.25 Disco fever.... add another project to the pile

[lloyd-ss]

I'm thinking that the hammer and spring are most of the power difference compared to my Hayabusa.... especially since my FPE with the 25 gr. is slightly higher than yours at 2000 psi.... Have you ever bothered working out all the port areas to see where the greatest restriction is?.... When I did my Hayabusa I started from 75% of the barrel (ie 0.188") and worked back from there, just making sure nothing else was more restrictive than the barrel port.... That was easy with the transfer port and valve exhaust port (also 3/16").... The valve throat is 1/4" with a stock 5/32" stem.... That works out to 0.049 - 0.019 = 0.030 sq.in. and a 3/16" hole is 0.028 so no problem there.... I use a bolt probe that is 1/8" (smaller than the valve stem).... so again, less restriction than the barrel/transfer port....

Have you noticed any tendency for the pellet to hang up on the 0.199" port?.... just curious.... I wonder where the optimum is for diameter/flow compared to TP volume losses.... I know where that is on .177, it's a stock 0.140" port.... Even angling the port / streamlining seems to decrease velocity unless you are REALLY pushing the limits.... but then that's what we are doing, right?.... If you're not having pellet loading problems perhaps I should be looking at 80% of bore for the barrel port.... or maybe an elongated hole that is only 75% of the bore wide but as much as one caliber long (ie 0.188" x 0.25")?.... That should flow about the same as a 0.216" round hole.... let's call it 7/32".... Then again, I wonder if the increased TP volume would hurt?....

BTW, I gave the Condor owners a gentle "poke" on the CAF, telling them what you were doing and that you had achieved over 77 FPE with a 43 gr. EunJin.... and asking them what a Condor can do with that same pellet.... no answer so far.... Maybe somebody here will chime in.... I'm thinking we must be getting pretty close.... 

Regardless, I'm darned proud of what we are achieving on only 2000 psi.... pretty remarkable.... Really good idea using an O-ring to measure valve lift.... make SURE you tell us those results.... and you're right this is a BLAST, and it's what forums such as this are for.... sharing ideas in the best of possible ways....

Bob

Bob, I'll answer as best as i can, from the beginning of your email.

Regarding the heavy hammer and spring, I think that made the biggest difference at the high end of the pressure range where the tank pressure works harder against the head of the valve.  Now that all the porting is more or less maxed out, I can drop back on the spring and hammer and see what happens.  I also think the extra light return spring helped.

With the various port restrictions I think streamlining the ports might be as important as port size.  Trying to get the air to turn the corners and make an ugly U-turn.  I have heard that contouring the bottom of the bolt probe can be helpful.  With my current setup, I calculate that at 2000psi, I use 8 cc's of air out of the tank with each shot.  8 ccs will fill the 25 cal barrel for 10" of length.  That's a lot of air at that pressure, so I am guessing that a little extra volume in the passages after the valve won't hurt too much.  A tall breech block adds extra volume and I think those shoot just as fast as a short breech block??

I deburred the inside of the barrel where the .199 hole enters it such that the pellets go in easily.  I haven't shot this gun for accuracy, so I don't know if I have a problem there or not.

This is good stuff!
Lloyd

[workinmanVA]

Great job lloyd. Those are some great #s. Smoken. Have you got to shoot a good shot string? I was lookin back through my notes and when I had the lite valve return spring I was getting higher #s but my standard deviation doubled. That was before I installed the rear cocker. I'm tempted to put the lite spring back in.
One of the nice things about the rear cocker is being able to fine tune the hammer weight ,screw one off and screw a lighter one on. I've been curios about the max travel of the valve stem. The stock valve stem length from the back of the valve to tip of the valve stem is around .330 +/-. My guess would be around .200 .. I like the o ring trick I'm going to have to try that.
Any way its exciting to see your accomplishments. Its a good feeling when thinks work out.

[rsterne]

I'd be really shocked if the valve stem travel is 0.200".... but I've been wrong before.... IIRC poppet valves flow 100% when the lift is 1/4 of the seat diameter....

Area = Pi R^2 = Pi x R x R
Circumference = Pi x 2 x R
Curtain Area = Circumference x Lift = Pi x 2 x R x  L
For Curtain Area equals Seat Area.... Pi x R x R = Pi x 2 x R x L.... solve for L.... L= Pi x R x R / Pi x 2 x R.... L = R/2

If the seat diameter is 0.3" the lift only needs to be 0.075" for full flow.... After that additional lift only increases the duration not the flow rate.... at least in theory....

