Disco Double Gets Cothran Valve

[rsterne]

I installed an SSG this morning, using a new Disco spring (at least I think that is what it is) and set the preload to give about 5 lbs. of force and then adjusted for a slight gap.... No joy, about 200 fps.... I adjusted the gap to zero, about the same.... but as soon as I set "negative" gap (ie some preload on the valve stem when uncocked) the velocity jumped to nearly 1000 fps.... I pulled it apart and adjusted the preload to 7 lbs. and at that setting I could barely get any gap without the spring going coil bound, which takes about 12 lbs. of force, making the gun about as hard to cock as I find acceptable using the bolt handle.... At 3000 psi I still had only about 500 fps, so I drained the reservoir down to 2500 and it shot over 900, so I thought "finally, I will get a curve".... I filled the reservoir 100 psi at a time, and the velocity suddenly dropped at about 2800 psi.... I then filled to 3000 and shot a string with the following results.... I was using the 78 gr. JSBs, and later added the data for the bullets.... The pressure curve only applies to the pellets, of course....



It's pretty clear what is going on here.... When the pressure is over about 2800 psi, the valve is not opening properly.... It is only using about 40 psi per shot, and the velocity is varying all over the place between 570 and 680 fps.... As soon as the pressure drops below the magic number, the valve opens the way it should, and the velocity jumps way up into a stable operating zone.... It uses about 150 psi per shot from then on, with the velocity dropping as the pressure drops....

I was hoping to see a further improvement in the efficiency using the SSG, and indeed that did occur.... Previously I was getting 0.88 FPE/CI with the pellets, and now it was 1.03 FPE/CI.... although the average power was down slightly from 166 FPE to 160 FPE over the first 3 "proper" shots.... What continues to disappoint me is that I still have been unable to get anything that resembles a bell-curve.... When I dial the hammer strike down below a critical point, the velocity drops like a stone because the valve is outside its proper operating range.... When the valve IS inside its proper operating range, then all I can get is a "first shot fastest" shot string....

Without changing anything on the gun I tried 3-shot strings with each bullet, starting at 3000 psi.... The starting velocities were as before, and actually the drop per shot was slightly less, because the gun was using less air per shot with the SSG.... It only used 440 psi for 3 shots, so obviously it's working.... The efficiencies increased to 1.30 FPE/CI with the 127 gr. and 1.38 FPE/CI with the 154 gr, with slightly increased power, up from a 3-shot average of 208 FPE to 213 FPE.... There was only a 21-26 fps drop over the 3 shots (3% ES), which means a very usable 3 shots for hunting with either bullet.... The combination of the Cothran Powerhouse Valve and an SSG produces a great setup for my Disco Double in .357 cal, capable of over 200 FPE for 3 shots within less than a 3% ES.... Of course in a single tube Disco, with half the air, you would only get two, with a greater loss in velocity between the shots (maybe 2 within a 4% ES?).... A third shot would be at significantly lower velocity, although OK for a follow-up if required....

Bob

[Sfttailrdr46]

Bob would that set up work better as a bottle baby using a 90ci carbon fiber bottle for more usable shots and better consistency ?

[moorepower]

Smaller T.P.?

[rsterne]

Adding air would increase the number of shots, and tighten up the velocity drop between each shot.... but I see no reason why it would change the basic declining shot string into anything else.... Right now I have just over 15 CI of air onboard, so a 90 CI tank at 3000 psi should give six times as many shots within the same 3% ES.... let's call it 20 usable shots at 200 FPE.... The exception would be if you had a 4500 psi bottle and then regulated it down to 3000 psi.... Then you should get no significant velocity change, shot to shot, more than what you get with any regulated gun, or possibly even tighter, because once the valve opens, IT OPENS !!!....

