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 Post subject: Re: Tale of Two BRods
PostPosted: Sat Jan 28, 2017 5:03 am 
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Joined: Wed Oct 17, 2007 3:27 am
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Location: Winnipeg, Canada
Thanks for the bullet photo, Bob.

Those don't look much different from the home cast .357 r/n's we recovered from the trap when I was a whole lot younger. 96O's velocity (iirc) for the mild target loads we had worked up. Our trap eventually evolved into a sand berm- which was actually a lot less mess than wood. We kept the heavy steel backing plate though...

-D.S.

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 Post subject: Re: Tale of Two BRods
PostPosted: Sat Jan 28, 2017 6:51 pm 
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Location: Coalmont BC
I went back to the tube with the Cothran valve today, looking to experiment with varying the lift at 1900 psi.... First I had to make a hammer with an adjustable striker position.... This is the second MDS Nylon hammer I have made, and this time I decided to make it even lighter, with an aluminum core.... They are pretty simple to make, actually.... I started with a piece of 3/4" aluminum, faced it off and drilled it through with a #3 drill (the tap size for 1/4"-28 threads) and then used a 1/2" end mill to create the spring cavity 3/4" deep.... I then knurled the outside longer than the hammer, but leaving a smooth area I could hold in the lathe chuck.... I bored a piece of MDS to a few thou over 3/4", but about 0.010" smaller than the OD of the knurling, faced both ends square to a length of 1.25", and pressed them together to form this....

Image

I then turned the OD to finished diameter to slide smoothly in the tube, and holding the stub of aluminum in a collet I drilled and tapped two 8-32 holes, one for the cocking pin and one for a "brake" to prevent the adjusting screw from adjusting itself.... I then faced off the length at the nose, added the taper for the sear, and tapped the nose through to 1/4"-28, ground the cup point off a 1/2" long setscrew, and installed it with the allen key hole on the inside.... I then dropped a 1/8" long piece of 1/8" diam. Delrin in the front hole, against the side of the adjuster, and topped that off with a 1/4" long 8-32 setscrew, tightened against the Delrin, pressing it into the threads of the adjuster.... Now I can adjust the striker from flush 3 turns in either direction, which is greater than the length of the valve stem on the Cothran valve, so I can go from full lift to no lift in (just under) 6 turns.... Here is the finished hammer, which weighs 28.4 grams (exactly 1 oz.)....

Image

I had a small scrap of energy absorbing polymer sheet about 3/32" thick from Lloyd, which he used in his Cothran valve experimenting, and cut a washer from it with a 3/8" hole (for the striker to clear) and the OD a snug fit in the tube, and installed it hard up against the back of the Powerhouse valve as a bumper to stop the hammer and absorb it's energy.... I removed the spring guide from my SSG, so that I could access the lift adjuster from the back with a long 1/8" allen key, and using the same hammer spring as before, reassembled the gun.... I set the preload on the spring to zero (with the striker just touching the valve stem) with the striker set flush with the face of the hammer.... Assuming the hammer hits the bumper, the valve stem will be driven open about 0.108", and at 0.036" travel per turn of the adjuster, that means that 3 turns out (proud of the hammer face), the striker will push the valve fully open, and backed out about 2.5 turns below flush, it won't open the valve at all.... That was the theory, anyways....

I reassembled everything, tethered the gun at 1900 psi, loaded up a mag. with 45 gr. JSBs, and took a shot.... I had no idea if the valve would cycle properly at these settings or not, and smiled as the shot went through the Chrony at 978 fps, exactly the same as the plateau velocity I had before.... I checked the velocity with more spring preload and it stayed the same, as expected, then reduced the preload, no change at 1 turn of gap, a few fps less at 2 turns, but at 3 turns of gap it dropped to 284 fps.... OK, so there is the normal "Cliff" for the Cothran valve.... I set the preload at 1 turn negative (ie 1 turn of gap), and started adjusting the lift by moving the striker in and out.... When I increased the lift (striker extended), the velocity stayed the same, as expected.... Then I started backing it out, below flush, to reduce the lift on the valve, while leaving the valve cycling properly by virtue of having lots of hammer strike.... I was hoping to be able to have much finer control over the velocity, to give me the ability to control the velocity on the Powerhouse valve without having to change the pressure.... Unfortunately, I was disappointed....

As I recessed the striker into the valve, at 1 turn below flush the velocity held steady at 978 fps.... I figured, "OK, this valve doesn't need much lift".... At 2 turns below flush, I got 321 fps.... much like I did when changing the preload a turn, the valve either cycled or it didn't.... At 2.5 turns out, or further, the valve didn't open, exactly as I had hoped, no shot at all occurred.... I tried various combinations with more preload on the spring, to make sure that I had LOTS of hammer strike, and could not get away from that annoying cliff.... If I was just 1/6 turn (one flat on the allen key) more recessed than where the velocity was steady at 978 fps, the velocity was unstable, it could be anything from under 600 to over 800 fps.... As I recessed it more, the velocity range dropped, but it was still not stable, fluctuating 100 fps or more, shot to shot.... If anything, I think the velocity was more stable with the valve lift greater, and reducing the preload.... I'm pretty frustrated at this point.... Travis suggested I try a heavier valve spring, but I don't have anything suitable, unfortunately.... At this point, I still have no way to reduce the velocity when using a Cothran valve, other than by dropping the pressure....

