Author Topic: Tale of Two BRods  (Read 14892 times)

rsterne

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Re: Tale of Two BRods
« Reply #105 on: February 23, 2017, 06:14:58 PM »
Travis sent me a replacement poppet for my Cothran valve, as I think I went too far with the 0.035" vent hole.... because I lost some of the peak velocity.... I tried this one with the stock 0.020" vent hole, but with no metering rod, and it acted normally.... The velocity on the plateau (.30 cal. 50 gr.) was the same as stock, ie removing the metering rod didn't change it.... I had no strange effects like hammer bounce, but remember, I am running a #210 90D O-ring on the back of the valve to limit how far the hammer can drive the stem to about 0.075".... and the O-ring then stops the hammer, absorbing any excess energy.... However, with the small 0.020" vent, the valve was back to being "on or off", with a flat plateau and then a sudden cliff, losing the small amount of adjustment you can get with a bigger vent....

I had previously tried a 0.028" vent hole, which worked fine, and then increased it to 0.035", which IMO is too big, because I lost velocity on the plateau.... I think this means that I cannot get enough dwell to max. out the power.... Both the previous sizes were achieved by a pointed burr in a Dremel, which is difficult to get to the correct size, requiring that you keep checking the hole with a piece of wire of the diameter you want.... A couple of days ago, in a local Home Hardware, I saw some Cobalt twist drill that were 1/32" (0.031"), which is in between what I had tried, so that is what I used to enlarge the hole in the new poppet.... Travis had told me that the shaft of the poppet was very hard, and to grind it with a Dremel.... but I picked up a pair of these drills (they were $6.49 for 2) in the hopes that the cobalt steel would be hard enough to do the job.... I used the smallest collet in my Dremel, and ran it at the lowest speed (still probably 5000 PRM), and it drilled the hole out perfectly.... You have to be careful not to break such a small drill, but it was a lot easier to do than using the pointed burr, and the hole will be the same size, every time....

I was very pleased with the results.... I haven't lost any of the peak velocity on the plateau with the 1/32" vent hole.... This compares with a 35 fps loss with the 0.035" hole.... I haven't yet had the opportunity to see how much of an adjustment range I can get with this size.... ie what the knee looks like.... but I'm sure it will be similar to the 0.028" hole, which gave me some degree of adjustability before the steep dropoff.... Plotting that curve, and checking the efficiency on it, will have to wait for another day....

Bob
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oneshot61

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Re: Tale of Two BRods
« Reply #106 on: February 28, 2017, 06:28:41 PM »
That's what I did mine with. But I set up one night and twisted it slowly with my fingers until I broke through. I still broke one of the bits, which is very easy to do. I think my stronger valve spring has taken a bit to much off the top power though. Looking forward to your results.

rsterne

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Re: Tale of Two BRods
« Reply #107 on: March 11, 2017, 07:06:56 PM »
The first trials with the new stem with the 0.031" (1/32") vent hole were a bust.... I removed the metering rod, and over a matter of a few shots at 2900 psi the velocity started to drop.... I had to keep increasing the hammer strike, and even that wouldn't put me back to where I started.... I pulled the valve apart, and found that the seat had a deep groove in it.... I can only assume that by removing the metering rod the valve was closing with such force it destroyed the rather soft seat on the poppet.... Either that, or there was a leak in the force reduction piston O-ring, which put a lot more force on the poppet.... I don't know why I never experienced this with the original poppet, it was still in good shape.... The solution was to disassemble both poppets and build one good one from the two sets of parts....

The poppet is made in three pieces, the hollow stem, the seat, which looks like a nylon washer, and the tubular front part, which is the sleeve of the force reduction system....The two metal parts are threaded together using 10-32 threads and glued, which make the poppet difficult to take apart without scarring the surfaces.... By using the two chucks on my lathe, and turning the headstock backwards by hand, I was able to take the poppets apart.... I used the new stem, with the 1/32" vent hole, and the old seal, turned over to use the flip side, which was perfect, and one of the front sections.... I reassembled it with a drop of blue Loctite and let it cure overnight before assembling and pressurizing it.... It sealed up perfectly first try.... I had installed the stock metering rod, and set about testing it....