If you tuned the gun for 200 FPE shots on regulator, at let's say 1.30 FPE/CI using an SSG.... that works out to 154 std. CI per shot.... Using a 90 CI tank, filled to 4500 and shot down to 3000, that is 103 bar x 90 CI = 9270 CI / 154 CI = 60 shots by my calculations.... The question is, do you really want a 90 CI tank hanging off the end of your gun?.... If I could fill the lower tube of my DD to 4500 psi, and had a regulator between the two tubes, I should be able to get 5 shots at 200 FPE, without having the velocity dropping off.... Lloyd has already shown how well the Powerhouse valve can work in a regulated PCP....

moorepower, I have the ability to just turn a screw and restrict the porting on the gun.... see reply #19 on the previous page.... I have just done that.... I screwed the adjuster in 4 turns, which blocks off about half the barrel port.... 3000 psi fill, 78 gr. JSBs....

635 - 931 - 916 - 905 fps.... pressure remaining 2540 psi.... 3 "good" shots avg. 917 fps (145 FPE) at 1.03 FPE/CI.... First proper shot at about 2940 psi....

I then screwed it in another turn, same fill and pellets....

877 - 867 - 860 - 852 - 840 fps.... pressure remaining 2340 psi.... 5 shots avg. 859 fps (127 FPE) at 0.92 FPE/CI.... starting at 3000 psi (like the bullets)....

As you can see, the increased backpressure caused the valve to open properly at a higher pressure, just like a heavier bullet does.... However, there is still no sign of self-regulation.... The flatter strings are due to less pressure drop per shot as the FPE goes down.... but they are still first shot fastest.... I see no point in going further, I was getting more power, better efficiency, and a proper curve with my original valve.... I can get 10 shots inside a 4% ES bell-curve at 126 FPE average with the same pellets on a 3000 psi fill.... 834-867-842 fps at 0.98 FPE/CI, and that was without an SSG.... or 9 shots at over 130 FPE, or 7 shots at 135 FPE....

Bob

[oldpro]

 This is exactly why I didnt go with a balanced valve all the data Ive seen and Ive seen a lot shows no way to get a proper bell curve and only declines with pressure. Even the best balance valve set up I have seen wasnt nearly as efficient as a standard self regulating valve. The only advantage I see is less cocking effort and thats all. Lloyd at the GTA fun shoot had a balance valve using a diaphragm That I do believe could be made to be the best of both worlds with a little work. Lloyd if your reading this Id love to get more details on building that diaphragm valve.

[rsterne]

The balanced valve in the Slayer certainly can be tuned for a bell-curve.... The efficiency I am getting from the Cothran is better than I have achieved with my own valve, and at higher power to boot.... We are only scratching the surface on balanced valve designs, and I fully expect them to get better and better with time.... The biggest drawback is that they require more precision to make, and more development work so that they do the job intended and are tuneable.... I think the Cothran valve would be hard to beat in a regulated PCP, but unless I can figure out how to get a bell curve on it, then in an unregulated PCP it's just a way to get big power, for a big bore, with a light hammer strike.... That in itself is a big plus, of course, and it's what Don designed the valve to do.... For that purpose, it's hard to argue with success....

Using this valve in a .257, regulated at 3000 psi and fed from a 4500 psi bottle would be a VERY interesting project indeed.... Lloyd was getting some spectacular results at just 2200 psi in .357 cal with the MRod version.... Check out reply #185 on this page.... http://www.gatewaytoairguns.org/GTA/index.php?topic=109718.180

Bob

[Gippeto]

Bob, would you mind pulling the stabilizer piston (small rod) out of the valve stem and trying again? Might get a little hammer bounce, but betting it'll close quicker too. Closing force from the valve stem is nullified by the contra piston being at atmospheric until it can fill past the stabilizer piston.

If that pans out, might try sanding a couple thousandths off the stabilizer piston....

Al

[oldpro]

As you can see, the increased backpressure caused the valve to open properly at a higher pressure, just like a heavier bullet does.... However, there is still no sign of self-regulation.... The flatter strings are due to less pressure drop per shot as the FPE goes down.... but they are still first shot fastest.... I see no point in going further, I was getting more power, better efficiency, and a proper curve with my original valve.... I can get 10 shots inside a 4% ES bell-curve at 126 FPE average with the same pellets on a 3000 psi fill.... 834-867-842 fps at 0.98 FPE/CI, and that was without an SSG.... or 9 shots at over 130 FPE, or 7 shots at 135 FPE....

