How Unregulated PCPs Work

[rsterne]

I was explaining something in another thread, which made me look at this graph a different way, and I wanted to share it with you.... Here are the curves of velocity vs. hammer spring preload for my Grizzly at two different pressures, with three different pellets....



You can see that in every case, once you reach a certain preload, the velocity plateaus and no more velocity can be obtained.... As you reduce the preload, the velocity starts down slowly (what I call the knee of the curve), and then falls in a more of less linear fashion as you reduce the preload further (the downslope).... Think for a minute what is happening where two curves for the same pellet cross, for example the two red lines representing the 45 gr. JSB pellet.... The dotted red line (the velocity at 2900 psi) crosses the solid red line (the velocity at 1800 psi) at about 930 fps, when the preload is about 2.7 turns out from flush on my particular RVA.... This means that at that preload, if you started shooting at 2900 psi and shot a string down to 1800 psi, the string would start at 930 fps, climb to a higher peak, and then drop back to 930 fps at 1800 psi.... As you go to a heavier pellet, you need a bit more preload to get the velocity the same at both 2900 and 1800 psi, particularly when you go from pellets (45-50 gr.) to a 70 gr. bullet....

While this really won't help you tune an unregulated PCP, understanding what is going on sure can't hurt....

Bob

[rsterne]

I made up a couple of graphs that might help explain what happens in an unregulated PCP, and how changing the hammer spring preload changes the shape of the shot string.... Consider these graphs....



The upper graph show what happens at constant pressure as you change the preload.... There is a separate line on the graph for each 200 psi from 3000 down to 2000.... Each curve has a plateau, and knee, and a downslope.... If you pick one hammer spring preload, and plot the velocity at each pressure, you get the curves on the lower graph.... The black line is a typical bell-curve, and corresponds to the preload marked "Tuned" on the upper graph.... The other lines show what happens if you increase or decrease the preload.... Note that if the preload is quite a bit too high, the velocity falls every shot as the pressure drops (red line).... However, if the preload is reduced a bit from there, but still more than what we would normally use (orange line), you can get a few shots over a very narrow ES, at near maximum velocity.... That is an excellent way to tune a Big Bore, as it gets near maximum power without vertical stringing, and is more efficient than a declining shot string.... The light blue and dark blue lines show what happens when you reduce the preload from ideal.... Note, you can use those settings if you reduce the fill pressure.... The light blue line would be usable with about a  2800 psi fill, and the dark blue line below about 2600 psi.... In both cases you could continue below 2000 psi to complete the bell-curves....

Bob

[MicErs]

Good Stuff!  Thanks! 

[SpiralGroove]

Nice Bob,
Thank God for the RVA

[Rivers3Plinker]

Love your tech posts Bob. Really appreciate sharing the knowledge with us.

[jwthe2nd]

Hi rsterne,

Thanks for sharing so much of your findings.
I have a .22 AA S510 carbine FAC and would like to tune it to give as flat a curve as possible using 16 grain pellets at about 850 fps muzzle velocity.  Is it better to start with reducing the hammer mass first or to reduce the hammer spring preload first by grinding?  Followed by adjustment with transfer port opening to fine tune.


Thanks.

JW,
Fellow Canadian

[rsterne]

Hammer weight changes the momentum, and hence the dwell, but not the lift.... Hammer spring preload changes the energy and momentum, and hence both dwell and lift, and also how easy it is to crack the valve against a given pressure.... If the gun is fairly close to what you want, you need likely only change the preload.... Only if you need to drastically reduce the velocity because you have a "first shot fastest" setup do you need to worry about the hammer weight.... speaking in general terms.... Even then, preload may be the only adjustment you need....

Bob

[jwthe2nd]

Thanks very much rsterne.  It is much clearer when the variables are isolated.