As I found previously at 1900 psi, the velocity was right back up to where it was with a stock vent hole.... In .25 cal, there was very little adjustability, basically a plateau and a cliff.... In .30 cal, there was a slight rounding of the edge of the cliff, but the valve was still rock stable just above the dropoff.... With 70 gr. bullets, I did lose a few fps, so there is some indication that this is the largest vent hole you can use in .30 cal at this pressure without losing performance.... I would not go larger, because with the 0.035" vent I lost about 20 fps.... I did some short strings (1 magazine) tethered to my 500 cc tank with the output regulated at 1900 psi, and the efficiency was slightly better than with either the stock vent hole, or with the 0.035" vent, so I think I hit the sweet spot for the vent size, at least when regulated at 1900 psi.... Here are the results....

.25 cal. 34.2 gr. JSB.... 987 fps (74.0 FPE) @ 1.00 FPE/CI, with an ES of 13 fps....
.25 cal 46.4 gr. Cast RN.... 878 fps (79.5 FPE) @ 1.04 FPE/CI, with an ES of 7 fps....
.30 cal 44.8 gr. JSB.... 971 fps (93.7 FPE) @ 1.11 FPE/CI, with an ES of 12 fps....
.30 cal 49.3 gr. Daystate.... 949 fps (98.6 FPE) @ 1.17 FPE/CI, with an ES of 12 fps....
.30 cal 70.0 gr. BBT.... 824 fps (105.7 FPE) @ 1.35 FPE/CI, with an ES of 11 fps....

The .25 gr. pellets required 6 turns of gap on the SSG, the .30 gr. bullets required 5 turns, and the other three all were shot at 5.5 turns of gap.... I can probably get a bit smaller ES by using a lighter hammer, a weaker hammer spring, or a bit less preload on the SSG, to reduce the gap.... I was running the 51 g. MDS hammer with the steel core....

I then took the gun all apart, stripped down the valve, and replaced the metering rod with one I had sanded down 0.003" smaller.... I had heard this was something worth trying, and it produced some interesting results.... First of all, it created a curve where the plateau used to be.... With no gap in the SSG, the velocity dropped about 15 fps with the .25 cal 34 gr. JSBs, and about 40 fps with the .30 cal 50 gr. JSBs.... It then increased until there was 3 turns of gap, and then curved downwards to about 6-6.5 turns and was in the basement when 7 turns out.... I think the lower velocity at heavy hammer strike may be because of the hammer bouncing off the rubber O-ring, although why it should show up only with the undersized metering rod, I have no idea.... The total adjustment range between peak velocity (at 3 turns) and when it rolled over to head into the basement was about 100 fps, so I was quite excited that I would finally be able to get some adjustment in velocity with the Cothran valve without having to play with the pressure.... Unfortunately, for nearly all of that 100 fps range, the velocity was unstable, with the ES increasing as you approached the cliff.... In other words, with the undersized metering rod, I got a slight knee, but not a usable one, as the ES quickly became unusable.... Just before the velocity took a dive, the ES could be as much as 130 fps from one shot to the next.... I fiddled with it, trying various adjustments, but could never get the ES as low as with the stock metering rod, so I took the gun apart and put the stock rod back inside the valve stem....

I tried a few more tests with the stock metering rod and the 1/32" vent hole, and I am very pleased with this combination at 1900 psi.... As long as you stay just above the cliff, in the area where the velocity is 10-20 fps below the maximum at that pressure, you are rewarded with a narrow ES, lots of power without maximum report, and decent efficiency.... I don't think there is anything I can do to get a better tuning range than I have for the Cothran valve, in .25 or .30 cal, when regulated at 1900 psi....

Bob
  • Coalmont, BC

rsterne

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Re: Tale of Two BRods
« Reply #108 on: March 12, 2017, 11:52:07 AM »
I took a few shots today tethered at 2900 psi, with the following results....



