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PostPosted: Thu Jul 18, 2013 7:37 pm 
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I swapped out the tank for the other one that I had set up for 1500 psi.... I wasn't quite sure what to expect, as normally that is too much pressure for stock hammer spring preload on a QB78.... As it turns out, with the smaller plenum of the QB79, the pressure drop during the shot compensates for the extra pressure at the beginning of the shot.... allowing the stock hammer spring to work OK.... Here are the results of the Stage 2 tune running at 1500 psi....

Image

As before, the starting pressure was 2000 psi, and the ending pressure was 1000.... I got 16 shots averaging 918 fps (26.8 FPE) down to 1400 psi before the velocity started to drop below 900 fps.... The fact that the drop in velocity starts 100 psi below the regulator setpoint indicates that this tune is operating very close to the "knee" of the curve where I like to tune my regulated guns.... The efficiency was higher than at 1200 psi, working out to 0.80 FPE/CI, and I should be able to get 42 shots @ 27 FPE on a 3000 psi fill.... That a pretty serious hunting platform....

When I did the calculations for the pressure during the shot for the 12 cc Plenum, I get 836 psi at the end of the shot, and an average pressure of 1168 psi during the shot.... That gives a pretty good indication of why the stock hammer spring preload is working so well with this setup.... It also confirms that running 1200 psi on the Stage 2 tune is running less efficiently than it could because the hammer energy is too much for the 880 psi average pressure.... I now have no doubt that reducing the preload a bit on that tune will increase the shot count with little or no velocity loss.... I'm going to have to give some thought to what is the easiest way to accomplish that, as fitting an RVA to a QB isn't the easiest thing to do, and I want it adjustable, I don't want to just clip the hammer spring....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Thu Jul 18, 2013 9:24 pm 
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Just out of curiosity I flung a few 18.1 gr. JSB Heavies through the Chrony.... They did 855 fps (29.4 FPE).... That's 3 FPE better than the best I ever did with a QB79 at 1500 psi with those pellets previously.... so the new "Maxi-Valve" certainly works great.... Also, so far, it doesn't seem to leak, despite not having the O-ring "squeezed" between the valve halves.... I would say the "Maxi-Valve" idea is a complete success in the QB79....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Jul 18, 2013 9:51 pm 
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Nicely executed Bob! I need a vacation to work on my own air guns!!!!

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PostPosted: Fri Jul 19, 2013 7:38 am 
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Bob.

Very cool results. I agree the tiny little plenum is killing the efficency.

What about the tank block? with some judicious trimming and hogging out would it be possible to pick up some serious plenum area? I don't have one in my hand but looking at web pics it seem like there might be an additional 100% in that block.

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PostPosted: Fri Jul 19, 2013 9:58 am 
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You could definiitely get a bit more volume, but the problem is the O-rings and mounting screws end up with very little room between them for air passages.... The current air hole from the reg is only 1/8".... If the block had been designed with proper screw placement (further from the end of the tube), it would be even less.... I came to the conclusion that the search for a couple of cc's wasn't worth the time and effort.... Pushing a QB79 to these limits is really kind of a waste, you would be far better off to start with a QB78 and reverse the tank under the forestock like this....

Image

By using the entire '78 tube as a plenum you get about 50 cc, which is more than you need for .22 cal, and even allows you to go .25 cal (as above).... Here is how that gun shot....

Image

All it takes is for some enterprising machinist to produce the tank blocks.... The drawing are free to the right guy (nudge, Sean, cough) and as a bonus, the same block will fit a 2260 tube with just a different adapter in the end of the tube (drawings also done)....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Fri Jul 19, 2013 5:41 pm 
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rsterne wrote:
You could definiitely get a bit more volume, but the problem is the O-rings and mounting screws end up with very little room between them for air passages.... The current air hole from the reg is only 1/8".... If the block had been designed with proper screw placement (further from the end of the tube), it would be even less.... I came to the conclusion that the search for a couple of cc's wasn't worth the time and effort.... Pushing a QB79 to these limits is really kind of a waste, you would be far better off to start with a QB78 and reverse the tank under the forestock like this....

Image

By using the entire '78 tube as a plenum you get about 50 cc, which is more than you need for .22 cal, and even allows you to go .25 cal (as above).... Here is how that gun shot....

Image

All it takes is for some enterprising machinist to produce the tank blocks.... The drawing are free to the right guy (nudge, Sean, cough) and as a bonus, the same block will fit a 2260 tube with just a different adapter in the end of the tube (drawings also done)....

Bob



I have this set up [ bought the project ] and it works wonders.