C'mon Lloyd, stick an O-ring on that hammer stem.... we're dyin' here.... 

Bob

[lloyd-ss]

Bob,
Prepare to be shocked.  I have to run to work in a few minutes, but I put an o-ring on the cocking rod to act as a tell tale.  I think I had estimated my valve stem protrusion at about .140.  Well, I think it is probably more like .160.  I had shortened it at your suggestion to get more hammer travel distance.  I took several dry fire shots at about 1400-1200 psi and the o-ring consistently showed about .160 of travel until I backed the spring preload out several turns.  Then I was getting about .130 of valve travel.  I cranked it back in a little and got a 51 ftlb shot (25.4 gn at 953 fps) with about .150 travel at 1200 psi.  I think my valve stem might be too short and I am limiting the power to a measly 77 fpe at the higher pressures, LOL.  I will have to spend more time with this tonight and get some accurate measurements!
Not a hint of hammer bounce, but that's another story.
So the extra travel distance is not necessary for the air flow, but it is what increases the dwell.  Makes sense.
Lloyd

[rsterne]

Be interesting what you find when you fit a longer valve stem (or will you shorten the back of the valve like a 22XX?).... Mind you, I've never played with a 4 oz. hammer either....  

You greedy guy!.... I only told you I shortened the stem 0.060"....  

Bob

[lloyd-ss]

OK, I have cleaned this thing up a little since last time, but she is still a mean one.  Bob, yes, I did get greedy with the stem length, but that version has a home made valve stem and head that is about .100 shorter than stock.

So what I have done recently is 1) increase the hammer stroke, 2) regain the original .250 (approx) max valve stroke, 3) incorporate a much stronger and adjustable spring, 4) incorporate a much heavier hammer that is adjustable in weight.


Here is what I came up with.  Some of the ideas are new, some are copied from other folks, and some are variations of what others have already done.  Can you imagine how hard it would be to do something like 20 years ago before the "information highway?"


This is a pic of most of the parts that go into a set up with a .770 hammer stroke.  Stock is about .600.  I also has room for a fatter and longer spring, and is adjustable for preloading the spring.  It also has a big hammer, plus a cocking pull rod with the additional weight of the cocking knob.
Notable in this pic is the cocking rod that has a drawer pull as a cocking knob. The hammer and rod are 70 grams and the knob is 35 grams.  Because the knob is tied directly to the hammer, changing the knob will change the hammer weight.  We were discussing a similar situation on another gate earlier today.




Here are all the pieces assembled together.  The threaded insert that the cocking rod goes thru adjusts the spring preload. The drawer pull has a nice sharp edge to scrape the hide off your thumb when the hammer fires forward.  Gotta fix that!




Here is how I got the extra hammer stroke length.  In the stock config, the sear catches on the bevel at the front of the hammer.  In the new config, the sear catches at the same place.  OK, no diff.  BUT The new hammer has cup and the surface that contacts the valve stem is recessed  in that pocket.  That way, the hammer doesn't hit the valve stem until the front of the hammer is well past where it used to strike the stem.  About .200 farther, but I have a little piece of dowel pin in there that will drive the valve stem down past flush at the end of the valve body.  That is to compensate for a short valve stem (shut up) and I am again giving up a little hammer travel.  If I make a new valve, I will make the stem longer so I don't have to jury-rig the hammer stroke.



More coming shortly!!

Lloyd

 

[lloyd-ss]

Have you ever wondered how far the valve actually opens when it is struck by the hammer?  Yea, me too.
I was able to do it with this setup by putting an o-ring around the cocking rod.  You re-adjust it with each shot, and then, shooting the gun moves the cocking rod and hammer as a single unit.  The o-ring is pushed back on the rod at the maximum valve stroke for that setup.   Then the hammer bounces and the gap left at the o-ring shows how far the valve actually opened.

The picture sequence shows an interesting phenomenon that we had all heard about but probably didn't have any proof of.

The hole in the adjuster that the cocking rod slide thru is too small for the o-ring so it stops the movement of the o-ring.
Look at the last 2 pics.  In the second from the bottom,


The air is at 2200psi  and the valve has only gotten open to .076 of travel before the HPA forces it shut.