As you can see, the pellets that were in the mid 900s at 1900 psi are Supersonic at 2900.... I tried some heavier bullets, sized down to fit, and the biggest surprise was the 66 gr. in 25 cal breaking 120 FPE.... I have a feeling that the enlarged vent hole in the stem is hurting the performance of the 109 gr. in .300 cal, as I would have expected more FPE from it instead of less.... After all, the porting is the same as my DAQ, which shot over 180 FPE with the same bullet at the same pressures.... If you correct for my shorter barrel lengths, my .25 cal is about on a par with Lloyd's original experiments with the MRod with the Cothran valve, as is the 80 gr. bullet in the .30 cal, but the 109 gr. is lagging.... It could also be the O-ring bumper on the back of the valve that I have used to limit the hammer travel, I have, after all, done several things to throttle back the valve to work better regulated at 1900 psi.... I guess it's not to surprising that some or all of those changes have combined to knock the top end off the power at the top end.... in fact it would have been surprising NOT to see that....  ::)

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #109 on: July 31, 2017, 08:26:30 PM »
I know it has been a while since I posted in this thread, but summer is an impossible time for me to work on airguns because the Motel is too busy.... During the winter, I worked with Travis and Jim designing an adjustable regulator to go inside their drop down tank block, as used in the new COBRA.... I just received one in the mail today, and it is gorgeous.... It threads right into the WAR tube for the FLEX/WARP/COBRA....



I got both tank adapters, one for 5/8"-18 UN threads, and one for 18 mm x 1.5 mm threads, so I can use any tank.... The burst disc is on the output side, and is a 3K, as I won't be using more than about 2200 psi.... However, I understand the output can be adjusted higher than that, depending on the Bellevilles installed....



It has room for two gauges, the upper one reads output pressure, and the lower one tank pressure.... The adjusting screw is an allen set-screw on the bottom of the block.... It moves the HP seat towards the piston (CW) to reduce the output pressure and away from the piston (CCW) to increase it.... I can't wait to try it out on the tube I have the Cothran valve installed in.... It will be the perfect arrangement for that, because you must adjust the pressure to adjust the velocity....

Bob
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Alan

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Re: Tale of Two BRods
« Reply #110 on: August 01, 2017, 04:20:17 AM »
I have one of these adjustable regulators on the Cobra version I bought from Travis earlier this year. The results of the team effort are rather evident, and thanks to all who made the device possible.

Once you play with it a bit, it is really easy to readjust the pressure—handy if you shoot different weight pellets. The only issue, and Travis warned me about this, is when you do adjust the regulator, GO SLOW! Itsy-bitsy movements is all it takes! Even a 5° rotation can make the adjustment too much!

I want to add something here. Bob, you truly are an unsung hero. I've never met you in person (I'd like to sometime), but your dedication and thoroughness in answering questions is legendary here, and on just about every other shooting-related web site out there. If I could bestow a gift on you, it would be a Doctorate of Letters in airgun technology! You sure have earned it!
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Tale of Two BRods
« Reply #111 on: August 01, 2017, 08:28:49 AM »
Thanks for the kind words, Alan.... When I started in airguns a decade ago, there was very little GOOD information on the Forums, people were not documenting and sharing their ideas, for the most part.... I struggled trying to get helpful information (with a few important exceptions, by people I will forever be in debt to).... so I decided early on to do the best research I could, and share all my results, good and bad, as accurately and completely as I could.... I am delighted that effort has been valued, and I hope it will continue to survive, despite Photobucket's recent efforts to derail the knowledge sharing on our Forums....

