Air Gun Home :: View topic - The Importance of Volume

rsterne · Moderator

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.
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.... Mr. Green

Bob

Sholo · Veteran Member

Hi Bob:

If you wanted to use a full length 78 tube, could you not use a reverse mounting block like the one you made up for your Hayabusa? Then the rifle would balance a little better, plus get you your extra volume. Yes? No? Maybe?

Todd

rsterne · Moderator

YUP, that could be done.... and I may make one at a later date like that.... It requires a custom reversed block.... and I want to do this in a manner that could be done with as little machining as possible using mostly QB parts....

Bear in mind, going from a 30cc volume to a 90 cc volume won't gain much compared to the work involved....

Bob

Alstone · Moderator & Site Supporter

I like the way you have opened up the back end of the valve, I usually drill extra holes around the base, but your idea would give a lot better flow and neater.

Al

rsterne · Moderator

I figured it out myself.... and then found out that Archer airguns use a somewhat similar method.... although not as extreme.... Rolling Eyes

Bob

badshot · New Member

Rich in Michigan was doing his Qb valves like this a number of years ago.

rsterne · Moderator

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.... Mr. Green

Bob

rsterne · Moderator

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

SMP · Silver Status Member

Bob is obviously tiring of the wing/feet method of dressing grouse.

He's going to hit them so hard he just has to pick up the breast as he walks by Mr. Green

rsterne · Moderator

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.... Mr. Green

Bob

rsterne · Moderator

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.... Mr. Green

Bob

Doc Sharptail · Member

That transfer port modification looks like a good one- the milled recess in the valve....

What type of cutter accomplishes a job like that, and is it with-in the capability of a guy with access to drill press and dremel?

Thanks in advance...

Regards,

Doc Sharptail

SMP · Silver Status Member

Hi Doc

Oddly enough as I read this post I just happened to have the cutter in my pocket to do some work with it.

It's a 1/4" end mill cutter. I get mine from Busy Bee.

I use mine in the drill press all the time. With a cross sliding vice you can do all sorts of trickery.

rsterne · Moderator

HI Doc....

Like Sean says, use a 1/4" end mill to produce the square-shouldered recess in the breech and valve.... You can do it in a drill press provided you can hold the work rigidly and accurately centered, plus with the hole vertical so that you get the recess the same depth on all sides.... All it takes is care in setup and a cross-sliding vice really helps there....

One trick you can use to get the hole centered is to find a drill that fits the hole accurately and chuck it backwards.... If it's too long, you may have to sacrifice one by breaking the fluted portion off.... Using it as a probe in the chuck, you adjust the position of the workpiece until the drill slides in and out then you lower the chuck without dragging on the sides.... Then you know you are centered perfectly....

One added tip.... Check the position of the valve, breech and barrel ports relative to one another.... Often they don't line up all that well, and you can benefit from moving the holes over slightly to improve alignment.... If you look at my modified barrel port in the top photo you will notice that I had to move it to the right (towards the muzzle) to get it to line up with the hole in the breech.... Look at the position compared to the O-rings (it's no longer centered).... A bit harder to see is the recess in the valve, it was moved toward the muzzle slightly as well (only a few thou), again to align it with the hole in the main tube and breech....

Bob

Doc Sharptail · Member

Thanks for the replies, guys. There is a drill press vise- it ain't a cross-slider, but good enough for scope mount holes in an old marlin or 2. I've used rod blanks in the past for lining up things with some semblance of precision, so I'll be on familiar ground here.

No busy bee tools here, but there are a couple of machinist's havens- I'll find what I need....

Regards,

Doc Sharptail