The Importance of Volume - QB 78 vs 79

[rsterne]

I have done some experimenting before on the effect of the volume between the regulator and the valve on performance.... For minimal degradation in the power, I use a rough rule of thumb of 1 cc per FPE for the amount of air required.... As you reduce the volume below that point.... and since the regulator can't react fast enough to keep the pressure at the valve seat at the setpoint.... the average pressure during the shot drops.... and therefore so does the velocity and power.... This volume can only work, however, if the inlet side of the valve can flow air faster than the outlet side.... If the valve has a lot of restriction on the inlet side (as, for example a piercing pin, a small inlet hole, or (as in the Disco) a restrictive gauge port.... then this volume can't work anyway, and this rule won't apply to the same degree, if at all.... Guns with such a setup won't care a lot whether the volume between the regulator and the valve is large or small.... However, if in the search for power you have removed those restrictions.... then the volume downstream of the regulator is critical at high power levels....

First of all, let me show you the modifications I made to my QB 78 to get maximum power from it.... Here is a photo of the valve and ports....



The upper parts are stock, while the lower parts are the ones I modded.... Here is what I did....

1. Removed the filter and screen, internal sleeve and piercing pin from the valve.
2. Machined two large slots in the side of the valve to eliminate any restriction on the inlet side.
3. Machined a new spring seat in the front end of the valve (and on the poppet) and fitted a longer, lighter spring.
4. Milled a small 1/4" recess at the valve exit and into the breech and replaced the transfer port with a 1/4" OD poly tube (0.163" ID instead of 0.104").
5. Rounded the inside corned of the valve exhaust port to streamline the flow, and drilled the throat (inside the seat) out to 0.218".
6. Drilled out the barrel port from 0.134" to 0.166".
7. Installed 3 washers (3/16" total) for preload on the hammer spring (the spring was only 0.047" wire, the older QBs were 0.051").
8. Drilled out the front and bottom of the bolt probe to 5/32" to increase flow.
9. I should also have replaced the valve poppet with a Delrin one as the reshaped stock one is about to fail at HPA pressures (to come).

After all these mods were done, and with four washers on the spring, I got over 1050 fps (35 FPE) with 14.3 gr. pellets at about 1800 psi (unregulated, just filling the tube).... With 21 gr. pellets I got about 40 FPE, and with 32 gr. EunJins I got over 45 FPE.... With just three washers in place, using 18.1 gr. JSB Heavies, I got about 35 FPE at 1700 psi. but just a few shots.... Since I wanted to be able to use the QB 79 tank block and a regulated tank to get a reasonable shot count.... and incorporate a 1.8K burst disc for safety.... I had to reduce the pressure to 1500 psi.... I ran two sets of tests today, one with the regulator feeding the QB 78 tube through a short remote line.... and the other using the QB 79 tube and tank block with the tank and regulator mounted directly.... The tube in the QB 78 has 6 inches between the front of the valve and the fill fitting (43cc).... The QB 79 has virtually no space between the tank block and the valve.... The valve itself, machined away as it is, has a volume of about 7 or 8 cc.... When you take into account the volume of the hose (on the 78) and the passages in the tank block (on the 79) I think a fair estimate of the total volume between the regulator and the valve seat would be about 55 cc on the QB 78 and about 10 cc on the QB 79.... Here are the results, using JSB 18.1 gr. pellets and a 1500 psi regulator setpoint....

Tube......QB78.....QB79

Low..........872........805
High.........885........817
Avg fps.....880........811
Avg FPE....31.1......26.4
No.Shots.....40.........39
Total FPE...1245....1031
PSI Used...1550....1550
Vol Used....1390....1390
Efficiency...0.90.....0.74

A few things are immediately obvious.... The smaller volume of the QB 79 tube reduced the power by 15%.... but the shot count did not increase.... That means the efficiency suffered as well.... I have a spreadsheet that I use for estimating the average pressure during the shot based on the volume, and it tells me that with the QB 78 tube the average pressure should be about 1430 psi.... but with the QB79 tube only about 1210.... That is almost exactly the 15% reduction in power I saw in the results.... The spreadsheet estimates that if I eliminated half the volume of the QB 78 tube (ie shortened it by 3") the average pressure should be about 1370 psi (producing about 30 FPE).... a loss of only 4% from the full length tube.... That would give me a volume of about 1 cc per FPE.... Incidently, adding 6" to the QB 78 tube (ie doubling it's volume) would only increase the average pressure to about 1460 psi, a gain of only 2% (0.6 FPE).... You can see how extra volume buys you very little, but insufficient volume loses you a lot of power....