Bob
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oldpro

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Re: Tale of Two BRods
« Reply #112 on: August 01, 2017, 11:56:19 AM »
 Photobucket arrrrgggghhhhhhh!!!!!!!! :( >:(
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rsterne

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Re: Tale of Two BRods
« Reply #113 on: January 16, 2018, 09:14:11 PM »
This project has been on the shelf over the summer, both for lack of shop time and because I have been waiting to received one of the new ART/SS valves to try.... The hope is that it will match the Cothran, both in performance and ease of opening.... with the added benefit of being able to adjust the velocity with hammer strike, something you can't do with the Powerhouse valve.... you have to change the pressure to change the velocity.... Anyways, the wait is over and my valve arrived today, many thanks, Travis....  8)

The valve is a stock JSA "SS Balanced Valve" as they supply for the MRods.... It comes with a 3/16" exhaust port and the stock recess for the transfer port, which would not work in my BRods, as they are set up for a 3/8" OD transfer port with a 1/4" exhaust port.... Travis assured me that there was plenty of meat to enlarge the ports, and there certainly was.... I usually angle my exhaust ports about 20 deg. but with that big a port and no experience with this valve, I choose 15 deg. instead, and I'm glad I did.... At 20 deg. with a 1/4" port, I think I would have broken into the back of the valve and ruined the seat.... The valve throat is 0.281", which works out to the same area, once you subtract the 1/8" stem, as the 1/4" exhaust ports.... This is the same size throat as what I had on my modded MRod valve with the PEEK poppet, but the area is smaller than the Cothran valve.... I didn't want to drill out the throat at this time, but I did blend the new 1/4" exhaust port into the throat with a spherical Dremel burr.... There is certainly lots of port area for lots of flow....

I got the valve installed in the tube for one of my BRods and it seems to be holding pressure.... I will leave it overnight and plan on doing extensive testing tomorrow, tethering the gun at 1900 and 2900 psi in both .25 and .30 cal to get the baseline preload curves and assess what the opening forces are like compared to the Cothran valve and my modded MRod valve.... This should be FUN !!!

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #114 on: January 17, 2018, 03:45:17 PM »
Here is the first set of results for my BRod .30 cal with the ART/SS valve installed.... I tethered it at 3000 psi (a more accurate, digital gauge says my regulator was not set for 2900 as I previously thought), and tried the 50 gr. JSB/Daystates and also the 67 gr. NOE BBTs.... I tried all the SSG settings from maximum preload (way too much), through zero gap, to 10 turns of gap (over 1/2").... I then repeated the procedure with the 45 gr. JSBs and 50 gr. JSB/Daystates tethered at 1900 psi.... Lastly, I connected up my 500 cc bottle which has a regulated 1900 psi output and shot one 7-shot magazine of the 50 gr. pellets at several gap settings to find the pressure drop and calculate the efficiency.... Here are the results....



First some comments about this valve.... IT OPENS EASILY.... nowhere near the hammer strike required of my modded MRod valve.... I will be doing the same testing procedure with my Cothran valve to get an idea of the relative hammer strike required, but don't have that data yet.... It is much more responsive to changes in hammer strike than the Cothran, although still exhibits a VERY sharp velocity drop if you try and tune it for tiny sips of air (ie low velocity).... This sudden drop could well be due to the jet size installed in the inlet of the valve, I only got one with the valve, presumably a "stock" one, whatever that is.... and I can tell you it's pretty tiny.... With that tiny jet, and the huge drilled out exhaust port, I may be operating the valve outside its normal parameters.... in fact you can almost bet on it.... I hope Travis has some ideas to try and will send me some other jets to play with.... not that it really matters, I never tune my guns on the downslope anyways.... If I wanted a lower velocity, I would reduce the regulator setpoint and move back up onto the knee of the curve....

I am using an MDS hammer with a 3/4" steel core that weighs 51 grams.... The hammer spring is 0.48" OD x 3.00" long, and is made of only 0.051" wire, so has a spring rate of just 8 lb/in.... It is currently set up on the SSG with 0.60" of preload (4.8 lbs.).... and when set to zero gap, with the hammer stroke I have of 0.79" takes a maximum force of only 11 lbs. to cock the gun.... If you notice from the data in the graph, I can run the SSG gap at about 6 turns out (0.33") at 1900 psi (cocking force 8.5 lbs) and about 3 turns out (0.165") at 3000 psi (cocking force under 10 lbs) with hardly any velocity loss.... Those settings are basically at the top of the "knee".... With the 50 gr. pellet at 1900 psi, at 6 turns out, the velocity is 922 fps (94 FPE) and the efficiency is 1.04 FPE/CI.... The knee is quite long and gentle, making velocity adjustment quite insensitive, until you approach the "cliff".... Just before you get there, the velocity seems to get unstable, and the ES increases.... and then suddenly you are teetering on the edge of the cliff, where just a quarter turn more gap and the velocity tanks to under 400 fps....