Bob

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 Post subject: Re: Tale of Two BRods
PostPosted: Sat Jan 28, 2017 7:54 pm 
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Kind of suspect that valve is blowing open Bob. If you want it to act more like a "normal" poppet, increase flow to the contra piston.

Just a thought....

Al


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 Post subject: Re: Tale of Two BRods
PostPosted: Sat Jan 28, 2017 9:39 pm 
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Travis at WAR had a conversation with Don, and he said if you open up the contra piston too much the valve will close so fast it can self-destruct.... so he doesn't recommend it.... FWIW.... Travis has found the best setup so far is the lift limiter combined with a much heavier valve spring.... I think you may be right, however, providing I don't go too far on the metering rod (contra piston).... I'm trying not to do something I can't undo, ATM.... I did have a look at it today, and there is a "step" in it, the back end is smaller in diameter.... I haven't measured it yet....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Sun Jan 29, 2017 9:24 pm 
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I took a break from tuning today to work on the Tactical version.... I measured up the length, selected a hole in the adjustment range of the AR style stock that would give me about 14.5" LOP, and then made the new end plug / stock adapter....

Image

As always, I am astounded how long it takes to make a simple part from scratch, designing on the fly.... about 5 hours for this, believe it or not....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

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 Post subject: Re: Tale of Two BRods
PostPosted: Mon Jan 30, 2017 7:47 pm 
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I installed a taller sear from Lloyd in the PRod trigger group today and installed it on one of the tubes.... Since the PRod tube is thinner wall and smaller diameter, the sear would otherwise not project far enough through the Mrod sized tube to catch the hammer.... Once that was done, then I made a foregrip out of a piece of 1-1/4" ABS black plastic pipe.... It is just the right thickness to fit between the tube and the barrel, and I notched it around the receiver.... It fills the entire space between the trigger group and the drop block....

Here is what the Tactical version looks like with the PRod trigger group, the AR style stock, and the tubing foregrip.... I'm quite pleased with how it turned out....

Image

As shown, with the 500 cc bottle (no regulator) it weighs 7 lbs. 3 oz.... I have never shot a Tactical style gun before, and beauty isn't exactly a strong point, IMO.... but the ergonomics are interesting.... I mounted a scope in Medium height rings, and the cheek weld is really solid and the perfect height for me, with the stock mount lined up with the center of the MRod tube.... That's nice, because that is how I made the mount, and it's a lot easier than making an offset one.... I find the high toe of the stock strange, compared to what I am used to, but the toe of the butt sits nicely in the pocket of my shoulder, so it is actually quite comfortable.... I can see the practicality of a Tactical stock, and how you could quickly grow to like it....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Mon Jan 30, 2017 10:19 pm 
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Entering the Tactic Cool ergonomics of guns there Bob. You sure your up to it?
Build is looking good. Now I just gotta read what you have all done....lol

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 Post subject: Re: Tale of Two BRods
PostPosted: Tue Jan 31, 2017 6:33 pm 
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Today I made the cheekpiece for the wooden stock....

Image

It is made from a piece of 1-1/4" ABS pipe cut in half and mounted on two short pieces of aluminum tubing.... This stock has a full length groove that is 1.25" in diameter to fit the main tube, that goes all the way to the back.... The pieces of aluminum tubing are mounted in that groove with wood screws, and then the plastic pipe is mounted to the tubing brackets with 6-32 low-profile SHCSs.... This places the top of the cheekpiece the thickness of the tubing (1/8") above the top of the main tube, which is identical to the AR style stock on the "Black" version.... I'm really happy with the cheek weld using that height and medium scope rings.... This completes the work on the wooden stock....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Tue Jan 31, 2017 10:12 pm 
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After assembling the wood version with the .25 cal barrel and the Cothran valve, I decided to try some of the 41 gr. BBTs that I cast a week ago.... I tried some as cast, and they were a bit tough to load, they were better at 0.254", and at 0.253" you could feel them seat in the chamber, but you didn't have to push very hard.... I will test different diameters for accuracy at a later date, but for now I am using them sized to 0.253" for my Chrony work.... The hammer is the 28 gr. MDS hammer I made with the aluminum core and an adjustable striker.... I tethered the gun at 1900 psi, set the striker flush with the end of the hammer, and set the hammer spring preload to zero, and started testing.... I am still using the same spring, but without the SSG, so that I can adjust the striker through the middle of the spring and rear adjuster bolt....