You might ask why not use the entire length of the QB 78 tube.... Well, first of all, I have to shorten the tube to fit the QB 79 tank block anyway.... Secondly, the shorter I make the tube, the better the rifle will balance.... as using a longer tube compared to the QB 79 moves the tank forward.... It's a matter of how short can I cut the tube without losing too much of my hard earned velocity gains.... I'm going to go with a tube length that will give me my 1 cc per FPE.... ie about 30 cc total including the valve.... There is little point, IMO, in tuning my QB 78 for much beyond 30 FPE because the shot count will just disappear.... and it would be difficult to do without running pressures higher than what can be handled without overloading the 1.8K burst disc.... I have already discussed in other threads about why it is so important to limit the pressure when using a QB 79 tank block because of the attachment safety issues....

Anyway, I learned what I needed to know.... My QB 79 will be reassembled into a non-PAL .22 cal HPA rifle and put up for sale.... I will be shortening the tube on my QB 78, making a new delrin valve stem, customizing the stock to fit, and tweaking it from there.... I'll post the final results in this thread just to see how theory and practice agree....  

Bob



 

[redone1992]

Another great post Bob lots of good info.

[lloyd-ss]

Bob,
Thanks for the helpful information.  I think you need to call yourself an "airgun systems engineer."  The interrelationships in these guns is complex, but usually predictable.  The poor efficiency is a good lesson on opening up the flow on the downstream side of the valve and making the pre-valve/post-reg chamber volume become the choke point.  I realize it was an experiment to quantify the results.  I did not notice how the eff of the QB79 was before the porting was done.  I imagine it was noticeably higher.

I like your rule of thumb of 1cc of pre-valve/post-reg vol for each FPE.   

Good info.
Thanks,
Lloyd

[SAADE]

Your killin me Bob, I can picture all of that great work on the lathe and the mill and...

Good comments on the flow:efficiency:pressure relationships, I often think of these issues in liquid forms(hydraulics), it helps me reason through the "small-tube / big-tube" problems and pressures vs volume etc etc.

[rsterne]

I do have a few efficiency numbers from the QB 78 at various stages.... Stock 43 cc tube, unregulated (total vol. about 49 cc - 51 cc with slots).... 14.3 gr. pellets, strings to 90% of peak velocity....

Stock gun with only the flow through bolt drilled out to 5/32" (front and bottom)....

Avg. velocity - 579 fps
Avg. energy - 10.7 FPE
No.Shots - 34
Total energy - 363 FPE
Pressure range - 1000 psi (1600 to 600, peak at 1200)
Air used - 206 CI
Efficiency - 1.76 FPE/CI

Filters removed, barrel ported, poly transfer port....

Avg. velocity - 678 fps
Avg. energy - 14.6 FPE
No.Shots - 18
Total energy - 263 FPE
Pressure range - 900 psi (1500 to 600, peak at 1100)
Air used - 185 CI
Efficiency - 1.42 FPE/CI

Valve Mods - no piercing pin or sleeve, slotted body, beveled poppet, new spring & seats, 0.218" throat, ported....

Avg. velocity - 814 fps
Avg. energy - 21.0 FPE
No.Shots - 9
Total energy - 189 FPE
Pressure range - 800 psi (1400 to 600, peak at 1100)
Air used - 172 CI
Efficiency - 1.10 FPE/CI

3 Washers on hammer spring (higher pressure fill)....

Avg. velocity - 981 fps
Avg. energy - 30.6 FPE
No.Shots - 6
Total energy - 183 FPE
Pressure range - 1000 psi (2000 to 1000, peak at 1800)
Air used - 214 CI
Efficiency - 0.86 FPE/CI

Note that the regulated version using the QB 78 tube tested above was about the same power level (31 FPE) and slightly greater efficiency.... but I was using heavier pellets.... Still, that is pretty consistent results between the regulated and unregulated versions.... which shows that the "plenum" volume (ie in between the regulator and valve seat) is adequate.... I may be able to increase the efficiency slightly by taking a washer out of the hammer spring.... as it's a bit too hard a strike for only 1500 psi....

Bob

[rsterne]

Well, I completed the QB 78 conversion to regulated today.... I shortened the main tube by just under 3" so that the tank block and part of the regulator are hidden by the stock.... which I hollowed out to suit (pics to follow).... The new reservoir is 3.15" long from the tank block to the front of the valve, and including the valve the volume is about 30cc.... The projection from my previous data was just over 30 FPE.... and I'm please to say that I got 40 shots at 30.5 FPE with JSB 18.1 gr. Heavy pellets.... for a total of 1219 FPE.... The string averaged 871 fps with a high of 875 and a low of 867 fps until the last 2 shots.... I stopped shooting at 40 shots (857 fps, 2% below the peak), and the pressure remaining in the tank was 1400 psi, so I used 1600 psi (1434 CI) of air, and the efficiency works out to 0.85 FPE/CI.... Here is the shot string....