The maximum performance (plateau velocity) with this valve is a few fps better than my modded MRod valve or the Cothran valve at 1900 psi.... and virtually the same with the 67 gr. BBT at 3000 psi.... but with the 50 gr. pellet at 3000 psi it was about 30 fps faster (1100 fps).... However, it also had virtually no velocity drop as you increased the SSG gap until it suddenly fell off the cliff.... almost acting like a Cothran valve.... However, this is so far out a normal condition (50 gr. pellet at 1100 fps) it doesn't really matter.... it's more an anecdote.... One other thing of note.... If I reduced the SSG gap to negative (ie introduced preload) the air consumption, report, and recoil all went "stupid".... I can only assume that the closing forces on the balanced poppet are so small that they are overwhelmed by the 5 lbs. preload on the SSG, and it increases dwell to the point the valve is still wide open when the pellet leaves the muzzle.... and YES, it's THAT dramatic.... When I was testing the pressure drop with 2 turns of preload, I actually got 640 psi of drop in my 500 cc bottle in just 3 shots.... compared to 370 psi drop in 7 shots with zero SSG gap.... Pretty much BOOM when you pull the trigger, complete with 6" or more of muzzle jump....  :o :o

The 8 lb/in. spring is OK for use at 3000 psi, but realistically I need to either reduce the preload on the SSG, or fit an even weaker spring for use at 1900 psi.... The SSG gap is too large at that pressure, and I might find a velocity difference shooting up or downhill from gravity slowing the hammer.... This change would make the gun even easier to cock, which seems silly, as it's sooooooooo easy now.... I can tell you, I'm SURE impressed at the reduction in cocking force with the ART/SS valve.... Travis and Lloyd sure came up with a winner here.... I can see that I will end up converting all my Big Bores and bullet shooters over, to get rid of the massive, long stroke, hard to cock hammers.... Incidently, I resized a 109 gr. Lee HP in .308 cal down to 0.300", and although it was still hard to chamber, I got 811 fps with 2 turns of preload (159 FPE) at 3000 psi.... accompanied by a huge roar and muzzle jump.... Yes, this valve can move a LOT of air....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #115 on: January 18, 2018, 11:46:38 AM »
Here is the data for the .25 cal BRod using the ART/SS valve.... The methodology was the same as for the .30 cal.... except the pellets used were the 25.4 gr. Kings and 34.1 gr. King Heavies at 1900 psi and a 52.7 gr. RN .25 ACP bullet at 3000 psi.... Here are the results....



The SSG gap where the gun fell off the cliff was virtually the same at the same pressure, about 7 turns at 3000 psi and 9 turns at 1900 psi.... I was using the same hammer and spring, all that changed was the upper (receiver and barrel).... This gun drives the Kings way too fast, nearly Supersonic at 1900 psi, but the King Heavies look to be a great choice at this pressure.... At 7 turns of gap (just over 3/8") the velocity is 966 fps (70.7 FPE) but the efficiency is a bit low, at only 1.01 FPE/CI.... Still, this is pretty impressive performance at only 1900 psi, and should yield about 32 shots at 70 FPE from the 500 cc bottle.... nothing to sneeze at....

As a bullet shooter at 3000 psi, the gun is solidly over 100 FPE, peaking at 115 FPE.... This is the highest FPE (by a whisker) I have seen with the .25 cal BRod at this bullet weight, and with a 60 gr. bullet would be even higher I am sure.... I didn't do any efficiency testing at that pressure, as I am short of cast bullets at the moment, and don't want to stop testing to make more.... Once again, if you crank in actual preload on the SSG, the gun roars and kicks as it blows air out the muzzle after the bullet is long gone.... It appears obvious that this valve (at least with the big ports) needs either an SSG or SSS to have any chance of behaving in a civilized manner.... It does offer some range of velocity adjustment, and the efficiency, like the Cothran valve, is best when operating just above the cliff.... However, with the ART/SS valve, that range is a lot broader and easier to tune to....

