QB79 Ninja - The $200 PCP

[rsterne]

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

[Millipede]

i was wondering when you would put a delrin stem in there..  i just ordered a couple from mountainair for my own qb79 since i noticed how deeply indented the original stem seal was the last time i had it apart

[Ribbonstone]

HAve swapped to Delrin stems in a couple of conversions, will probably end up with them in all of them sooner or later.  Do use the original at first, and when that blows, will switch to a Delrin stem.  Some of the rifles are still going strong on the issue valve stem, even after a couple of years of use but all of mine run at 800-850psi which probably doesn't stress them much.

[neric]

Very nice post.  Really enjoyed the read.  I may have to dive into this project myself.

[rsterne]

I've used factory valve poppets up to 1600 psi with good success.... The last time I had a problem I disassembled the brass poppet and just replaced the seal material with a Delrin washer I made.... and promptly LOST a few fps.... I have NO explanation as to why, the results I got this time were much more satisfying.... I certainly wouldn't have bothered replacing the stock seal for use at 1100-1200 psi, as CO2 sees those pressures any time it gets hot outside.... However, slimming up the poppet seems to be advantageous, so I'm glad I did.... I doubt if swapping out the poppet for a Delrin one (even a slimmed one) would have changed the performance in a Stock or Stage 1 gun.... other than it might actually waste more air if you don't back off the hammer spring preload to compensate for the additional (and unnecessary) lift with the harder material....

If you are modding a QB78, with the larger plenum volume, and running over 1200 psi, I can see the Delrin poppet adding to the performance when using a stock hammer spring preload.... This is because it is easier to knock off the seat, leaving more hammer energy to produce lift and dwell against the higher average pressure in the larger tube.... What I'm saying is that with a Delrin poppet the velocity with a stock hammer strike would likely max. out at a higher pressure....

Bob

[ericw]

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

I find this experiment to be real interesting Bob.  Have you ever considered trying to make the poppet head more aerodynamic to improve air flow? If making it a bit smaller has this much effect it makes me wonder.   Also, it is interesting to note that with near zero compression of the poppet, the energy required to open it also goes to zero (E=Force * distance).   

[rsterne]

The valve spring is 0.32" diameter, so I made the poppet the same diameter.... You can't go a lot smaller on the seat end, although you could slim it a few more thou.... I don't think using a "coke bottle" shape like they do for the fuselage in supersonic aircraft is worth it (and might be worse).... besides that is to allow for the cross sectional area of the wings (the "area rule")....

While you may think the compression on the poppet is "near zero", let me assure you it isn't zero.... I measured it on a Disco with a Delrin poppet and got about 0.007" of compression at 1200 psi.... The seats are about the same diameter, so at 1200 psi, the force is about 79 lbs, plus 7 for the spring for a total of 86 lbs.... To calculate FPE, you use half the force (the average, it's zero when compressed), so you get:

FPE = 86/2 x 0.007/12 = 0.025 FPE of hammer energy to "unstick" the valve.... If you triple the compression of the seal material (to 0.021", maybe a good guess) you would get 0.075 FPE of hammer energy sucked up before the valve even cracks with a stock (soft) seal....

I haven't calculated the hammer energy in a QB, but in a Disco, the Delrin poppet not worthy up about 18% of the available hammer energy (0.29 FPE) just to unstick at 2000 psi....

Bob

[rsterne]

Here is the results of preliminary pellet testing indoors at 20 feet....



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

[rsterne]

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....



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

[supertech77]

nice shooting,    i would say its dialed in at 20feet very nicely.. 

[rsterne]

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....



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

[Millipede]

Hey bob, for those people who want to emulate this but can't make a hammer spring adjuster; how much of the spring should we cut off to give us the same power and shot count as yours?

Also can't you adjust the output of a regular ninja tank just by adding a shim or two to the regulator?