Bob
The efficiency I am getting from the Cothran is better than I have achieved with my own valve, and at higher power to boot.... We are only scratching the surface on balanced valve designs, and I fully expect them to get better and better with time.... The biggest drawback is that they require more precision to make, and more development work so that they do the job intended and are tuneable.... I think the Cothran valve would be hard to beat in a regulated PCP, but unless I can figure out how to get a bell curve on it, then in an unregulated PCP it's just a way to get big power, for a big bore, with a light hammer strike.... That in itself is a big plus, of course, and it's what Don designed the valve to do.... For that purpose, it's hard to argue with success....
 Your loosing me here Bob I'm getting mixed data. I have seen the Slayers shot strings also and it's not a bell curve at all and it's the best balanced valve system out there period. I think people will be disappointed installing balanced valves in their Marauders thinking it's going to be some fantastic upgrade. What they will see is big power gains in the expense of efficiency and a declining shot string. Now I'm not saying there's not a place for it there is but only in big bore Guns where a curve isn't needed and knock down power is.

[rsterne]

Al, I'm trying to wrap my brain around whether or not that would help the valve self-regulate or not?.... I can understand the "close quicker" idea with a smaller (or no) stabilizer rod.... but what do you think will happen as the pressure changes?.... We need the valve to open MORE/LONGER as the pressure drops....

I realize the purpose of the stabilizer rod is to prevent the valve from being driven open a second time by hammer bounce, right?.... Perhaps by using an SSG to help eliminate the hammer bounce we might be able to do away with it?....

The whole purpose here was to wring out a stock Powerhouse valve, and I've pretty much done that.... If Don has any idea how to make the valve self-regulate, I wish he'd chime in....

Bob

[QVTom]

Travis buddy you are dead wrong; we do have a bell curve and they are very tunable.

Tom

[rsterne]

Travis, just to be clear, here are my findings at the present time....

The Cothran valve produces big power, with a light hammer strike.... This makes it perfect for a big bore conversion.... what it was designed for....
It is not fussy as to hammer strike required, but there is a minimum hammer strike for a given pressure, and going below that the velocity drops suddenly....
It produces that power with good efficiency, and when tuned as close to the cliff (where the velocity drops off) as possible, with greater efficiency than I have achieved otherwise....
I have not been able to get the valve to "self-regulate" (produce a bell curve).... The velocity slowly declines with each shot as the pressure drops....
If the pressure is steady (regulated or tethered) the shot to shot velocity is very stable, but depends almost completely on bullet weight, pressure, and barrel length....
For a given barrel length and bullet, the velocity in the stable part of it's operation is a function of pressure.... but can be varied by changing the port size.... NOT hammer strike....
Since the valve has stable velocity at a given pressure, it should be great in a regulated PCP.... but you would tune the velocity by changing the pressure (or port size)....

In conclusion, the valve does what it was designed to do very well.... give big power, efficiently, with easy cocking.... It is NOT the answer for producing a bell-curve shot string, in my experience.... I think it shows promise for regulated operation, and Lloyd certainly showed that in his tests.... but I think the primary tuning method will be the regulator setpoint.... or actual valve lift control as Lloyd experimented with....

Bob

[Motorhead]

Regulated .... I'm listening 

It would be interesting to know if power your wanting can be made on say 2 - 2.2k working pressure ?
That is I'm assuming your double is currently running on 3k fills and would take a regulator set to @ 2K range.

[Buldawg76]

Regulated .... I'm listening 

It would be interesting to know if power your wanting can be made on say 2 - 2.2k working pressure ?
That is I'm assuming your double is currently running on 3k fills and would take a regulator set to @ 2K range.

Bob
+1 on that as well. very interested in the valve in a regulated and bottled Mrod.

Mike

[moorepower]

Can you fit a heavier valve spring to it?