I checked the velocity first with the 34 gr. JSB King Heavies, and it was 978 fps (73 FPE) as expected at this pressure... The first shots with the BBTs were 918 fps for the 41.7 gr. FN (78 FPE) and 938 fps for the 39.5 gr. HP (77 FPE).... Leaving the striker flush, I backed out the preload a turn with no change, and then 2 turns, and the velocity tanked.... I then turned the preload back in a turn, it was back to normal, and then I backed out the striker 1 turn below flush (recessed into the hammer).... The velocity was normal, so I backed it out another turn, and the gun did not fire.... The hammer is hitting the bumper on the back of the valve, and at this combination of preload and recessed striker, the valve is not opening at all.... I left the striker recessed, and started adding preload to the spring, and by adding a couple of turns I was able to get it to fire weakly.... After some experimenting with different settings, I decided to set the preload 1 turn out from zero (ie a slight gap between the hammer and spring), and to set the striker 1 turn below flush on the face of the hammer.... At this striker setting, the hammer is hitting the bumper when the valve is open about 0.070", and it really won't matter how much preload I add, it won't open further, at least in theory.... I am hoping that when I connect it back up to a bottle so that I can check the efficiency, I will have found an improvement, both from the heavier bullets, and from limiting the valve lift.... which may reduce the dwell slightly....

The experimenting I have done so far with an adjustable striker mirrors what happens with changing the preload.... Basically the valve is either cycling properly, or it isn't and falls off the cliff.... At 1900 psi the velocity with the 34 gr. JSBs is a bit higher than desired at 978 fps.... while these BBTs are a fraction lower than I want.... I would like to see the FN solids at just under 950 fps, and the HPs at just over, for a solid 80 FPE.... I think bumping the pressure to 2000 psi would do the job.... However, to get results I can compare to my previous ones, I will leave the 500 cc bottle regulated at 1850 psi for now.... and connect it back up for some efficiency testing....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Wed Feb 01, 2017 8:47 pm 
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I did some measurements of the efficiency of the .25 cal version with the Cothran valve today.... I tethered the gun to the 500 cc bottle, with it regulated at 1800 psi, the same as I did previously.... I was using the 28 gram MDS hammer with the aluminum core and adjustable striker, and the same spring but without the SSG in place (so that I can adjust the striker), and the energy absorbing polymer (IsoDamp) bumper on the back of the valve.... With this setup the hammer stops when it hits the valve (bumper), and the amount the valve opens is dictated by the position of the striker, relative to the face of the hammer.... If the striker is flush, the valve can open about 0.108".... if the striker protrudes, it can open further (up to the 0.200" limit of the valve stem protrusion).... and if recessed the valve opens less (recessed 3 turns the valve can't open at all, no matter how much hammer spring you use).... With relatively light preloads, the valve quits opening if the striker is recessed about 2 turns into the hammer face.... ie the gun will not fire at all with the striker recessed more than 2 turns....

As I experiment with the adjustable striker, I am coming to some interesting conclusions about the Cothran Powerhouse valve (whether they are correct or not, I don't know).... I think the valve, once opened a small amount, and with enough backpressure in the exhaust port, "blows open".... From actual lift measurements I made on the Disco version, when cycling properly it opens about 0.060"-0.120".... while when it has fallen "off the cliff" it is opening about 0.020" or less.... Normally, a valve opens pretty much in a linear relationship to the hammer strike, the Cothan does not.... The interesting part is, that with the adjustable striker in this gun, I can get the valve to cycle reliably (providing I have enough hammer strike) with the striker recessed into the hammer face 1.5 turns, which in theory means it can only open the valve about 0.050".... Now perhaps the bumper is compressing, and the valve stem is getting pushed a few thou further, but I don't think there is any way it is getting to 0.060".... and yet the measurements I made with the Disco valve (which is identical inside) say that the valve is opening that far or more.... This has led me to the conclusion (unproven) that once the valve opens about 0.030", it blows open to provide twice that much actual lift.... Right or wrong, that is what I THINK is happening....

Like most valves, if you hit them way too hard, they waste air.... In my first tests with the Cothran valve, whacking it hard didn't produce any more power, but I could make the gun use up to 80 psi per shot, with an efficiency of less than 0.50 FPE/CI.... This is usually an indication that the valve is still open after the pellet has left the muzzle.... As you approached the cliff (from the plateau), the efficiency came up to around 0.65 FPE/CI, and then just before you got to the cliff it increased to just under 1.0 FPE/CI, using about 35-40 psi per shot with the 34 gr. JSB King Heavies.... I was curious what would happen to the efficiency using the adjustable striker to restrict how hard you could hit the valve, regardless of the hammer spring preload.... I wasn't disappointed....