Then in the last pic, the gun has been fired at  1600 psi and the stroke stopped at .150

I also took at shot at around 1200 psi and the valve had opened up more than .200

I will be working on getting some shot strings that have some useful data.  With the adjustability of the spring and hammer, it will be tough to zero in on a "best" setup, but there will be lots of "good" setups.

Lloyd

[rsterne]

Lloyd.... I gotta hand it to you.... You definitely think "outside the box".... I love the recessed hammer face (gonna steal that one for my Hayabusa).... For my "big-bore" Hayabusa I planned on moving the valve ahead to get more hammer fall.... With that idea, I may not need to....

The idea of using an O-ring on the cocking rod to measure valve lift is one of the best demonstrations of the KISS principle I have seen in a very long time.... BTW, remember I said that max flow on a poppet valve occurs when the lift = 1/4 diameter.... How big was the valve throat again?.... 

Now you're gonna have the problem I have on my Hayabusa.... too many variables.... 

Bob

[lloyd-ss]

Bob,
The recessed hammer face just kind of came to me when I was figuring out what I would have to move to get more hammer stroke.  Definitely use it!  When I re-make the valve stem with more sticking out, I can replace that little steel dowel pin in the hammer with a flush one, but I think the net result will be the same???

I stole the o-ring idea, but it is perfect for this.  The hole in the valve is .281 and the valve stem is .139.  But the front end of my valve is very un-areodynamic and needs to be redone, I think.  It's all about chasing the bottle necks.  HPA flows more like water than air, so I have opportunities for improvement.

Yes, I agree on the too many variable thing.  And I have a hard time changing just one thing at a time, LOL.
Lloyd

[rsterne]

I have a hard time changing just one thing at a time

I noticed !!!

must resist.... mmuusstt rreessiisstt.... mmmuuusssttt rrreeesssiiisssttt.... 

Bob

[lloyd-ss]

Here is the latest data, but I am not too thrilled.  A lot of work, but not much additional gain.  I know there are some bottle necks that I need to figure out.   

So I took these shots with various pellets to get a feel for what might be there.  I am starting to run out of pellets and need to order some.
The .25 Air Venturi has straight cylindrical sides and is just about impossible to load without hammering them into the breech.  Has anybody used these Air Venturi .25 and do you have the same problem?  My barrel is actually a barrel liner for firearms so might be working against me.

So the shots were with one Air Venturi 50gn, two eunJin 42.8 gn, and the rest were Barracuda 30.2 gn.  My grain weights might look odd, but I weighed the pellets to get an accurate weight for the tests.  The total hammer weight is about 105 grams.  With a few of the adjacent shots, I removed the cocking knob, which cuts the hammer weight by about 35 grams.  And to be honest, I dont think removing the extra hammer weight helped or hindered.  There fore, it will probably be best to leave it off.  Less wear and tear on everything.  Still, the stock weight is about 53 gns, and even without the knob, the new hammer is 75 grams, so almost a 50% increase.

So, the power level seems to be stalled    ,it likes heavy pellets, but is really inefficient with light pellets    .  It uses lots of air, but that is to be expected at these power levels.  After all, that's why added the second tube, LOL.   

So what to do next?  I think the valve head is like driving down the road with a sheet of plywood stuck flat against the front of your car.  Not aerodynamic at all.

So, any suggestions from anyone?  First thing is I need is more pellets, or maybe soft cast bullets.  The thought of using bullets does  turn this thing into something else, but for the sake of experimentation and knowledge, maybe I need to go there.  Still, those big EunJins might be plenty of weight.  What do you think      I know mellowroadman over on the Big Bore gate had cast some nice .25 bullets.

Thoughts anyone..... Please.....
Thanks,
Lloyd

Various shots with heavy hammer and spring.  Various pellet weights.  Starting and ending pressures recorded, but the 63 psi per shot is just an average.  Efficiency stinks, but that's what it costs.  Actual valve openg was .085 with the first shot, and slowly crept up to about .200 with the last shot.

