FPE vs Caliber and Port Sizes

[enazle]

Starting out at 3500 psi it peaks around the 10th to 12th shot.  The dwell depth in Inches is .24 and the hammer weight is 120 grams total.
I miscalculated the area at throat and it is .037 not .049.  I'll try to open it up to a size "G" which will give me the .049 area with a .125 stem.  The stem may be choking the port at the peak of the dwell so I will try to remove a little more material leading into the port to offset the stems intrusion.  Aignt this fun.

[rsterne]

The dwell depth in Inches is .24

If you mean that the lift is 0.24", that is a LOT.... Typical lift on PCP valves seems to run about 0.06" at high pressure, increasing to twice that at the bottom end of the usuable bell curve.... How are you measuring that, with an O-ring on a sliding spring guide or what?.... You can't simply measure how much the valve can physically move and assume that is the lift.... Once your lift is greater than 1/4 of the throat diameter, the flow RATE doesn't increase, just the dwell (open tme)....

Bob

[enazle]

Exactly, o-ring on the spring guide.  Then that probably confirms my theory of the plunger stem choking the port. Agree?

[rsterne]

I'm not sure what you mean by the stem choking the port.... it's always in the throat of the valve, no matter how far it opens, and easy to calculate the remaining area.... I am astounded at the amount of lift you are getting (0.24").... At what pressure are you getting that?.... 3500?..... 3000?.... or zero?.... The higher the pressure the less lift, of course....

Bob

[enazle]

I'm not sure what you mean by the stem choking the port.... it's always in the throat of the valve
The stem is turned down to .125 back to the base of the port. At the peak of the lift the full diameter of the stem enters the area underneath the port.  However, this is not a problem at full pressure because the lift is less than .06"

Here are the Port and valve opening. The stem is turned down to .125 back to the base of the port.


This is after 12 shots and velocity with 27.8 gr domes is 785fps.


This is the amount of the lift with the 20th shot at its peak at 1075 fps with the domes.


The remaining air dumped at shot 26 and 812 fps.

Here is the hammer and springs for reference.


I really think I have plenty enough spring. I have a couple disco valves coming from Crossman so I can make different modifications if you have any suggestions?

[rsterne]

I've never seen such a large knob (that sounds rude!).... nor have I seen that much lift occurring either.... You are working in a tuning regime unfamiliar to me.... What do you estimate the pressure is at shot 20 and 1075 fps (71 FPE).... Would it be possible to graph the shot string (velocity vs. pressure), I'm really curious....

Bob

[enazle]

No gauge on this one.  Ha ha the knob is a Lowe's special.  The hammer, rod and Pull weigh 4.25 oz. The Springs are a 55 wire 7/16 x 2" outer and a green disco inner spring. I know the answer is in the poppet and valve seat.  I know you did some research and testing in this area, did you find a sweet spot as to valve seat dimensions? Currently the seat is about .02" and digs into the poppet. Could this be my issue?
Here is a picture of the poppet and valve seat.  I noticed the seat is not centered due to drilling out the throat to 9/32".  I think i can center it a little better.  How wide should I make the seat in you opinion?

[rsterne]

The poppet in your gun look just like the one in my .30/.35 cal Hayabusa.... When I ran 3000 psi with a 5/16" throat the Delrin starts extruding into the throat.... One of my airgun buddies who studies a lot on materials says that Delrin will extrude at a force of about 260 lbs (IIRC).... interesting that it's the force, not the pressure that is important.... I can't quite wrap my brain around that.... However, there was certainly a dramatic difference in the extruding when I went from a 9/32" throat to 5/16" at 3000 psi.... I was using a 0.030" seat, but it was only raised about 0.015" above the flat front of the valve, and the head of the poppet extruded right over it until the valve was sealing over the outer edge.... ie a diameter of over 3/8".... That increased the force to over 340 lbs, and really munched the head of the Disco poppet I was using.... In addition, I was having great difficulty opening the valve, the performance was way down compared to using the smaller throat.... just not enough hammer strike.... I was running a 145 gr. hammer with 1.45" of travel, driven by a 0.36" diam. 3" long spring made of 0.051" wire....

Bob

[enazle]

Here is a brand new poppet and the re-worked valve seat.


I didn't notice any difference other than it might have helped a little on the initial lift. I will dismantle the gun in a couple days see if there is an impression in the poppet face. 

The current spring I'm using has a compressed length of .77" leaving a distance to the poppet of .5675 at full compression. Which would indicate a pre-load of .70 at coil bind. It will be Thursday before I can do any testing but I might try to lesson the pre-load or  lengthen the spring.

