QB78 on HPA - Hammer Spring Question

[rsterne]

I've been working on the QB78 I recently purchased that was holding CO2 but not working.... The hammer strike was too weak to open the valve at all at over about 85*F.... I tore it apart, couldn't find anything wrong, polished everything up and reassembled it and it worked like a charm.... I assumed it was some dirt hanging the hammer up....

Anyway, I made an HPA fill fitting for it and proceeded to do some testing as a non-regulated PCP, just using the stock air tube (which is about 44cc).... Before the initial testing I pulled the filter out of the valve (there was no screen), drilled the bolt flow-through out to 5/32", and I got 34 shots within 10% of peak (23 within 4%), shooting from 1600 psi down to 600, and the velocity peaked at 603 fps at 1200 psi.... I had heard that QBs had a very strong hammer spring, but there was no question it wasn't capable of doing the job at over 1600 psi at all.... the velocity fell to only about 300ish there.... The efficiency at this stage was a staggering 1.96 FPE/CI....

Next I deciced to get rid of the very restricted transfer port, so I bored a shoulder in the valve and breech and fitted a poly port.... This gained 100 fps, but dropped the shot count down to 18 within 10% and 13 within 4%, and dropped the top end pressure for the 10% range down to only 1500 psi (with the bottom still at 600).... The peak velocity was now 701 fps at only 1100 psi.... and the efficiency was still very good at 1.58 FPE/CI....

Today I pulled the valve out and did a major chop job on it.... I machined two 5/16" wide slots about half way down the sides of the valve to effectively eliminate the valve having any restriction to the flow at all.... I got rid of the peircing pin, inner sleeve and washer, I machined spring seats in the end of the valve and on the poppet, fitted a longer spring, tapered the poppet and the surrounding housing, drilled the exhaust valve throat out to 7/32" and rounded the corner on the exhaust port.... Basically, the biggest restriction is right at the valve seat, everything ahead of that is wayyyyyyyyyy bigger.... The idea was to increase the average pressure during the shot by using the entire 44cc tube as the valve....

I'm pretty pleased with the results, especially at the low pressures involved.... The shot count dropped to only 9 shots within 10%, and 5 within 4% of the peak velocity, which was 851 fps at just 1100 psi (fill to 1400, refill at 600)....    .... The efficiency is still 1.28 FPE/CI, a fraction better than what a Disco returns at the same power level.... Here is a graph of the results so far.... JSB 14.3 gr. Exact Express pellets....



OK, so I've got a gun that breathes VERY well, producing 23 FPE at only 1100 psi.... The shot count is short, but a regulated tank would fix that in a big hurry.... Here is the problem.... There has to either be something wrong with this hammer spring (not hardened?).... or all the rumours I have heard about the hammer spring in a QB being good to about 2000 psi (some say more) aren't true.... Comments?....

Bob

[rsterne]

It's been an interesting afternoon.... I discovered that my hammer spring is 0.046" wire instead of 0.051", hence the weak hammer hit.... I found out that there is room for five 1/16" thick #10 washers on the spring guide.... I tried various setups and decided that three washers (3/16") under the back of the spring gave me what I wanted.... Here are the results (red line)....



As you can see, the velocity now peaks at 1800 psi, at 1016 fps with 14.3 gr. pellets (32.8 FPE).... Since I plan to run a tank regulated to about 1600 psi and fitted with a 1.8K burst disc the hammer spring is now up to the job.... Here are the peak velocities with heavier pellets....

18.1 gr.  940 fps   35.5 FPE - The efficiency is 1.16 FPE/CI with this pellet....
25.2 gr.  841 fps   39.6 FPE
32.1 gr.  762 fps   41.4 FPE

During experimenting I actually tried 2 washers, then went to 4 before deciding on 3.... With 4 washers, I got the following results....

14.3 gr.  1050 fps  35.0 FPE
18.1 gr.  971 fps    37.9 FPE
25.2 gr.  875 fps    42.9 FPE
32.1 gr.  800 fps   45.6 FPE

Nearly 46 FPE is a whack of power from a QB.... I would imagine that with a higher pressure tube and a stronger hammer spring the gun could at least equal my Disco, where I got 52 FPE in .22 cal.... I did some quick calculations, and using a 13 CI tank regulated to 1600 psi the gun should be good for about 40 shots at 35 FPE....  