[rsterne]

hmmmmmmmmm.... Interesting results.... My first comment is that unless you are going to regulate this beast, you either need to increase the fill pressure or reduce the hammer strike.... The velocity starts out high and declines from there.... That indicates you need to balance things better.... I am getting almost identical results from my Hayabusa with the Baracudas.... 66.3 FPE at 1900 psi.... but at a lot higher efficiency (~1.1 FPE/CI).... Like I was discussing in another thread, when the heavy pellets show a large advantage over the medium weight ones.... the gun is likely an air-hog.... Your efficiency numbers, from my experience, are showing that trend.... so I would back off on the hammer strike, lose a bit of power, and gain some efficiency.... If you can throw those Kodiaks downrange in the mid 900's you'll have a heck of a rifle.... Have a look at this graph....



Notice what happens with the last 2 turns on the hammer spring preload.... With the light pellets there is NO gain in velocity.... and even with the heavy pellets the gain is very small (~3%).... Then look at the efficiency curve.... You are using nearly 50% more air and gaining nothing.... Now check out this graph for an unregulated gun....



Notice that the lightest pellets start at their highest velocity, and as the weight increases, you begin to get a bell-curve.... By the time you get to EunJins, the bell-curve is well established.... For the curves on the left (at maximum hammer spring preload), here are the efficiencies....

Hobby 11.9 gr. - 0.78 FPE/CI
Express 14.3 gr. - 0.83 FPE/CI
Exact 15.9 gr. - 0.87 FPE/CI
Heavy 18.1 gr. - 0.90 FPE/CI
Kodiak 21 gr. - 0.95 FPE/CI
Rab. Magnum 24.8 gr. - 1.03 FPE/CI
EunJin Dome 28.7 gr. - 1.05 FPE/CI
EunJin Pointed 32.4 gr. - 0.94 FPE/CI (IIRC, they were a tight fit)

With 18.1 gr. Heavies at reduced hammer spring preload, the efficiency increased to 1.14 FPE/CI in the case of 2 turns out.... Your shot string looks like the ones for the light pellets in the above graph.... and the efficiencies are in that range as well (~0.8 FPE/CI).... When you look at shot count, if you set a 4% velocity variation as the standard.... using the Baracudas starting at 1013 fps.... you only have 7 shots above 972 fps (including the one 42.8 gr).... starting at 2137 psi and ending at 1693 psi.... IMO, better to lose a bit of velocity, and gain efficiency and shot count.... 

It's a hoot going for broke and seeing what your new toy will do.... and I'm as guilty as any of savouring that moment.... Then, unfortunately, you have to back the gun down to what is reasonable and practical.... With the power you are getting from the platform, the only way I can see to possibly take advantage of the potential FPE is to try much heavier "boolits".... A good fitting, aerodynamic 50-60 grain bullet might prove just the ticket.... but I have no experience to draw from on that....

Bob

[lloyd-ss]

Yup, that's my style.... go for broke first, and get everything maxed out.  Then back the velocity down to 80 or 90% of max, and that will cut the air usage almost in half.  But a gun developed that way will never be efficient in the low power ranges because the air passages will all be over sized.  But I could leave it stock if that's what I wanted, LOL.

I think the fact that i was able to drop the hammer weight from 105 grams down to 70 grams with no appreciable change in velocity shows that the spring is overpowering the hammer and valve.

Like you said, I do want to try and get some more top end power out of this by going to a little higher fill pressure. I have done the calculations and I will  replace the (3) 8-32 screws that hold the valve in place with (3) 10-32 screws.  That should be a strength increase of about 40%.   The tube and other components all calculate well within the desired safety factor.

Lloyd

[Rescue35]

I have done the calculations and I will  replace the (3) 8-32 screws that hold the valve in place with (3) 10-32 screws.  That should be a strength increase of about 40%.   The tube and other components all calculate well within the desired safety factor.

Lloyd

Lloyd, I love the progress you are making. I think I know what my winter project might be. Thanks to you and Bob for the ideas and information.

I highlighted the last paragraph from your post because I think it is important for someone thinking of undertaking a project that involves high pressure to understand that the math comes first. From the shear stress on the bolts to the stress on the thin walled cylinders there is a lot going on when a PCP is just sitting there fully charged.

I am not trying to discourage anyone. If someone cant find a formula or is uncertain of performing the calculations feel free to PM me. I am more than willing to help as are many here.

I took the time to do the calculations when I converted my 2260. Remember, safety FIRST, and happy shooting.