With the ART/SS valve using a 1/4" exhaust port, my BRods will be great with pellets when regulated at 1900 psi.... and are capable of shooting with bullets in both calibers at 3000 psi.... I'm pretty please with the results for initial testing.... I do need to try less preload on the spring to try and reduce the gap on the SSG, particularly when regulated.... I hope to get to that soon....

Bob
« Last Edit: January 18, 2018, 12:09:31 PM by rsterne »
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rsterne

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Re: Tale of Two BRods
« Reply #116 on: January 18, 2018, 04:25:37 PM »
I reassembled a BRod with the Cothran valve this afternoon just to assess the comparison between the required hammer strike with it and the ART/SS valve.... To my surprise, the new ART/SS valve was easier to open than the Cothran....  :o .... Even with the preload reduced to only 0.40" (3 lbs.), the Cothran valve fell over the cliff at a 3 turn smaller gap than the ART/SS valve, with the same pellet at the same pressure.... Now BOTH these valves are a LOT easier to open than a conventional PCP valve of the same dimensions.... That is the big advantage to balanced valves, you can use huge ports, throat and poppet, and not have to end up with a gun that is impossible to cock....

I rechecked the Cothran valve, and it remains stubbornly "ON" or "OFF".... You really have no control over the velocity with the hammer strike, you need to change the setpoint pressure to do that.... Yes, the Cothran valve uses less air when tuned just above the cliff, but it cant be turned down a bit before it ceases to cycle properly and the velocity drops to near nothing.... It is when operating just above the cliff that the ART/SS valve can be tuned for good efficiency while remaining stable (unless you get too close to the cliff, as mentioned earlier).... That region with the Cothran valve is very narrow, and when you get close to the cliff the velocity becomes unstable, although if you get it just right, it also exhibits improved efficiency.... It is just way fussier to adjust than the ART/SS valve....

To make large adjustments to the velocity with the ART/SS valve (at least in my large ported version) you still need to alter the setpoint pressure.... However, having a 100-200 fps range of adjustment without changing the pressure sure makes it easier to find a suitable tune.... It seems that the heavier the bullet, the wider the velocity adjustment range.... I suspect that with a stock ported ART/SS valve the adjustment range might be even wider.... It looks like I may have to find a weaker spring to reduce the SSG gap down to the desired level when running regulated at 1900 psi.... but before I make that change, I think I will completely remove the front jet and see what happens....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #117 on: January 19, 2018, 12:47:19 PM »
Today I tested the ART/SS valve without any jet on the inlet side.... just about a 1/8" hole there.... I still had the preload on the 8 lbs. hammer spring on the SSG set to 0.40" (about 3.2 lbs).... I tested the .30 cal version with the 50 gr. pellets at 1900 psi (only), and realistically there was no significant difference with the jet removed.... The valve still took very little hammer strike to open it.... So, I then swapped out the MDS hammer with the steel core (51 grams) for one with an aluminum core that only weighed 27 grams.... For some unexplained reason I lost about 10 fps from the maximum velocity (which I would not tune for, it would be too inefficient).... but as expected I now needed less SSG gap to get to the cliff.... Here are the results, compared to where I started with this valve (black line)....



Changing to the 0.40" preload from 0.60" is what moved the top of the cliff from 9 turns of gap to 8 turns.... I didn't plot that curve because it essentially tracked the black line except for the earlier cliff.... You can see that with no jet installed (purple line) there was less of a "knee" to the curve.... still some, but there was less velocity adjustment before you got to the cliff.... The same setup but with the 27 gram hammer (the orange line) shifts the cliff to only 5 turns of gap, but I can still reach the plateau with 1 turn of gap.... This gives me an adjustment range of 4 turns, and makes the amount of gap where I would adjust the gun just about where I like to see it.... around 1/8"-3/16".... The only way to reduce it further would be a lighter hammer spring, or reducing the hammer stroke.... I can actually DO that with the aluminum cored hammer, as it has a steel setscrew insert for the striker (currently set flush).... One other thing I noted with the very light hammer.... The range of velocity adjustment between plateau and cliff increases significantly.... leading me to believe that light hammers and this valve would be a great combination....

