/replies so far. Thanks to everyone, as I'm still wrapping my head around all this pcp stuff.

[Ribbonstone]

 "This is particularly noticeable when tuning on the downslope, where hammer strike is very light and the gun is operating on tiny sips of air.... When the lift and dwell are very small, any tiny variation in hammer strike will result in a large ES...."

That's about as short and clear an explanation for the "spikey" behavior of the low end of tuning as I've read.