Bob

[Jay]

Bob, truely you are the guru of learning about HPA work from, please don't ever find another hobbie.

[Bullit]

Bob,
I'm with Jay.  I enjoy reading your posts.

[Donny]

I am really interested in this thread, wanting to do this to my .177 qb. Id like to push my 8.4 jsb's to the 900-950 fps mark, keep us posted on the final build, thanks

[redone1992]

Im collecting parts to do a qb i just got a ninja tank and reg 13ci.
I have all ready set the reg 1000 psi to start with and 1.8k burst disk
on the down stream side.
Now waiting on the qb to get here. 

[Ribbonstone]

Re: QB78 on HPA - Hammer Spring Question..am assuming your are pressurizing the tube to some medium air pressure and shooting as as a low pressure PCP.  The short shot string is pretty common (how may shots inside of 20ps?),  Can get a quite powerful gun for a couple of shots, co2 valves are pretty well timed to let a larger volume of medium pressure gas out at a hit.


So long as you refill ever few shots (in order to keep the velocity variation in a reasonable spread), can shoot your  QB on the coldest of winter days and not loose a bit of power.

Which is exactly how I got hooked.  Was pumping up standard QB's to co2 type pressures (under 1300psi) and tuning them to run well...run fast...at that air pressure for 3 to 5 consistent shots. Got addicted o the sped running on air.

So i pumped the gun's tube to some medium pressure...lets say 900psi...and tune the gun to shoot well at that pressure.  Means you may only get a couple of shots while testing/tuning at  that pressure before having to re fill in order to adjust/test a few more shots,

THEN cut the tube, add a QB79 gas block, then add a 13CI HPA  bottle with the same output as the pressure you tuned for.

End up with a well tuned gun, running on your selected HPA bottle output pressure, and a whole lot of shots per fill.

The ones with the bottles attached are running on HPA...how fat is your choice; just tune them for the pressure you expect the HPA bottle to be regulated to.


It is a good bit of work...but in the end, they:

run on air, so temperature has a very minor role in performance
are consistent for a long string of shots.
can be as powerful as you'd care to tune for (although air use does decrease shot count)

[Donny]

So if i hook up a bulk fill setup regulated at 1200psi or charge the stock air tube to 1200psi on my .177 qb78 with a modded valve what kind of performance shoud i see, shot count, fps that kind of stuff, i only use jsb 8.4 express in this gun, thx

[rsterne]

If you look at the above graphs at 1200 psi you will see that my stock gun got ~600 fps.... the ported configuration got ~700 fps.... and with the extensively modified valve I got ~840 fps.... Those were with 14.3 gr. pellets in .22 cal so it's pretty hard to extrapolate that to 8.4 gr. pellets in .177 cal.... maybe 100-150 fps more?....

The number of shots you get with a remote tank will depend on your state of tune and the size of tank.... If you are just charging the stock air tube, you should see a bit more shot count than what I got.... I got 34 shots stock,  18 shots ported, and 9 shots with my modded valve.... using a 10% velocity window....

Make SURE your regulator is fitted with a 1.8K burst disc on the downstream side to protect the gun....

Bob

[Donny]

ok, il let you know how it goes, should i stick with the 1200 psi or go for the next step up, need to know before i order the reg, id really like to hit the 900fps mark, the valve is already a modified one but im not sure how much archer modifies them, its called their xp tuned valve

[SciGuy]

Donny,  with the modded valve and 1200 psi you should be over 900 fps.  You may need to shim the hammer spring, but most of the QB springs are strong and you may be OK.  

I had to shorten my hammer spring and reduce the pre-load to get my setup down to 800 fps using HPA at 850 psi and a highly modified valve from another tuner.  I also have the Archer bolt which added 50 fps.

[Donny]

Where did you get your valve? I still need to get the xp bolt myself.

[SciGuy]

The guy that made my valve is no longer in business. 

[Millipede]

Bob, truely you are the guru of learning about HPA work from, please don't ever find another hobbie.

i agree   

i usually end up bookmarking his topics