I'm extremely pleased with the results.... I also tested a selection of other pellets to get a feel for how the FPE changes with pellet weight.... 5 shot averages....



As you can see, this is a pretty typical PCP, where the FPE increases with the pellet weight.... With EunJin Pointed pellets, the gun was shooting 36.8 FPE, and I would expect at least the same shot count.... That would put the efficiency at or above 1.0 FPE/CI.... which is pretty good for a PCP at those power levels that started out life as an 11 FPE CO2 rifle.... The stock had some shipping damage right at the front, and with the mods I made the easiest way to make it look decent is to paint the front portion black.... Once I have it prettied up, I'll post a photo.... 

Bob

[Smackey54]

Excellent thread...and amazing results! Thank you!

[rsterne]

I swapped out the barrel today and installed a .177 barrel and bolt.... I left the barrel port stock (0.134") but I drilled out the bolt probe to match, front and bottom.... I tested several weights of pellets with the following results.... Notice that once again, the heavier pellets have more FPE energy....



From 3000 psi I got 40 shots down to 1400 psi which is where the velocity started to drop off.... Interestingly, that is the same number of shots as the .22 cal version.... Since the FPE is a lot less in .177, so is the efficiency.... Both the JSB Exact Heavy and Crosman Premier Heavy pellets went into one hole shooting inside at 20', closely followed by H&N 10.2 gr. Baracudas (not in the chart above, about 5 fps faster than the JSB Heavies, same energy).... The worst groups were, once again, from the H&N Rabbit Magnums, at 0.65" C-T-C.... I hate those pellets, they are the worst I have ever tested.... With the best pellets, the efficiency was only about 0.58 FPE/CI.... with the heaviest it was 0.67....

With the energy this platform delivers with the most accurate pellets, it would be simple to detune it to just under 20 FPE.... which would be a good setup for FT.... I would think you could get the shot count up a bit by doing that.... It is interesting that in .177 this gun only develops 2/3rds of the energy it does in .22 cal.... Too bad I don't have a .25 cal barrel, it might pack a pretty good whallop with one....

Bob

[rsterne]

Well I did a VERY interesting test today.... There is an ongoing argument about which is better, flow-through style bolts or ones with an extended probe that seat the pellet skirt just past the transfer port.... I've been favouring the probe style, but today I got to finally prove it.... I chucked the .22 cal QB bolt in the lathe, and machined off the front of the flow-through section, back to the rear edge of the transfer port hole.... I then drilled a 1/16" hole down about 1/4" past the O-ring groove and fitted in a piece of 1/16" music wire of the appropriate length.... I fitted a sleeve over the wire made from 3/32" OD x 1/16" ID - K & S brass tubing from a hobby shop.... I adjusted the lengths until the probe was 0.100" longer than the front of the original bolt to make sure that any pellet would be seated with the skirt clear of the front of the transfer port.... A couple of drops of crazy glue (plus a good fit) hold the pin and sleeve in place....

I stripped off the .177 cal barrel and bolt, and installed the .22 barrel and the now probed bolt and proceeded with Chrony testing.... I used the 18.1 gr. JSB Exact Heavy pellets which the gun particularly liked.... Here are the before and after numbers....

Number......... Flow-Through..... Probed
Avg. Velocity......871 fps...............903 fps
Avg. Energy.......30.5 FPE..............32.8 FPE
No. Shots..........40.......................37
Total Energy......1220 FPE.............1214 FPE
Efficiency..........0.85 FPE/CI..........0.85 FPE/CI

The probed bolt.... with NO other changes from the flow-through bolt.... increased the velocity by 32 fps and the energy by 2.3 FPE (7.5%).... The total energy per fill remained the same, which reduced the shot count by three shots.... There is no longer any question in my mind that a probed bolt is superior to a flow-through style in terms of performance.... at least in a higher powered PCP.... Just to double check, I also tested the RWS 11.9 gr. Hobbies, and the EunJin 32.1 gr. Pointed pellets.... the Hobbies gained 35 fps (1.9 FPE) and the EJ Points gained 20 fps (2.0 FPE).... so the results are not limited to the JSB pellets....

This QB just keeps getting better and better.... 900+ fps with 18.1 gr. JSB Exact Heavies is a thing of beauty.... especially when I watched all 37 shots stack into a hole measuring 0.3" across (0.08" C-T-C) on my inside range as I did my Chrony testing....  

Bob

[oldpink]

Wow, Bob!
You must be an engineer to exploit that much knowledge so well.
Fascinating information here.

[rsterne]

Naw, not even a wannabe engineer.... They make things too complicated.... 