If I set the striker so that it protruded 3 turns from the hammer face (the same thickness as the bumper on the back of the valve), then the bumper never came into play, and the valve acted just like it did before.... Whack it too hard, and the efficiency went into the tank.... When I set the striker flush with the hammer face, however, things changed.... In theory, the valve could only be driven open about half way, and that is exactly the way it acted.... That is far enough to deliver maximum velocity, but not enough to drive the valve so far open that it becomes an air hog.... No matter how much preload I used on the hammer spring, I couldn't get the gun to use more than 40 psi per shot.... Compare this to the 80 psi per shot it would use if you really whacked it before, and you realize that what the bumper does is stop the hammer before it opens the valve further than required, even if you use way too much hammer strike.... It's like having a "limiter" on the amount of air the valve can use.... This is obviously a very good thing, as it makes it extremely easy to not use way too much air.... However, 40 psi per shot is still not great for efficiency, it is only about 0.85 FPE/CI with the 34 gr. JSBs.... It was now time to explore what happened when you tuned closer to the "cliff"....

I now had the ability to approach the cliff in two ways.... I could do it by reducing the preload, or by reducing the initial valve opening, or both.... Believe me, I burned through a lot of pellets and bullets, and a tank of air, trying to figure out what was going on, and which worked better.... I tested with the 34.2 gr. JSB King Heavies, the 39.5 gr. NOE HPs, and the 41.7 gr. NOE FN.... Both the BBTs were sized to 0.253".... I determined that I could recess the striker as much as 1.5 turns into the hammer face and still have the valve cycle properly.... I tried various preloads with it set there, and with it set flush (and in between), and determined that all that happened was it took a slightly different amount of preload to reach the cliff, and the psi of air used for an 8-shot magazine didn't really change.... It was how far from the cliff you were that determined how much air you used.... However, I reasoned that the less lift I allowed with the striker (while still having enough to cycle the valve), the less sensitive the preload might be, and the less difference it would make in air usage if you used too much.... In reality, I don't think it mattered much, but I got some pretty decent efficiency numbers while operating in the 71-75 FPE power range....

Something else I discovered is that the BBTs used less air than the pellets.... I don't know if that is because they were tighter in the bore, heavier, and therefore building more backpressure, causing the valve to close faster, or some other effect.... but instead of 320 psi for 8 shots, they only used 280 psi.... and this was when operating up on the plateau, well above the cliff.... Since I was also getting 2-3 more FPE with the BBTs, this put their efficiency at over 1.0 FPE/CI, instead of under with the pellets.... As I backed off on the preload and approached the cliff, the pressure drop for 8 shots dropped to 270 psi for the pellets and 240 psi for the BBTs, while the velocity only fell off 2-3 fps.... There was about a 1/2 turn range of preload over which the change in velocity and air use was slight, making it much easier to tune just above the cliff, but with consistent velocity.... Here are the final results at 1800 psi....

JSB 34.2 gr.... 969 fps.... 71.3 FPE.... 1.00 FPE/CI
BBT 39.5 gr. HP.... 918 fps.... 73.9 FPE.... 1.17 FPE/CI
BBT 41.6 gr. FN.... 898 fps.... 74.7 FPE.... 1.18 FPE/CI

With the BBTs, I was getting a pressure drop on the 500 cc bottle of 240 psi for 8 shots.... With the regulator set at 1800 psi, that should work out to about 5 magazines (40 shots) at 75 FPE per fill (a total of 3000 FPE).... or about 35 shots with the JSBs at 71 FPE.... I am beginning to think that the Cothran valve is better suited to bullets than pellets.... and quite likely the heavier the better.... If I were tuning for the 34 gr. JSBs, I would probably reduce the setpoint to 1700 psi for 70 FPE.... while for the BBTs I would increase it to 2000 psi, where it should be a good, solid 80 FPE gun.... with a low ES and a decent shot count....

One thing for sure, anyone using a Cothran valve in an MRod might like to consider putting a couple of # 118 O-rings inside the tube, right up against the back of the valve (assuming a stock hammer with flanged, adjustable striker).... If you are using a hammer with a flush face, like the new WAR MDS hammer, a single one should prevent you from opening the valve so far that it will waste a ton of air.... If the valve is capable of opening 0.090" or so, it never needs to open more than that, from what I have seen.... The stem protrudes just under 0.200", so with an O-ring being 0.103" thick, and a flat faced hammer, you should never have to worry about opening the valve too far and dumping air after Elvis has left the building.... Playing with the preload to get close to the cliff and optimize the efficiency is all that would be left to do, once you get the pressure set to give the velocity you want with the pellet you are using....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Thu Feb 02, 2017 5:00 pm 
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OK, I think I am FINALLY beginning to understand how the Cothran valve reacts to different setups and calibers.... I replaced the .25 cal upper with the .30 cal upper today, with NO other changes.... It was still tethered to the 500 cc tank with a regulated output of 1800 psi, and the striker is still set at 1.5 turns below flush on the hammer.... This means it drives the valve open about 0.050", although it may blow open further than that if it wants to.... I initially used the same 2 turns of gap between the spring and the hammer that I started with yesterday, loaded a magazine up with 7 of the JSB 50.2 gr. pellets, and fired a shot across the Chrony.... about 400 fps.... OK, I half expected it would need a harder hammer hit with the larger bore, so I added a turn of preload.... 930 fps.... GREAT, so now I know that there is hardly any difference in how hard you need to hit the valve with a .30 cal compared to a .25 cal.... That is quite different than a conventional valve, which usually requires a significantly harder hammer strike as you increase the caliber, to fill the larger barrel volume....

