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I swapped out the tank for the other one that I had set up for 1500 psi.... I wasn't quite sure what to expect, as normally that is too much pressure for stock hammer spring preload on a QB78.... As it turns out, with the smaller plenum of the QB79, the pressure drop during the shot compensates for the extra pressure at the beginning of the shot.... allowing the stock hammer spring to work OK.... Here are the results of the Stage 2 tune running at 1500 psi....
http://i378.photobucket.com/albums/o...ps6fe7e17e.jpg
As before, the starting pressure was 2000 psi, and the ending pressure was 1000.... I got 16 shots averaging 918 fps (26.8 FPE) down to 1400 psi before the velocity started to drop below 900 fps.... The fact that the drop in velocity starts 100 psi below the regulator setpoint indicates that this tune is operating very close to the "knee" of the curve where I like to tune my regulated guns.... The efficiency was higher than at 1200 psi, working out to 0.80 FPE/CI, and I should be able to get 42 shots @ 27 FPE on a 3000 psi fill.... That a pretty serious hunting platform....
When I did the calculations for the pressure during the shot for the 12 cc Plenum, I get 836 psi at the end of the shot, and an average pressure of 1168 psi during the shot.... That gives a pretty good indication of why the stock hammer spring preload is working so well with this setup.... It also confirms that running 1200 psi on the Stage 2 tune is running less efficiently than it could because the hammer energy is too much for the 880 psi average pressure.... I now have no doubt that reducing the preload a bit on that tune will increase the shot count with little or no velocity loss.... I'm going to have to give some thought to what is the easiest way to accomplish that, as fitting an RVA to a QB isn't the easiest thing to do, and I want it adjustable, I don't want to just clip the hammer spring....
pharoh007 - Eric, D&L, and Trailmarker are all Ninja Dealers and should be able to give you a quote and delivery....
Bob
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Just out of curiosity I flung a few 18.1 gr. JSB Heavies through the Chrony.... They did 855 fps (29.4 FPE).... That's 3 FPE better than the best I ever did with a QB79 at 1500 psi with those pellets previously.... so the new "Maxi-Valve" certainly works great.... Also, so far, it doesn't seem to leak, despite not having the O-ring "squeezed" between the valve halves.... I would say the "Maxi-Valve" idea is a complete success in the QB79....
Bob
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Well, sometimes you pay for not keeping good notes.... I made an offset RVA, removed the rear cocking pin to convert to "cock on open", and machined a new downwards slot to hold the bolt back while loading the pellet....
http://i378.photobucket.com/albums/o...A/IMG_2133.jpg
The only problem was, that the version I previously did that on (in the photo above) had a shortened hammer and guide pin on the cocking block to allow the bolt to be drawn back further without the hammer hitting the cocking block when drawn back to the rear notch.... When I adjusted the RVA so that I could push the bolt down into the rear slot, the hammer spring didn't have near enough preload on it.... Rather than modify the hammer and guide pin, I made a spacer to slide over the guide pin on the cocking block to increase the hammer spring tension.... The spacer was 0.35" OD, 0.22" ID, and 0.30" long, so it adds that much preload, and allows the gun to function properly.... I then proceeded to test the velocity at various preloads, with the following results....
http://i378.photobucket.com/albums/o...ps9a15ffd6.jpg
You can see the typical plateau in velocity you get on a regulated gun when the hammer strike is more than required.... The velocity doesn't increase, the gun just uses more air.... One slightly unusual thing on this graph, is that the efficiency falls off on the right side as well.... For some reason, the power drops off quicker than the air usage when the gun is detuned below 800 fps in this Stage 2 configuration.... My choice with this setup, at 1500 psi, would be to tune the gun for just over 900 fps (26 FPE).... I would expect about 50-60 shots at that power level.... Pushing for more power than that with a QB79, even with the "Maxi-Valve" simply wastes air....
Bob
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Here are the results of a full string starting at 3000 psi using the Stage 2 "Maxi-Valve" setup running at a regulator pressure of 1500 psi.... I set the RVA to 2.5 turns out, which set the velocity just where the "knee" of the curve starts, with the 14.3 gr. Crosman CPHPs....
http://i378.photobucket.com/albums/o...ps434961a3.jpg
The average velocity was 908 fps (26.2 FPE) with a high of 914 and a low of 902 for an ES of 12 fps (1.3%) over the first 58 shots.... Shot 62 was just outside a 2% ES and wasn't counted, and the pressure at that point was 1300 psi.... As is typical when you tune a regulated gun to the "knee" of the velocity curve, you can shoot a couple of hundred psi below the setpoint before the velocity takes a dive.... The efficiency worked out to 1.07 FPE/CI, which for this power level in a QB79 is pretty astounding....