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PostPosted: Fri Jul 19, 2013 10:13 pm 
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Getting 32 excellent shots (48FPE) out of a .25 with THAT small a reservoir is incredible, Bob! Well done. Maybe I SHOULD be rethinking that .25 Hatsan of "Gordy's - HA! He was only taking about 18 shots - yuk! (I'm sure he was no using the full reservoir- but I won't mention that)

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PostPosted: Sat Jul 20, 2013 12:38 pm 
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rsterne wrote:
You could definiitely get a bit more volume, but the problem is the O-rings and mounting screws end up with very little room between them for air passages.... The current air hole from the reg is only 1/8".... If the block had been designed with proper screw placement (further from the end of the tube), it would be even less.... I came to the conclusion that the search for a couple of cc's wasn't worth the time and effort.... Pushing a QB79 to these limits is really kind of a waste, you would be far better off to start with a QB78 and reverse the tank under the forestock like this....

Image

By using the entire '78 tube as a plenum you get about 50 cc, which is more than you need for .22 cal, and even allows you to go .25 cal (as above).... Here is how that gun shot....

Image

All it takes is for some enterprising machinist to produce the tank blocks.... The drawing are free to the right guy (nudge, Sean, cough) and as a bonus, the same block will fit a 2260 tube with just a different adapter in the end of the tube (drawings also done)....

Bob



I would buy a couple of blocks straight away Shaun :mrgreen:


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PostPosted: Sat Jul 20, 2013 12:59 pm 
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Well, sometimes you pay for not keeping good notes.... I made an offset RVA, removed the rear cocking pin to convert to "cock on open", and machined a new downwards slot to hold the bolt back while loading the pellet....

Image

The only problem was, that the version I previously did that on (in the photo above) had a shortened hammer and guide pin on the cocking block to allow the bolt to be drawn back further without the hammer hitting the cocking block when drawn back to the rear notch.... When I adjusted the RVA so that I could push the bolt down into the rear slot, the hammer spring didn't have near enough preload on it.... Rather than modify the hammer and guide pin, I made a spacer to slide over the guide pin on the cocking block to increase the hammer spring tension.... The spacer was 0.35" OD, 0.22" ID, and 0.30" long, so it adds that much preload, and allows the gun to function properly.... I then proceeded to test the velocity at various preloads, with the following results....

Image

You can see the typical plateau in velocity you get on a regulated gun when the hammer strike is more than required.... The velocity doesn't increase, the gun just uses more air.... One slightly unusual thing on this graph, is that the efficiency falls off on the right side as well.... For some reason, the power drops off quicker than the air usage when the gun is detuned below 800 fps in this Stage 2 configuration.... My choice with this setup, at 1500 psi, would be to tune the gun for just over 900 fps (26 FPE).... I would expect about 50-60 shots at that power level.... Pushing for more power than that with a QB79, even with the "Maxi-Valve" simply wastes air....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Jul 21, 2013 12:26 am 
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Here are the results of a full string starting at 3000 psi using the Stage 2 "Maxi-Valve" setup running at a regulator pressure of 1500 psi.... I set the RVA to 2.5 turns out, which set the velocity just where the "knee" of the curve starts, with the 14.3 gr. Crosman CPHPs....

Image

The average velocity was 908 fps (26.2 FPE) with a high of 914 and a low of 902 for an ES of 12 fps (1.3%) over the first 58 shots.... Shot 62 was just outside a 2% ES and wasn't counted, and the pressure at that point was 1300 psi.... As is typical when you tune a regulated gun to the "knee" of the velocity curve, you can shoot a couple of hundred psi below the setpoint before the velocity takes a dive.... The efficiency worked out to 1.07 FPE/CI, which for this power level in a QB79 is pretty astounding....

I'm extremely pleased with the results of this testing.... I think it would be very difficult to get more power than this from a QB79 without major changes to allow running a burst disc higher than the 1.8K which is required for safety the way the gun is produced.... While more power could be produced from a QB78 because of the larger tube, when you consider the simplicity of just screwing a tank onto a QB79 to convert it to a PCP it's a pretty impressive combination.... I will be re-fitting the 1200 psi standard SHP Pro regulator for final testing shortly....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Jul 21, 2013 2:36 am 
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The shot strings are nice and flat with a regulated bottle. Why don't manufacturers install a regulator in the airguns with integrated airtubes? They seem cheap enough to manufacturer and fit.

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PostPosted: Sun Jul 21, 2013 10:25 am 
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Good question.... Part of the answer is that for best performance, you need a fair amount of air "on deck" in a plenum between the regulator and the valve.... about 1 cc per FPE being ideal.... That detracts from the reservoir volume, so you trade off fewer shots for a flatter string.... OR you have to have a separate tank and plenum like my reversed block design....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Jul 21, 2013 8:25 pm 
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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....

Image

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

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jul 22, 2013 12:13 am 
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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....

Image

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

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Mon Jul 22, 2013 9:02 am 
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That's a great shot string Bob great work well done. :wink:
One question ? how much of a deference was the shot sound decibel from stock to stage 2

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