I topped up the 500 cc tank, leaving my Great White tethered to it with the valve closed, so that I could use the big gauge to read the pressure drop in the 500 cc (same as I have been doing with all these tests), shot a couple of shots to stabilize everything, recorded the pressure, cranked in 8 turns of preload, and shot a 7 shot mag. across the Chrony and recorded the new pressure.... It dropped 380 psi.... Bear in mind that similar tests without the striker recessed showed as much as 80 psi pressure drop per shot, so the recessed striker certainly saves a lot of air if you have wayyyyyyyyy too much hammer strike.... I backed the preload off to 4 turns, the velocity dropped only 3 fps, and the pressure drop for 7 shots was 300 psi.... I then set the preload to zero, still the same velocity and pressure drop, so the gun was acting exactly the same as it did yesterday with the .25 cal barrel on it, at the same settings.... even the same pressure drop, although this was for 7 shots (at 97 FPE) instead of 8 shots (at 71 FPE).... I then backed the preload out another turn (1 turn of gap, now), lost another 2 fps, but the pressure drop was now only 280 psi.... One more turn out (which worked fine with the .25 cal), and I was back down to ~400 fps, so I added 1/2 turn, and the valve was back to cycling solidly and reliably, at 926 fps (96 FPE), with only a 240 psi pressure drop for 7 shots.... This works out to 1.33 FPE/CI, a very respectable efficiency for a .30 cal gun at this power level.... I only needed 1/2 turn more preload in .30 cal than I did in .25 cal....

So to summarize, when set about 1/2 turn of preload above the cliff, so that the valve cycles solidly, reliably, and with low ES (well under 1%), and shooting pellets.... I am getting 8 shots in .25 cal at 71 FPE on 280 psi of air (1.00 FPE/CI).... and 7 shots in .30 cal at 96 FPE on 240 psi of air (1.33 FPE/CI).... In my Disco Double, at the same pressure, in .357 cal, I was getting 128 FPE at 1.53 FPE/CI (78 gr. pellets @ 862 fps).... I think it is pretty clear what is happening here.... The Cothran valve, when cycling properly but operating just above the cliff.... has a relative constant dwell, so providing the barrel is long enough that the valve is closing well before the pellet reaches the muzzle, it is dumping a relatively constant amount of air into the barrel.... Yes, it dumps a bit more into a larger bore, but relative to the bore volume, it is less, which means the valve is closing sooner in the larger caliber, leading to higher efficiency.... This behaviour also explains why we were getting such high FPE/CI numbers in .357 cal but pretty miserable numbers in .25 cal. and the .30 cal is right in between....

This leads me to a pretty obvious conclusion.... The Cothran valve is a way better choice for larger calibers and heavier bullets.... ie for high FPE applications.... However, as we gain a better understanding of it, we can tame it down for use in regulated PCPs as low as .25 cal....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Tue Feb 07, 2017 1:01 am 
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Location: Coalmont BC
Today, after much prompting by Travis, I started modifying my Cothran valve, to try and improve it's efficiency in my .25 cal regulated BRod.... It is important to realize that these changes may not work (in fact are unlikely to work) in an installation that is prone to hammer bounce.... In my gun, I have an energy absorbing bumper made from "Iso-Damp" on the back of the valve, an extremely light MDS hammer of only 28 grams, a recessed striker that only pushes the valve stem about 0.050", and an SSG.... This allowed me to consider removing the small metering rod (counter-piston) which lives inside the valve stem.... There is a debate as to the purpose of that device, but I believe it to be to prevent the valve from re-opening on a second, or subsequent hammer strike, by acting as a "leaky check valve", slowing the HPA that fills the poppet head during the cycling of the valve from dumping back too quickly into the exhaust port.... I have been told that removing it will cause everything from uncontrollable hammer bounce, to various burps or flutters, to extreme air use, to even destruction of the valve itself.... However, since Travis agreed to supply me with any parts I destroyed I decided to remove it.... I tethered the gun at 1900 psi and started testing.... What happened?.... Exactly NOTHING.... Zip.... NADA....