[Norm_m]

Rescue35 is right on!!! As intersting as this thread is, ANYONE THINKING OF BUILDING  THEIR OWN PCP NEED TO KNOW THE DANGERS. The pressures that are present in all PCPs, be it a custom built one or one you buy of the shelf can cause extreme harm if you do not pay attention to what you are doing. Things like being disturbed while filling your PCP and taking your eyes off the gauge even for several seconds can cause pressure to rise above safe levels. So everyone out there be safe and be careful we do not want to see anyone hurt or worse.
Norm

[0351_Vet]

Great Thread and Thanks for warning us all of the dangers involved with PCP's. I think this thread is a marvel of very firearms educated people pushing the envelope because THEY HAVE THE KNOWLEDGE. I personally wouldn't attempt any of the above myself, but it is certainly educating. Thanks Bob & Lloyd for the informative thread.....and Norm for the stern warning on PCP's.

Again....GREAT THREAD

Edited for bad grammar......

[lloyd-ss]

Thanks guys for reinforcing the SAFETY aspect of PCPs.  That always has to be number one in your mind. 

The pcps we buy are well engineered with all the calculations to back up their safety.   Many of us like to work on our guns, and there are plenty of great mods you can do that don't affect safety.  But unless you can honestly say that you know exactly how a change will affect the safety, I would not do it.  Rescue35 has graciously offered to take a look at folks ideas, but it all boils down to YOU doing the right thing.

Norm's comment about paying attention while filling your gun is EXTREMELY important now that 4500 psi is so available. Recommended fill on a disco is less than half that. Each time you overload a metal part, it is weaker the next time.  Please be careful.

Lloyd

[lloyd-ss]

I was thinking about the discussion on going for broke on a design (my style) and got to thinking I'd stop changing one thing after another and take a closer look at where I was right now.  
Well, I am glad I did.  In fact I am kinda tickled about it.  
One thing I had noticed about the heavy hammer (and then only1harry had mentioned it elsewhere) was about beating things up with too much hammer.  During one session, I had removed the 30 gram cocking knob, which left the hammer at a mere 75 grams, yeah.  Bottom line was that removing the extra hammer weight didn't drop the velocity much at all.

So I replaced the cocking knob with a 10-32 wing nut, and used the long spring and backed the preload way off.  I played with it to get the curve to start where i wanted and the velocity where I wanted it, and ended up with a very nice classic inverted bathtub velocity curve.  And a very pretty one, I think.  And the efficiency went way up, too.

Filled to 2250 psi, shot down to 1500 psi,  310cc dual tank
Barracuda 30.2 gn (weighed them)
23 shots between 903 fps and 960 fps.
18 shots with a 4% spread of 921 to 960 fps.
18 shots between 57.0 fpe and 61.8 fpe.

I was going to do some shooting for accuracy and was all set up to shoot in the woods behind my garage, but then got a long phone call and it was getting too dark to shoot.  Got off 3 shots at 43 yards.  2 of them touched and one was 1-1/2 " away, so I have no idea what I've got. But I will find out.  Was using a bug buster 6 x 32 which likes lots of light.
Lloyd
P.S.  I am attaching the pics a different way, so I hope it works.

edit note- I took a total 5 psi readings for the string and averaged them in my table.

[rsterne]

Excellent results.... and the efficiency at ~1.4 FPE/CI is great as well.... If the Baracudas will group for you, you are in business.... I'd try the 25.4 gr. JSB Kings with the same tune for accuracy.... and also backed down into the mid 900's (~50 FPE).... Since those are the only pellets with a good BC and a .25 cal PCP is the king of long distance critter whacking.... you don't have a lot of other choices....

PS, if you plot the velocity and energy curves on an axis that starts at 0 fps it looks a lot flatter.... 

Bob

[lloyd-ss]

I managed to get a tiny bit of shooting for accuracy done with the gun set up as in the chart above.  I pushed my target a little farther into the woods to get a full measured 50 yards.  It runs downhill pretty noticeably.
After it was all settled in, I did a 6 shot group with H&N Barracudas at about 940 fps for 59fpe.  Leapers Bug Buster 6x 32 scope. The group was 1.25 c to c, so definitely not what I am looking for, but it's a start.  And I have only tried the Barracudas.

50 yard lane down into the woods behind my garage. Target circled in red.


This guy was trying to block the lane. Yes, those are antlers.  The 2nd one had just trotted off.
Darn pests!


the 50 yard target