I did one "efficiency run" by shooting one 7-shot magazine with the velocity set to just over 900 fps with both setups.... Small changes in the exact velocity make big changes in the efficiency at around that velocity with these big ports.... but essentially I believe there is little difference in the efficiency if the velocity is the same with the jet removed.... HOWEVER, it does give you less velocity adjustment just above the cliff, and if you are tuning for efficiency I think you would struggle to get the best combination of power and air usage without the jet.... If all you are interested in is pure power with a light hammer strike, however.... this valve is a BEAST.... It requires MUCH less hammer strike than even the Cothran Powerhouse valve in the testing I have done so far.... I'm impressed....  8) 8) 8)

Bob
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oldpro

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Re: Tale of Two BRods
« Reply #118 on: January 19, 2018, 08:27:16 PM »
 Your Data is a welcomed sight from many hours of hard work. Thanks Bob.
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Re: Tale of Two BRods
« Reply #119 on: January 21, 2018, 07:35:52 PM »
Today I assembled the regulated tank block I got from Travis back in the summer.... It went well, and after a short learning curve installing shims, I was able to adjust it from 1000 psi up to over 2000.... I didn't try and go higher, but I am sure it will.... It has a 3K burst disc on the output side, and I didn't want to push it.... The adjustment is quite sensitive, you don't have to turn the 10-32 adjusting setscrew very far to move the output by 100 psi.... maybe 1 flat on the allen key or less....

Since the Cothran valve cannot be adjusted using hammer preload (or SSG gap), and needs to have the pressure adjusted to change the velocity, or retune for a different pellet weight.... it was a perfect match for an externally adjustable regulator.... Since there are two excellent pellets made by JSB that are quite different in weight (25.4 gr. and 34.1 gr.) I decided that to install the .25 cal upper on the lower that used the Cothran valve and the adjustable regulated tank block and I installed a 500 cc, 3000 psi bottle on it.... My initial adjustment of 2000 psi drove the 34.1 gr. JSB King Heavies at over 980 fps, but I used the opportunity to find out where the cliff was, and where the SSG gap needed to be adjusted to be close to the cliff, but solidly on the plateau, where the valve is operating stably.... That was with 2 turns of gap.... Then I shot the tank down to about 1700 psi so that I could drop the setpoint adjustment....

When increasing the setpoint pressure, you back out the 10-32 screw, which lowers the HP seat, moving it away from the piston and Belleville stack.... The seat opens, allowing more air to flow to the output side, raising the pressure and pushing the piston towards the (now lower) seat, against the Bellevilles, compressing them until the seat closes at the now higher pressure.... This puts no additional load on the seat, and can be done at any time.... When you want to decrease the output pressure, however, you need to drop the tank pressure below not only your current setpoint, but also below where you want the new setpoint to be.... The reduced pressure on the piston allows it to move away from the seat, and this allows you to turn the 10-32 screw in without putting additional pressure on the seat and possibly damaging it.... I wanted to start at 1800 psi and work my way back up, so I drained the tank down to 1700 and turned the 10-32 adjuster in until I felt slight additional resistance when the seat contacted the piston.... If you do this carefully and slowly, you can feel when they make contact.... When I added more air to the bottle, the gauge on the output side of the tank block moved up slightly, to about 1800 psi.... I shot a few shots, and the velocity was down to about 920 fps, so I simply backed the 10-32 adjusting screw out a fraction at a time, shooting a couple of shots at each setting, until I got my target velocity of 950 fps.... I then shot a couple of shots at each SSG gap setting, and recorded the results, which are on the graph below in red....