The valve functioned exactly as before, the same velocity, sound, air use, everything.... I was shocked, but it does reinforce my belief that its primary purpose is anti-bounce.... which I have already addressed by a combination of four different methods.... I did notice one thing that changed, if I leaned on the hammer strike by adding preload (instead of gap) to the SSG.... thereby vastly overdriving the hammer.... the velocity DROPPED slightly, accompanied by a high-frequency "flutter".... I don't know how else to describe it.... It sounded like hammer bounce, but much faster.... The strange thing was, it didn't use more air.... As I said, removing the metering rod may not work in many installations, if they have any possibility of hammer bounce.... but in my gun, set up the way it is, it made no difference, when the SSG was adjusted normally, with a bit of gap.... The velocity was stable until you got to the cliff, and then dropped like a stone, same as always....

OK, so the other thing Travis (and others) have tried is drilling out the tiny vent hole in the valve stem.... This hole lives in the exhaust port, and allows the hollow head of the poppet (which is the diameter of the "force reduction" piston attached to the front of the valve) to be at atmospheric pressure between shots.... This "blanks off" the equivalent area of the poppet, drastically reducing the opening force required to crack the valve.... When you fire the gun, the pressure in the exhaust port rises, HPA flows through that hole, through the valve stem (past the metering rod) and pressurizes the inside of the poppet, providing a greatly increased closing force for the valve.... Without this vent, the valve would stay open, and likely vent the entire reservoir.... With it, the valve cycles in a timely manner, allowing lots of air to escape and produce copious amounts of power.... The way Don builds the valve, I would estimate the dwell time at roughly 2 mSec.... and if you have way too much hammer strike, it can be even more.... Limiting the valve lift keeps that more constant, preventing the valve from being a real air-hog, which is why I like that modification....

In stock form, that vent hole is tiny, only about 0.020".... Logic dictates that enlarging it will make the valve close faster, reducing the dwell.... and that should lead to increased efficiency.... The stem is HARD, and Travis warned me not to try drilling it, and I didn't have small enough drills anyways.... so I did like he did, and used a thin pointed diamond burr in my Dremel and "countersunk" the hole, gradually opening it up at the same time.... The smallest diameter, doing it this way, is just at the very inside of the vent hole, and I found I was causing tiny burrs inside the stem, which I had to keep cleaning out with a #31 twist drill (0.120"), turning it by hand.... I did this in case I had to drop the metering rod (0.118" diam.) back in the hole, I didn't want it to get stuck.... Anyway, I used a couple of sewing needles with the points ground off as go-nogo gauges, and when one that was 0.028" would fit through the hole (but one measuring 0.030" wouldn't) I quit grinding.... I chose 0.028" because it has TWICE the area of the original hole.... I have no idea it that is optimum, but I wanted to start smaller than what others had used, which was 1/32" (0.031") or larger.... I left the metering rod out, reassembled the gun, and repeated my testing.... All I can say is WOW !....

First of all, it took a bit more hammer strike to open the valve.... not a lot, but a couple of turns less gap on the SSG.... Secondly, instead of a completely flat plateau, with only a few fps drop just before you got to the cliff, and then nothing.... I actually saw a "knee" on the curve.... It wasn't long, the sensitivity to SSG gap was still there, but compared to stock it was day and night.... Here are my results....

Image

Note that the blue (stock) and black (no metering rod) lines virtually lie on top of each other.... At 3 turns of gap on the SSG, the velocity is at maximum, at 4 turns, it has only dropped a few fps, and shortly after that the velocity became unstable (400-500 fps ES) and at 5 turns out it was in the basement.... With the larger vent hole, compared to the velocity at zero gap, there was about a 10 fps loss at 1 turn out, and then about another 15 fps in the next turn.... At 2 turns of gap, the velocity was 25 fps below the peak, and the ES was still less than 1%.... At 2.5 turns of gap, the average velocity dropped another 40 fps, to what in a conventional valve would be the middle of the knee, but the ES had increased to about 2%.... At 3 turns of gap, the velocity was down 200 fps, but the shots were all over the place, with an ES of over 5%.... The valve was becoming unstable, although nowhere near as bad as a stock one.... At 4 turns out, the valve was no longer cycling properly, it was in the basement....

I then tethered the gun to my 500 cc bottle, regulated at 1800 psi, for some efficiency testing.... I left the bottle connected to the gauge on my Great White (valve closed) so I could record the pressure drop for a 8-shot mag. and tested each half turn out, starting at zero gap.... At that setting, the velocity was maximum for this pressure, at 968 fps, and the drop 280 psi, for 0.97 FPE/CI.... This was basically the same as I had before, with the 34 gr. JSB Heavies.... At 1 turn out, the velocity was 962 fps (220 psi) and 1.22 FPE/CI.... At 1.5 turns, 951 fps (210 psi) and 1.24 FPE/CI.... and at 2 turns of gag, 943 fps (200 psi) and 1.28 FPE/CI.... This is far better efficiency than I have ever had before with the Cothran valve in my .25 cal BRod.... At 2.5 turns out, the velocity (809 avg.) was becoming unstable, and it was much worse at 3 turns out (701 fps), so I didn't record the efficiency, figuring it was meaningless....

