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 Post subject: Extending a QB79 Tube
PostPosted: Fri Apr 10, 2015 9:55 pm 
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Joined: Thu Sep 11, 2014 10:59 am
Posts: 482
Location: West of Ottawa
ive read lots of posts on QB79 tuning and in particular, Bob's $200 PCP thread. The main limit it seems with a QB79 is the lack of volume on the tank side of the valve. Once the valve opens, the pressure drops substantially.
I have just bought a QB78 tube which I plan to use on a QB79. I'm thinking that adding 2" to the qb79 tube length would substantially increase the performance. Has anyone done this? What results did you achieve?
I don't really want to extend the tube much more than 2" as it would start getting very front heavy.
Any advice is appreciated.

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- Doug
.................
QB79 .177 (PCP) X 2


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PostPosted: Fri Apr 10, 2015 10:33 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8979
Location: Coalmont BC
I used a tube halfway between a QB79 and QB78 on one of my builds (so about 3" longer than a QB79), and gained almost as much as you would with a full QB78 tube, on a 30 FPE build.... adding the 2" of length is definitely worthwhile, unless you are building under about 20 FPE, in which case it isn't worth the trouble.... One thing to consider, however, is that you really should mount the tank block into the stock with the two lower screws the way it is done in the QB79 for extra safety.... This is particularly true if you are using the stock QB79 tank block, as the mounting screws are really too close to the end of the tube for proper strength.... You can fix that by moving the screws back to in between the O-rings and using higher tensile screws.... otherwise you should make sure the tank block is fastened to the stock.... In any case, make sure that the regulator is equipped with a 1.8K burst disc so that the gun can never see more than 1800 psi pressure, which is the maximum it would see on a hot day with CO2....

PS the gun still balanced OK with the 3" longer tube....

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: Sat Apr 11, 2015 9:25 pm 
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Joined: Thu Sep 11, 2014 10:59 am
Posts: 482
Location: West of Ottawa
Thanks very much, Bob. I'll be sure to mount it into the stock securely. I'm thinking of moving my .25 cal LW stock over to this build and use a 1500 Psi regulator. Currently it is on an unregulated tube and it's got quite the bell curve to it and not a lot of shots.

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- Doug
.................
QB79 .177 (PCP) X 2


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PostPosted: Sat Apr 18, 2015 12:55 pm 
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Joined: Thu Sep 11, 2014 10:59 am
Posts: 482
Location: West of Ottawa
I got around to playing with this today. I had a .177 cal qb79 with an 850 psi tank. It was shooting around 770 Fps.
With no other changes except for lengthening the air tube by 3", it went up to 840 Fps.

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- Doug
.................
QB79 .177 (PCP) X 2


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