[Gippeto]

Al, I'm trying to wrap my brain around whether or not that would help the valve self-regulate or not?.... I can understand the "close quicker" idea with a smaller (or no) stabilizer rod.... but what do you think will happen as the pressure changes?.... We need the valve to open MORE/LONGER as the pressure drops....

I don't use springs in my valves and rely on the stem to create the closing force. I use larger diameter stems to accomplish this and do get bell curves. With the contra piston nullifying/reducing the closing force (I'm thinking) you're getting excessive dwell...same as if you had a too heavy striker hit, and it's showing up as the descending shot string. I'm thinking with the stabilizer piston removed, the closing force will be more dependent upon throat pressure...same as in a standard poppet arrangement.

I realize the purpose of the stabilizer rod is to prevent the valve from being driven open a second time by hammer bounce, right?.... Perhaps by using an SSG to help eliminate the hammer bounce we might be able to do away with it?....

That's what I'm thinking...stabilizer rod is a double edged sword here. Helps hold the valve closed, but also helping keep the valve open. Have only used an SSG with one of my valves, but it works FANTASTICI...IMO.

The whole purpose here was to wring out a stock Powerhouse valve, and I've pretty much done that.... If Don has any idea how to make the valve self-regulate, I wish he'd chime in....

Bob

Edit...blue on blue is kinda hard to read lol

[oldpro]

Tom let's see that shot string with your balanced valve I'm certainly not trying to call you out it's just I haven't seen one true bell curve yet from a balanced valve. I think with a lot of work you could get a modified bell curve where a few of the first shots are slower than the rest but not a true tunable bell curve where the curve is in the middle and can be moved with fill pressure or hammer strike. I hope I'm wrong believe me I just haven't seen it.Further more I don't think it's even necessary to make a bell curve with a balanced valve because it will only be used in big bore hunting guns where a declining string is fine.

[QVTom]

There are vids on youtube made by Militaryflip.  Other than that you'll just have to take my word for it cause I'm finished and don't wish to take Bob's post off topic too far.

Tom

[gabi.nechita]

Bob
Don valve stem have a venting hole in transfer port and when you fired, part of the pressure is redirected in the valve head and force the valve to close faster . In this case, self regulated effect is canceled, in my opinion.
If vent hole will be inline with stem, at the end where the hammer knock the valve, maybe will see a bell curve string.
Hope to be clear enough. My English is poor

[oldpro]

 Sorry Bob I didnt mean to hijack your thread Im just looking for answers on how to make a balanced valve make a curve and I know if anyone could do it it would be you. If i could get a stable curve from a balanced valve man it would be the best of both worlds.

[rsterne]

gabi.nechita.... Yes, I understand you perfectly.... If the vent hole went straight through, instead of picking up the pressure from the transfer port, then the balance chamber inside the head of the poppet would remain at atmospheric pressure.... The only closing force, when the valve is open, would then be the valve spring, as I understand how the valve works.... plus a bit of drag from the flow around the head of the poppet.... I think the valve would likely just dump all the air in the reservoir, or nearly so.... but I could be wrong....

Gippeto's suggestion of removing the stabilizer rod from inside the stem has the opposite effect, it would increase the pressure inside the poppet head, or at least make that pressure occur sooner in the valve cycle.... This should make the valve close faster.... However, without the stabilizer rod, there is a very good chance that if there is a second hammer strike (due to hammer bounce) the valve would waste a LOT of air.... It's a balancing act, and I know Don spent a LONG time getting the diameter of the stablilzer rod just right.... However, that was before the SSG came along, so the answer today MAY be different than what it was during the valve's development.... That's what we are wondering about....

No problem, Travis, we pretty much have to discuss all balanced valves here.... Lloyd built the first balanced valve I saw, and you would have to ask him, but I think it had a bell-curve, and I know that Tom's does in the Slayer.... I also know that getting the proportions just right is a BIG challenge, made worse by having to deal with the changing pressure.... As I said, these are early days for balanced valves.... Five years from now I think they will be widely used, and a decade from now I would be surprised if most production PCPs didn't use them....

Bob