So, what are my conclusions?.... Well, for one thing, if you have cured all chance of hammer bounce, I think you can try removing the metering rod, I don't think it is necessary.... Secondly, doubling the area of the vent hole, by increasing the diameter to 0.028", decreases the dwell at a given hammer strike, and creates the beginning of a knee to the velocity vs preload/gap curve.... It gives you significantly more control of the velocity just above the cliff, and softens the cliff into a usable, although sensitive knee.... While the operating pressure is still by far the dominant factor in the velocity, you can now detune the velocity just a bit, saving air in the process.... I haven't got all the answers, but I think we're on the right track....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Tue Feb 07, 2017 10:38 pm 
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Location: Coalmont BC
Today I pulled out the Iso-Damp bumper I had, and substituted a #210 90D O-ring.... I did this because ANYONE can find one of those.... I set the striker FLUSH with the end of the hammer, because then you don't need an adjustable hammer, Travis' new MDS hammer with the flat face would work fine.... The hammer I was using was still very light, at only 28 grams, and I was still running the SSG.... I measured the distance the hammer can push the valve stem with the #210 O-ring in place, and even pushing on the hammer very hard, it only pushes the valve stem 0.075".... I think this is right in the sweet spot, the valve opens plenty far enough, but you can't drive it so far that it becomes an air hog.... I haven't tried this at 2900 psi, but at 1900 psi, it is SIMPLE and works GREAT !.... There was NO change in the settings required with the 0.028" vent hole and no metering rod, the performance was identical to with the Iso-Damp....

This afternoon I put the metering rod back, with a small spring under it, as Travis suggested.... I had it small end down, and the spring was from a Crosman 13XX Safety..... It went coil-bind just before the valve stem went flush with the valve, but that is about 0.100" past where the hammer stops.... Basically there was almost no difference compared to running the valve with no rod at all, running tethered at 1900 psi.... The vent hole was still 0.028", and the peak velocity was a about 10 fps lower at zero gap on the SSG, and also at 2 turns of gap.... At 2.5 turns out, the velocity was about the same as with no rod, but the ES was a bit worse.... At 3 turns out, the velocity was actually 13 fps higher than with no rod, and the ES had increased to over 150 fps over 4 shots.... Not only did I not see an increase in the adjustability of the valve between maximum and the cliff, I was seeing greater instability in velocity than without the rod at all.... I was not at all happy with the way it was performing with the metering rod in place, so I took it back out....

For the next tests, I increased the diameter of the vent hole to 0.0345" (ie between 0.034-0.035"), which TRIPLES the area of the original 0.020 hole in the stem.... I did not use the metering rod.... With the gun once again tethered at 1900 psi, I immediately noticed both a drop in velocity, and a decrease in the report.... I tested hammer spring settings from 8 turns of preload, through zero gap on the SSG, to 4 turns of gap, where it fell off the cliff.... Here is the results, plotted on the same graph I posted yesterday....

Image

There are a couple of things to note.... Although I didn't bother to plot it, once I added preload to the hammer spring I noticed the same high-frequency "fluttering" sound I heard yesterday, and with the larger vent hole, it was accompanied by a significant DROP in velocity.... The more preload I cranked in, the lower the velocity got, until at 8 turns of preload it was just over 900 fps.... While not a huge loss, there was no way I could get to the nearly 990 fps velocities I was getting with the same ports, at the same pressure, before I started enlarging the vent hole.... I think this is proof positive that enlarging the hole reduces the valve dwell.... I did NOT try removing the O-ring so that I could drive the valve to greater lift, I don't know if that would move the plateau up again.... but I am pretty sure that doing that would burn a lot more air.... Note that the velocity with the 0.035" vent hole actually peaked at 1/2 turn of gap on the SSG, and fell off both sides of that.... This is completely different than the way the Cothran valve was designed to work.... At 2 turns of gap, the ES had increased to over 50 fps, and at 3 turns of gap it was over 100 fps.... At 4 turns the valve had fallen off the cliff, as expected....

I then tethered the gun to my 500 cc bottle, with an 1800 psi regulated output, to check the efficiency.... I only shot two 8-shot strings, one at the maximum velocity setting of 1/2 turn of gap, and the other with 1.5 turns of gap, where the velocity was still stable.... All I was interested in was comparing the efficiency to what I had with the 0.028" vent hole.... At the higher setting, I got an average velocity of 921 fps (64.4 FPE) and an ES of 1.29 FPE/CI.... With 1 turn more gap, that dropped to 908 fps (24 fps ES at 62.6 FPE) at 1.25 FPE/CI.... Basically, I was getting the same efficiency as with the smaller vent hole, but at 5 FPE less power.... not exactly a great trade-off, IMO.... Yes, I could increase the pressure to get the power back, but I really doubt I would see enough of an increase in efficiency to make up for the decreased pressure headroom.... In this gun, in .25 cal, and at these pressures, IMO I have increased the vent hole too much....

