QB79 Ninja - The $200 PCP (PAL Required)

[rsterne]

Here is a photo of the valve I will be installing for my "Stage 2" mods.... The 1/4" wide aluminum ring is all that is left of the front part....



It was epoxied onto the rear portion using a #113 O-ring for a spacer (just touching).... When the JB Weld was 4 hours old (not sticky but not hard) I cut the O-ring and peeled it out of the groove and then cleaned up the front of the groove with a razor blade to remove any excess epoxy.... Once hardened, I will turn it around and face off and bore out the inside to increase flow into the poppet seat....

Bob

[sholo]

I like what you did to your valve! I'm assuming (?) you will be running an external spring on the rear of stem with an E clip?

[rsterne]

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....



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

[rsterne]

Richards only make one airgun stock, for the QB78, it is inletted for the main tube, trigger, stock mounting screw and barrel band, and fit well.... The outside of the stock, however was VERY rough and required a lot of sanding.... I was fortunate to be looking at a time when they had two QB stocks on their "Specials" page (not 2nds, returned or never picked up), both quite fancy pieces of wood.... This one had a deluxe buttpad, and ebony grip cap and forend (which I had to remove when I shortened it for the QB79....

Bob

[rsterne]

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....



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

[rsterne]

Nope, the valve normally only misses the tank block by a fraction, so I'll fashion a spring seat on the block and use a longer spring.... I just measured the "on seat" force of a stock QB valve, and interestingly it's 7 lbs., the same as a Disco valve.... so I just have to match that....

Bob

[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

[rsterne]

I finally had a chance to do some testing outdoors today, so I tested all the pellets scoring "green" in the last tests indoors that I expected to have a decent BC at 25 yards.... I measured the average velocity at the muzzle and at 25 yards, for 5 shots with each pellet, and then used ChairGun3 to calculate the Ballistics Coefficient.... Here are the results....



The groups were sorted as usual, with any group you could cover with a Dime scoring green, any group you couldn't cover with a Quarter scoring red, and in between no colour code.... I was surprised that all the JSBs were the only pellets that produced sub-dime sized groups.... and the best of those was the 15.9 gr. Exact Jumbo.... When you look at the JSB Exacts, the heavier the pellet the better the BC, as expected, and this results in a lot more FPE at 25 yards.... However, with the combinaton of good velocity and high BC, plus superb accuracy, I chose the 15.9 gr. as the pellet to use.... After sighting the rifle in 1/4" high at 25 yards, here is a typical 5-shot group....



The small squares are 0.1", and the black squares are 0.4", so the C-T-C size of that group is less than 0.25".... I can get 70 shots per fill at 25 FPE, and basically it just keeps dumping them in a ragged hole at 25 yards.... I think we can call this project an unqualified success, and this rifle has earned a place on my "wall of keepers"....

Bob

[rsterne]

I did some more work on the Stage 2 "Maxi-Valve" today.... The concept is to turn all the volume between the valve seat and the QB79 tank block into usuable valve volume.... Here are the results....



I machined a 0.10" long spring seat into the end of the poppet to fit the stock QB valve spring and then tapered the sides of the poppet on a 5* angle to reduce the OD to that of the spring at the seat.... The support for the front end of the spring is made from a 1.5" long 8-32 SHCS with the head turned down a bit to fit inside the QB spring.... I made a tapered brass collar to slide over the screw to form the spring seat, and then drilled out a piece of 3/16" brass K&S hobby tubing to fit over the screw as a spacer to set the height.... The tank block will be drilled and tapped on center for the 8-32 screw....

It turns out that in a stock QB valve, the installed spring is compressed to 1/2" long, so as long as I have that distance between the spring seats I will have stock preload.... It works out in my QB79 that I need the front spring seat 1.035" from the tank block to acccomplish that.... Here is a view of the inside of the valve, showing what the flow is like....



If you compare that to a stock QB valve, you will see a huge difference.... The ID of a stock valve is 7/16", my valve is bored out to 1/2", which doubles the clearance around the poppet.... The aluminum ring I glued on is 1/4" long measured from the O-ring groove.... After the epoxy hardened, I machined out the front of the valve on a 30* angle until there was only about a 0.020" lip remaining.... That took the taper about half way to the seat.... I then changed the angle to 15* and knocked off the corner to smooth the flow even more....

The throat of a stock QB valve is 0.198" and the stem is 0.116".... I drilled the throat out to 7/32" (0.219") so that the throat area is larger than the rest of the porting I did in Stage 1.... I also blended the bottom of the exhaust port into the throat using a 5/32" ball grinder in a Dremel.... The distance from the valve seat to the tank block is just over 2", which gives a basic volume of 14.5 cc.... but by the time you subtract the volume of the poppet, spring, and forward spring mount, I ended up with a whisker over 12 cc.... While it's not as much as I'd like, it shouldn't cause too much loss in efficiency until the power goes over 25 FPE.... I'd frankly be very surprised to see that power level, even with my "Maxi-Valve" installed, when running at only 1200 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