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rsterne · Moderator

I just had one of those "AHA!" moments while writing another post.... It's probably one of those things that lots of guys know, or at least allow for (I did), when tuning a PCP, but until moments ago, I hadn't really organized my actions into words, so here goes....

When you change the caliber, you need to make a corresponding change in the dwell (via hammer strike).... This is necessary to compensate for the larger (or smaller) bore volume.... It's a simple concept, but one I had somehow glossed over.... Here are my thoughts on the matter.... Let's take the three common calibers, and a barrel length of 24".... The bore volumes are:

.177 - 9.7 cc
.22 - 14.5 cc
.25 - 19.3 cc

For the moment, let's assume we start with a .22 cal PCP that is tuned for a beautiful, classic bell-curve starting at 2500 psi and down to 1500, all shots with a 4% ES, and peaking at 2000 psi.... What does that mean?.... Well, it means that the combination of valve dwell and pressure is balanced to produce velocities with that 4% range over that range of pressures.... At 2500 psi, the pressure is higher, but the valve is not opening for very long, so the volume of (higher pressure) air is smaller.... As the pressure drops, the valve dwell increases, and the valve releases more air but at lower pressure.... Eventually, the volume peaks, so does the velocity, and then as the pressure drops, the velocity declines with it....

Now let's assume we take off the .22 barrel, and substitute a .177 barrel.... It only has 2/3rds the volume, and yet our valve and hammer are balanced for the larger bore volume of the .22 cal.... Now that small amount of 2500 psi air is more than needed to make the velocity stay 4% below the peak it will be at 2000 psi.... and MUCH more than is required to balance the velocity at 1500 psi.... In fact, it may be enough volume so that the first shot (at 2500 psi) is the fastest in the string.... In order to rebalance the shot string over the same range of pressures, we need to REDUCE the hammer strike....

The reverse occurs if we install a .25 cal barrel.... Now the bore volume is 1/3rd larger than the .22 cal barrel, and the (relatively) small amount of 2500 psi air isn't enough to push the pellet to within 4% of what it will be at 2000 psi.... In fact, with the larger caliber, we would expect the peak of the velocity curve to move to a lower pressure, possibly down into (for example) the 1700-1800 psi range.... To rebalance the shot string over the original range of 2500 down to 1500 psi, we will need to INCREASE the hammer strike....

The same thing will happen in a regulated gun.... Most of you realize that I am a proponent of balancing the hammer strike against the regulator setpoint so that the gun is operating on the "knee" of the velocity curve.... which is basically the same thing as always operating at the top of the sweet spot (velocity peak) in an unregulated gun.... If you don't change the hammer strike and go to a smaller caliber, you will be moving up onto the "plateau" of the velocity curve, and therefore wasting air.... so reducing the hammer strike won't lose velocity, but will increase shot count.... When going to a larger caliber, you will be operating on the downslope of the curve, and increasing the hammer strike will move you closer to the "knee" and hence increase the velocity.... both of the above without changing the regulator setpoint....

I hope I've made the above argument clearly and understandably.... Sometimes things are obvious, and adjusted for, without knowing (or articulating) WHY they occur.... For me, until this morning.... that was the case....

Bob

rrdstarr · Member

How do you make the valve stay open longer(dwell) for the .25? A longer stem? A bigger transfer Port? A bigger volume valve? A lighter or heavier hammer?

rsterne · Moderator

"Dwell" is mostly a function of hammer strike.... You can increase hammer energy by increasing hammer travel or spring force.... Both of those increase momentum, and so will increased mass....

Increasing the pressure will increase the amount of air that can flow through the valve at a given lift, and through a given size of port.... Those can be made to increase the volume of flow required for a larger barrel volume.... but they are not part of the point I was trying to make.... The length of the stem has little to do with the lift of the valve, very seldom is it the limiting factor, and in fact lengthening the stem will usually decrease the hammer travel, decreasing the dwell....

Bob

rrdstarr · Member

Sorry Bob I don't have an engineering background, Just a retired old soldier who makes a living now machining parts for airplanes. Pictures help one hell of a lot.

Thanks!

radar · Silver Status Member

Do you find that expanded volume of air released (volume at S.T. and P., for example) should match the volume of the bore? Or have you gotten that far with the study yet?

rsterne · Moderator

The volume at STP is wayyyyyyyyyy bigger than the bore.... in fact the volume of HPA released runs anywhere from 10% to 100%+ of the bore.... If it's over 50% the efficiency is very low, however.... This means the valve typically closes when the pellet is 10%-50% of the way down the barrel....

A typical PCP runs about 1 CI of air (at STP) for each FPE produced, which is called the efficiency in FPE/CI.... So, a 50 FPE gun would use ~50 CI of air at STP.... The volume of a .25 cal barrel 24" long is only 1.18 CI....

Bob

Robw · Veteran Member

radar · Silver Status Member

rsterne · Moderator

Not sure what you mean by "share the statements".... To calcuate the bore volume, you divide the bore by two (to get the radius), square that, multiply by PI and multiply by the length.... If you are working in inches, multiply the result by 16.4 to convert to cc....

.25 cal:

0.125" x 0.125" x 3.14 x 24" x 16.4 cc/CI = 19.3 cc

Calculating the "optimum dwell" would require an understanding of the flow conditions through the vavle and ports at all the different range of lift values.... and a whole bunch of other things.... However, it isn't that difficult to "work backwards" from empirical data and deduce what is happening in terms of when the valve is closing relative to the position of the pellet in the barrel.... Lloyd Sikes, developer of the Rogue ePCP shared his spreadsheet with me, and it allows that.... My very much simplified summary above is based on working with his spreadsheet....

Bob

Robw · Veteran Member

Ah, that is what I thought. Thanks.

A statement is one half of an equation. ie: 3+4=7,

3+4 is a statement, 7 is a statement and

3+4=7 is an equation.

Most people mistakenly refer to a multiply value
statement such as 3+4 as an equation when in fact
it is not.