[enazle]

What will happen if I lengthen the distance between the poppet and hammer (Head Space).  Currently, it's about 9/16". 

The only way I can see to do this is to grind off the poppet stem a bit?  Mr. Sterne, do you have any advice as to a starting point?

If the Compressed Length (CL) is .77 and Free Length (FL) is 2", is the maximum acceleration achieved at 75% of (FL) or at 1.5"?  I think I read that somewhere?

Based on my math that would indicate a head space of about .75" needed or taking about 3/16" off the stem?

[rsterne]

I have never heard the term "head space", nor do I know where the maximum acceleration occurs.... I would think the acceleration would be the highest at full compression, as that is when the force is the highest, and F=ma (a = F/m).... The energy and momentum peak at the time of contact with the valve stem, provided that there is still "some" preload on the spring at that point, otherwise the momoentum and energy peak when the spring reaches it's free length (and hence has no more force left)....

The 9/16" you are referring to, is that the hammer travel (stroke) from sear release to first contact with the valve stem?.... It is about 0.58" in a stock Disco.... The stock valve spring in a Disco valve goes coil bound at 0.25" lift, and the stem sticks out 0.32", so I typically shorten my stem 0.070", which increases the stroke to 0.65", plus the 0.25" travel, and after travelling 0.90" the hammer hits the back of the valve at the same time the valve spring goes coil bound (and incidently all flow through it ceases).... On my current .30 cal project I additionally counterbored the front of the hammer 0.070", reducing the maximum stroke to 0.180", but I could have done that by shortening the valve stem 0.140" instead.... That increased the hammer stroke to 0.72"....

If you shorten the stem 3/16", wouldn't the hammer hit the back of the valve?....

Bob

[enazle]

Head space and hammer stroke are the same thing. I think I have a little less space because I re-faced the valve seat which move the end of the stem back towards the hammer.  .1875 might bottom out the hammer, so I will start with .10 removal.  However at 3000+psi I bet I don't get that much lift.  My goal is to get 3 shots at the highest fpe possible. Anything over 80fpe will be gravy.

A friend has a ranch toward Abilene, TX that has a very serious Hog problem.  I told him I was after a 80 to 100 lb hog. His response was, I may have to sift through two or three, 200lb porkers to get one that size!

[rsterne]

You remember you said you were getting 0.24" lift?.... Did you notice that the valve spring goes coil bound at 0.25"?.... That "choking" you think you are getting at high lift from the larger stem might well be no flow remaining between the coils of the valve spring.... Once coil bound, no air can get from the inlet side of the valve (reservoir) through the spring to the throat, right?....

BTW, what are you using for valve retaining screws that are safe at 3500 psi?.... The ones in your photo look like pan heads.... what is the thread size?....

For maximum FPE and penetration, I suggest you look at heavier pellets or cast bullets.... The 53 gr. .25 Auto bullet (which can be made with a hollowpoint at about 46 gr.) comes to mind.... Mr. Hollowpoint also makes bullets even heavier, but if you go too long you may have a stability issue, depending on the twist rate of your barrel.... Heavy bullets wlll up the FPE noticably....

Bob

[enazle]

Good questions.
The screws are 10/32 SS from Don C. where the tube came from.  The valve spring is a light gauge wire that binds at .15.  That big lift number is after 20 shots which my best guess is around 1800 to 2000 psi.
I only have a couple more fills before my tank drops below the 2900psi mark (meaning a trip to town for a refill).  Valve adjustments take a lot of air. LOL I will report back after the mod in a few days.

[enazle]

The results are in and I am pleased.  I shorten the stem .10", polished the valve seat and increased the spring energy and here are the results!
This is the first shot with 27.8 gr Benjamin Domes. 85fpe


I refilled and made these two shots with 53gr Hunters Supply Bullets

114 fpe


110 fpe

The only down side is there is very little adjustment as the these number are with the adjustment all the way out and it cocks just before coil bind.  However this is a hunting rifle so all I can say now is

HERE PIGGY PIGGY 

[enazle]

I just want to say thanks to Bob Sterne for the advice that he has shared freely with me on this project.  I hope some day you write a book, as I will be first in line to get one.

Thanks

E


2260 on Steroids
.22 cal & .25 Cal

[rsterne]

Starting pressure on those three shots?.... Well done!

Bbob

[enazle]

Right around 3100psi.  I have a 3000psi regulated Ninja that I'll get filled and use when I sight in the scope. The 28" TJ's barrel shoots groups better than I based on my earlier testing.

[rsterne]

So what did you end up using for all the port sizes.... throat, stem, exhaust port, transfer port, and barrel port?....

Bob

[enazle]

Here is the worksheet I used. All dimensions are in inches.