I'm extremely pleased with the results of this testing.... I think it would be very difficult to get more power than this from a QB79 without major changes to allow running a burst disc higher than the 1.8K which is required for safety the way the gun is produced.... While more power could be produced from a QB78 because of the larger tube, when you consider the simplicity of just screwing a tank onto a QB79 to convert it to a PCP it's a pretty impressive combination.... I will be re-fitting the 1200 psi standard SHP Pro regulator for final testing shortly....
Bob
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Today I refitted the stock Ninja SHP Pro regulated tank to the Stage 2 QB79.... I then shot 10-shot strings to check the velocity and air pressure drop for different hammer spring preloads so that I could calculate the efficiency.... Here are the results....
http://i378.photobucket.com/albums/o...pscdcebe24.jpg
As I suspected during the initial testing of this version, the stock hammer strike was too much, wasting a significant amount of air.... The original tune suggested about 50 shots on a fill, but if the efficiency could be bumped up to 1.0 FPE/CI I predicted 73 shots.... Now I'm pretty confident that can be met or exceeded, since at 4 turns out I was seeing an efficiency of about 1.15 FPE/CI.... Now when using only 10 shots, and a gauge with 100 psi increments, it's easy to have some errors creep in.... but that 10 shot string only used 220 psi, or 22 psi per shot.... while returning 845 fps (22.7 FPE).... I'm looking forward to running a full string starting from 3000 psi as soon as I have my SCUBA tank topped up....
Bob
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Here is the shot string with the hammer spring preload at 4 turns out.... This is for the Stage 2 tune with the "Maxi-Valve", using a stock Ninja SHP Pro regulated 13 CI tank filled to 3000 psi.... I was using 14.3 gr Crosman CPHP pellets....
http://i378.photobucket.com/albums/o...pscd2bcd5a.jpg
The average velocity was 847 fps (22.8 FPE), and over the first 80 shots the highest velocity was 854 fps and the lowest 841 fps, for an ES of 13 fps (1.5%).... Only the last shot dropped below 840, and it still fell within 2% of the highest, and the pressure at that point was 1070 psi.... That works out to an efficiency of 1.07 FPE/CI, interestingly the same figure as I got yesterday shooting at 1500 psi....
Getting 80 shots in a flat string at slightly better than stock Disco power is nothing short of amazing.... I think this is the way I'm going to leave this rifle.... It makes one heck of an entry level PCP.... Here is a summary of the three tunes using the Ninja SHP Pro Regulated 13 CI tank, shooting 14.3 gr. Crosman CPHPs....
Stock QB79: 135 shots at 609 fps (11.8 FPE)
Stage 1 Tune (top end work only): 105 shots at 738 fps (17.3 FPE)
Stage 2 Tune (Maxi-Valve): 80 shots at 847 fps (22.8 FPE)
Note that in order to get the extended shot count in Stage 2, I had to REDUCE the hammer spring preload from stock.... I have a feeling that the shot count in a Stage 1 tuned gun (and possibly even a Stock gun) could be improved a bit by the same method.... Although I chose to use and RVA and convert the gun to "cock on open", simply clipping a bit off the hammer spring would have the same result....
Although my Maxi-Valve, IMO, is getting just about all the performance possible from a QB79 at this pressure.... I would like to point out that there are commercially available valves from several sources that have the piercing pin and filters removed and the sides of the valve opened up to increased flow into the valve.... I'm guessing that they would be in the 800+ fps range when used with the porting done in Stage 1, when operating with this tank and reg.... I would highly recommend the Ninja SHP Pro 13CI Tank and Regulator combination as an inexpensive and yet excellent way to convert a QB79 to HPA.... All the same mods that are used in the CO2 versions also work to enhance the performance of the PCP version, which is a big bonus....
Bob
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Sometimes leaks can be a good thing.... I've had a slow leak in this gun virtually since new.... It lost about 600 psi the first day, then 3-400, then a trickle.... I tested everything with soapy water and couldn't find anything, which pretty much meant it was the valve.... I pulled the breech, and found a very small leak coming out the exhaust port, so it was the poppet that wasn't sealing on the seat.... I pulled the gun all apart, and on inspecting the seat with a loupe I saw a very small nick on the sealing surface, and a corresponding mark on the soft seal insert on the poppet.... The brass seat was easy to polish, but rather than take a chance on the seal in the poppet I decided to replace the poppet head with a Delrin one which is more suitable for HPA use.... I drove the stem out of the old poppet and reused the stem, and made a new poppet head from a piece of 3/8" Delrin.... Since I didn't have to worry about the OD of the brass poppet, I was able to turn the entire head down to 0.32", which is the diameter of the valve spring, and 0.050" smaller than the original.... I lapped the new poppet against the repaired seat, and reassembled the gun....