Bob

[lloyd-ss]

Bob,
Nice info from today's flow-thru vs plain probe.  I had thought about that in the past and the calculations always showed that the actual "flow" area of the probe was greater than a flow-thru.  The wall of the flow-thru would have to less than .015 thick for the numbers to work out.  Do you think that just the difference in cross sectional area made the probe perform better, or less congestion around the x-fer port?  How about a probe that is contoured to help the air turn the corner?
Neat stuff!
Lloyd

[rsterne]

The throat on my flow-through was 5/32" (0.157") diameter through the center.... as big as practical starting with the QB bolt.... The bottom hole was 5/32" wide (rounded entrance) and significantly longer than that (ie oval).... so more area than the 0.166" barrel port, especially in the direction of flow.... The transfer port was poly tubing, so the ID was about 0.161"....

The probed bolt, calculated back to the area of the corresponding circle, would be 0.195".... so yes, less restriction than the flow-through.... but the transfer port is really not much different than the flow-through, and a lot less than the area around the probed bolt....

The answer may be less congestion around the barrel port.... it may be a matter of just the area.... or a combination of both.... The fact remains that the straight probe is significantly better performance than the flow-through.... One less thing to worry about (until somebody gets different results)....

Bob

[Fisherdude]

Thanks for the post.  I have been disappointed with my QB 78. Although it is quite accurate, and fun to shoot, I can't shoot it whenever I want due to temperature extremes common where I live.  I have been thinking about switching to a PCP.  Your post has inspired me to reconfigure my QB 78 and keep using it.

Thanks again.

[rsterne]

Shimming the hammer spring will not make any difference if your HPA pressure is below a certain level.... On my QB78 that was about 1100-1200 psi, depending on the state of mods (ie how much flow was possible through the valve and ports).... Once the valve is fully open, clobbering it harder makes no difference.... I had to shim my hammer spring to make it work at 1500-1800 psi.... I'm probably "overshimmed" at 1500 and may take out one washer to see if it affects the air usage....

Bob

[rsterne]

Found out something interesting today when I was playing with my QB 78 regulated to 1500 psi.... There were three washers shimming the preload on the hammer spring, which is good enough for about 1800 psi.... Since I'm only running 1500, I wondered what would happen if I removed one washer.... Well, I lost 3 fps.... but the shot count (and the efficiency) went up....  Instead of getting 37 shots on 1600 psi, I now get 44 shots (at 32.6 FPE).... The efficiency has increased fro 0.85 FPE/CI to 1.00 FPE/CI.... NOW I'm happy.... the gun FINALLY has decent efficiency.... 

Bob

[Rescue35]

  Wow Bob, that is great. Thanks for all of the great information.

[rsterne]

Yeah, once again, too much hammer spring hurts the efficiency.... It's a no-brainer on a non-regulated PCP.... but it's a lot less intuitive on a regulated one.... This is the third gun I've increased the efficiency on now by carefully tailoring the hammer spring preload to the regulator output pressure.... I'm beginning to think I'm seeing a pattern.... 

Bob

[lloyd-ss]

Found out something interesting today when I was playing with my QB 78 regulated to 1500 psi.... There were three washers shimming the preload on the hammer spring, which is good enough for about 1800 psi.... Since I'm only running 1500, I wondered what would happen if I removed one washer.... Well, I lost 3 fps.... but the shot count (and the efficiency) went up....  Instead of getting 37 shots on 1600 psi, I now get 44 shots (at 32.6 FPE).... The efficiency has increased fro 0.85 FPE/CI to 1.00 FPE/CI.... NOW I'm happy.... the gun FINALLY has decent efficiency.... 

Bob

Bob,  I knew you weren't going to be able to live with that thing only getting .85 fpe/cuin.      What I have always found exciting about these pcps, regulated and not, is how tuneable they are, and how forgiving they are, too.  They might not be efficient, or shoot accurately, but they will almost always shoot.  Then if are so inclined, it can become a real science project to get the best efficiency, or shot count or accuracy or power that you are after.

I know we can count on you to get things tuned to a very useable and efficient state.
Good work,
Lloyd

[rsterne]

Yeah, well wait until you hear about todays results.... How about 60 shots from 3000 psi down to 1200.... high of 905 fps, low of 883, average of 896 with 18.1 gr. JSBs.... That's 32.3 FPE x 60 = 1936 FPE total.... Air used was 1800 / 14.5 = 124 bar x 13 = 1614 CI.... The efficiency was 1936 / 1614 = 1.20 FPE/CI.... I got the hammer spring setting perfect, the highest velocity was at about 2500 psi tank pressure (ie not the highest) and the velocity didn't fall off until 300 psi below the regulator setpoint.... The gun was self-regulating within the tiny fluctuation of pressure from the regulator creep....

Details in my "Dual Power Adjuster" thread to follow....

Bob