[Millipede]

Btw, thank you for sharing all of your experience with us, it is very helpful and also fun to read about :-)

[rsterne]

Yes to both questions.... How much to cut off the hammer spring is the question, but the solution is relatively easy, cut off a 1/4 turn at a time until the velocity starts to drop.... Since the spring rides on a guide, I don't even think you would have to worry about closing up the last coil, just put the cut end on the spring guide on the cocking block.... You can increase the pressure of the Ninja regulator by adding some thin shims, or by replacing the three shims inside with one more Belleville.... I covered that in this thread.... http://www.gatewaytoairguns.org/GTA/index.php?topic=49648.0

I would NOT go beyond 1500 psi on a QB79, and then only if you still have the 1.8K burst disc fitted.... The closer you go to 1800 psi, the more likely you are to pop the disc, I can tell you that for sure they fail occassionally at 1600.... In addition, any time you have gas in a QB79 (HPA or CO2) you should have the action bolted into the stock so that the two front stock screws are screwed into the tank block for additional safety....

Bob

[rsterne]

I finished and installed the stock today....



It's a Richard's Microfit Wildcat Thumbhome in Select Grade Black Walnut with Ebony Grip Cap.... beautiful figuring in the grain....



I had to shorten the forearm and inlet it for the tank block and regulator.... The finish is 7 coats of Watco Danish Oil, each coat wet sanded, ending with a white Scothbrite pad of 1200 grit equivalent and then a coat of Dark Minwax.... The scope is a Leapers 3-9 x 32 AO with Sunshade.... The gun weighs 8 lb. 7 oz. as shown.... The Air Stripper is from a Hatsan PCP, I threaded the barrel 1/2"-20 NF to accept it....

It's no longer a $200 PCP, however....

Bob

[jessered]

IT'S SO PURTY!!!!!

[rsterne]

I made one more slight change today before the final testing.... The off center RVA screw was canting the cocking block slightly, and I noticed a slight scuff mark on the spring spacer.... I made a longer, one piece cocking block, with an integral spring guide and spring spacer.... The new cocking block is 1.25" long, of which 0.25" is a spring spacer that is 0.34" diameter, the other inch being full diameter to eliminate it canting in the tube.... That allowed me to shorten the RVA screw and just tidy things up a bit.... It shouldn't have changed the performance at all, but in fact I picked up about 10 fps, and gained a bit of efficiency as well.... The Preload numbers have changed, as I can't put quite as much preload on the spring at maximum with the shorter spacer.... Here is a new graph of Velocity and Efficiency vs. Preload using the JSB 15.9 gr. Exact Jumbos pellets....



As per my usual practice, I selected a preload on the knee of the curve, just below the plateau, in this case 4.5 turns out.... I refilled the tank to 3000 psi and shot a string with the following results....



I got 80 shots between 844 and 856 fps, an ES of just 12 fps (1.4%).... and was able to shoot down to 1100 psi before the velocity dropped below 840.... The average was 849 fps (25.5 FPE) at an efficiency of 1.21 FPE/CI.... During shooting the string, I shot 5 shot groups and sighted in the scope in my 20ft. indoor range.... By the time I was done, it was punching group after group in the same place on the target that were only 1/4" across, barely bigger than the pellet.... I can't wait to try this rifle out at longer ranges.... This is just about the perfect velocity to shoot the 15.9 gr. Exact at, putting it right in the middle of it's sweet spot for the Ballistics Coefficient....

This has been an exciting project, and I'm extremely pleased with the final outcome....

Bob

[evandj]

Great results with a great looking stock to boot.

[Froggy.]

Remarkable performance at only 1200 psi!
Bob, can you give an estimate of the velocity increase with a 50cc plenum?...I'm busy with a qb 79 bullpup build and will be using a longer tube giving about 50cc plenum.

[Ribbonstone]

Post has made me appreciate the QB's once again. Think I'm destined to own two more.

As it is, they all run on air.  .177, 5mm, 22, and .25 from 11 foot pounds to 60. 4 HPA’s and two PCP’s.



Am going to undo #2 from the left. New (full length) barrel, modded valve, and a 1100PSI tank,a tune it to 20 foot pounds.

And will add a plain jane co2 version, as close to stock as I can stand.