The first test shots, with the RVA set at 4 turns out as before, were a bit of a shocker.... Instead of getting 847 fps (22.8 FPE), the velocity was in the high 880s with the occasional 890+ (25.0 FPE).... Obviously the new Delrin poppet was a big improvement.... Since the throat and seat is still the same size, I figure the increase must be from the decrease in head diameter and the corresponding easing of the flow around it.... There was another big change, and this one I expected.... I was able to reduce the hammer spring preload another 2 turns to get back to the "knee" of the velocity curve.... This is because the harder seal material compresses less, and that reduces the amount of hammer energy required to "unstick" the valve from the seat.... It sure is nice to see theory backed up by results....
Bob
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Here is the results of preliminary pellet testing indoors at 20 feet....
http://i378.photobucket.com/albums/o...ps09d136f6.jpg
I shot 5 shot groups from the bench, and then sorted the results by using two washers of 3/8" and 1/2" diameter.... If the 3/8" washer completely covered the group, it is coded green above.... If the 1/2" washer would NOT cover the group, it is coded red.... All others (3/8" to 1/2") are left unmarked.... The best group was the 15.9 gr. JSB Exact Jumbo, which was just a ragged hole, closely followed by the other JSB Exact Series pellets....
The Ballistics Coefficients listed are taken from testing over 25 yards with another gun that shot about 1 FPE more muzzle energy, so they will be very close in this gun.... Pellets shown in blue will be tested at longer range....
Bob
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Today I added a Hatsan Air Stripper to the barrel and did a few tests with the JSB Exact 15.9 gr. pellets which produced the best groups yesterday.... I had never tested a stripper before, so I set the gap between the stripper cone and the barrel at maximum (about 2 calibers gap from the muzzle) and then moved it in about 1/2 cal. for each group.... I shot 5 shot groups through the Chrony with the target still at 20 feet, not knowing what to expect....
Without the stripper, the velocity was 849 fps, and with the gap at maximum it seemed to be a couple of fps less.... Each time I moved the stripper closer to the muzzle it picked up a couple of fps, and by the time the gap was down to just over 1/16", the velocity was 855 fps.... I figure that when the gap is small, the stripper is focusing some of the muzzle blast behind the pellet and effectively lengthening the barrel a fraction, although not very efficiently.... The group size did, in fact, seem to vary as I adjusted the gap, although at this short range realistically the changes were very small.... I expect it would be easier to see results at longer ranges.... Since the smallest group seemed to be with a gap of just under 1/4", I decided to adjust the gap to exactly the same size as the head of the pellet (just because) so I clamped a pellet in some forceps and them used the head like a feeler gauge and adjusted the stripper cone so that the pellet just touched it and the muzzle.... Here is the 5-shot group I shot at that setting....
http://i378.photobucket.com/albums/o...psc3a09512.jpg
That is a .22 cal JSB Exact sitting in the hole.... The target won't meet the Feinwerkbau test where a pellet will hang in the hole with the target vertical, but it won't fall through either.... I figure the group is 0.03 - 0.04" C-T-C, or about 0.25"-0.26" OD.... In any case, I'm pretty pleased with it, and indeed the stripper seems to help....
Bob
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I tested the efficiency at various hammer spring preloads today, using the 15.9 gr. JSB Exact Jumbo pellets which are very accurate in this gun.... The new slimmer Delrin valve has picked up about 2 FPE, so it shoots the 15.9 gr. pellets at nearly the same velocity as it did the 14.3 gr.... I had to back the preload out several turns relative to the stock poppet because the harder Delrin is much easier to knock open than the soft seal on the stock valve.... Here are the results....
http://i378.photobucket.com/albums/o...psb290e1ce.jpg
The efficiency seems slightly higher than before, and the plateau starts at 6 turns out instead of 3.5 turns.... Based on the results, I tried a short string at 6.5 turns out, and the velocity was only about 10 fps below the plateau and the efficiency was about 1.10 FPE/CI.... I think that's pretty good for a QB79 shooting at 25 FPE, and it's likely where I will end up tuning it.... That should give me over 70 shots per fill....
Bob