You can see the typical "cliff" exhibited by the Cothran valve which starts just before 3 turns out.... The velocity at 3 turns out is very unstable, with an ES of nearly 200 fps, because the valve is about to fall over the cliff.... However, at 2 turns of gap, it is rock solid, with about a 1% ES.... From previous experience I know that is the optimum place to run a Cothran valve, with the lowest hammer strike that allows stable operation and low ES.... I tried the 25.4 gr. pellets, and the velocity was over 1050 fps, so to get them to work I knew I had to lower the setpoint pressure.... I started simply shooting the 25.4 gr. pellets and watched the velocity fall as the pressure dropped.... I was pretty shocked to discover that to get down to 950 fps I had to drop the pressure in the tank all the way down to 1400 psi.... I shot a few more shots, to drop the tank pressure to 1300, and then turned in the 10-32 adjusting screw until I could feel it contact the seat.... I added air to the tank, the output gauge barely budged, and the velocity settled in at about 910 fps at about 1300 psi.... I slowly backed out the pressure adjusting screw, shooting a couple of shot at each adjustment, and stopped when the velocity with the 25.4 gr pellet reached my target of 950 fps.... The gauge indicated 1400 psi.... and I confirmed that by shooting the tank down until the velocity just started to drop, while the tank was still connected to the accurate gauge on my Great White.... Yes, I was getting 51 FPE at only 1400 psi.... I ran through the gap adjustments, and recorded the other curve (in blue) on the graph above.... The optimum gap was at 3 turns out, and the cliff started at 4 turns out, 1 turn more than at 1900 psi.... As expected, the valve requires a bit less hammer strike at 1400 psi than at 1900.... I also shot a few of the 34.1 gr Heavy pellets at that same setting, and that velocity is shown on the graph above as the black dot (872 fps).... So, with the 34.1 gr. pellets, I was getting 57.6 FPE at only 1400 psi.... pretty remarkable, and no adjustment made from the 950 fps setting with the 25.4 gr Kings....

Now that I knew that 3 turns out was the optimum setting for the 25.4 gr. Kings at 1400 psi, I recorded the pressure in the 500 cc bottle (using the gauge on my Great White), and shot 8 shots, on regulator, at 1400 psi.... I averaged the velocity (952 fps = 51.1 FPE), and recorded the pressure drop, which was 200 psi.... That works out to 0.97 FPE/CI, which is pretty low, but we are in new territory here, getting 51 FPE at only 1400 psi.... I repeated the test using the 34.1 gr. Heavies, and as I expected, the pressure drop was exactly the same, at 200 psi.... Since the energy per shot increased to 57.6 FPE, however, the efficiency increased to 1.10 FPE/CI.... I then backed out the 10-32 pressure adjuster to increase the setpoint back to 1900, set the SSG gap at 2 turns, and shot another 8-shot magazine at that setting with the 34.1 gr. Heavies.... The pressure drop was just over 200 psi, so I recorded it as 210 psi, and with the average velocity of 950 fps (68.4 FPE), that works out to an efficiency of 1.24 FPE/CI.... pretty decent at that power level....

The pressure drop per shot for all these tunes runs just about 25-26 psi per shot from the 500 cc bottle.... If we use a fill pressure of 3000 psi, with the 1900 psi setpoint, that should give 42 shots at over 68 FPE with the 34.1 gr Heavies.... and with the 1400 psi setpoint, that should give 64 shots at 51 FPE with the 25.4 gr. Kings and nearly 58 FPE with the Heavies.... I think those are quite acceptable numbers, and tuning back and forth between the 25.4 gr. and 34.1 gr. pellets (both at 950 fps) requires only resetting the external regulator adjustment, and one turn on the SSG gap.... In fact, if you don't care about getting the full 64 shots at the 1400 psi setting, you don't even have to change the SSG gap, you can leave it at 2 turns, and just lose a few shots.... If you have the gun tuned at 1400 psi, you can shoot the Kings at 950 fps and the Heavies at 870 fps, for the same number of shots, with NO adjustment necessary....

Bob

  • Coalmont, BC