Ohhhhhhhhhhhhh Travis.... I need a new poppet.... *LOL*....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Fri Feb 10, 2017 4:11 pm 
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Location: Coalmont BC
As Travis requested, I ran another test today, with the 0.035" vent hole and the metering rod in place, upside down, with the small spring under it.... The rest was unchanged from the previous tests, I was using a single #210 90D O-ring as a bumper, with a flush front on the 28 gr. hammer, and the SSG.... The gun was the .25 cal. version, shooting 34.2 gr. JSB King Heavies, tethered at 1900 psi.... I started at zero gap, and checked the velocity over 4 shots, at each 1/2 turn of adjustment out from there until the valve fell off the cliff....

The results were pretty much as I expected.... The rod restricted the filling of the cavity in the poppet head somewhat, and the gun acted more like it had the 0.028" vent hole without any rod in place.... The velocity was slightly less at zero preload, peaking at 1 turn of gap at 975 fps, and then declining slowly to 2 turns, after which the velocity dropped quite quickly.... It did fall in a more linear fashion over the next turn of adjustment before hitting the basement at 3.5 turns of gap.... I was not at all happy with the ES, however.... It was great while the velocity was on the plateau at 970 fps, but as soon as the velocity started to drop, the ES increased.... At an average velocity of 958 fps, at 2 turns of gap, the ES over just 4 shots was 17 fps (2%).... At 2.5 turns of gap, the average velocity was 872 fps, but over 4 shots varied between 751 - 933 fps (20%)....

I tethered the gun to my 500 cc tank, regulated as before at 1800 psi, and ran one 8-shot string to check the efficiency.... It used 210 psi, producing an average velocity of 953 fps (69 FPE) with a 17 fps ES (1.8%) over the 8 shots.... This is an efficiency of 1.25 FPE/CI, basically unchanged from previous results at that velocity....

The greatest range of adjustability I achieved during these tests was with the 0.028" vent hole and no metering rod in place.... I had a solid 20-25 fps range over which the efficiency was over 1.2 FPE/CI, coupled with a low ES.... It was the closest thing to a conventional "knee" I have been able to achieve with the Cothran valve.... The only way to really adjust the velocity remains changing the pressure.... but that setup allowed me to decrease the velocity below the normally flat plateau the Powerhouse valve displays by up to 25 fps, accompanied by a decrease in the amount of air used.... Trying to tune further below the plateau, with any of these "improvements", resulted in unstable operation and a high to unusable ES....

I am going to try the 0.035" vent version (without rod) at 2900 psi with a heavier hammer.... and also plan to do some testing with it with the .30 cal top end.... once I top up my Great White....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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 Post subject: Re: Tale of Two BRods
PostPosted: Fri Feb 10, 2017 11:53 pm 
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Location: Coalmont BC
This evening I got a chance to do a little bit more testing.... I used the .25 cal version with the Cothran valve, with the 0.035" vent hole, but this time I put a 0.8" long piece of 3/32" brass rod inside the valve stem instead of the metering rod.... This is a loose fit, so it won't restrict the HPA entering the head of the poppet, but it will take up a lot of the volume inside the stem.... It is longer than the metering rod by 0.10", and the lenght just clears when the poppet is full open.... I also put the 51 gr. MDS hammer (with steel core) back in, so I did 2 things at once, not the best situation, but I'm running out of the 34 gr. pellets.... I tethered the gun at 1900 psi, determined where zero gap was on the SSG, and shot a couple of shots at each gap setting, increasing the gap until the velocity went over the cliff.... The velocity was pretty much stable, although the ES was poor, until I got to 4.5 turns of gap, when it started to decline, slowly at first, and then rapidly, falling over the cliff at over 6 turns of gap.... The velocity curve was quite similar to what I got with the metering rod in place with the spring, except it moved out 3 turns on gap, which would be due to the heavier hammer.... I tried one string tethered to my 500 cc bottle at 4.5 turns out, and 8 shots averaging 938 fps (67 FPE) used 220 psi of air, which is 1.16 FPE/CI, not quite as good as before....

I then tethered the gun at 2900 psi, and repeated the testing of velocity vs SSG gap.... At this higher pressure, I just nicely had enough hammer strike with the 51 gr. hammer.... I was able to max. out the velocity at 1/2 turn of gap on the SSG, just about perfect.... and the velocity was only 10 fps less than what I got in the very first tests with the valve in stock form.... With the 0.035" vent, however, there was some adjustment to the velocity before the velocity plummeted.... I didn't do enough shooting to determine what the ES was like, just a shot or two at each setting.... but it looks like the larger vent does pretty much the same thing to the curve at 2900 psi as it does at 1900 psi.... It would require a lot more testing to determine the optimum vent hole size at this higher pressure.... and the fact that I was down slightly on velocity tells me that the 0.035" hole is a bit too large.... I'm guessing that if I remove the 3/32" spacer rod, the dwell (and the velocity) will drop a bit more.... I may try that tomorrow....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.


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