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PostPosted: Tue Mar 31, 2015 5:26 pm 
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Joined: Thu Sep 11, 2014 10:59 am
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Location: West of Ottawa
I have just received a QB79 with an HPA 850 PSI regulated tank. Without any mods (except removing the filter from the valve) it shoots around 640FPS - sorry, not sure the weight of the .177 pellets i was using.

With a reasonable tune consisting of enlarging the ports and modifying the valve for better flow and volume, what kind of FPS could I expect for .177 at 850 PSI? Anything above 40 shots from 3000PSI down to 850 would be fine - more concerned with velocity at this point.

thanks

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QB79 .177 (PCP) X 2


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PostPosted: Tue Mar 31, 2015 10:04 pm 
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Location: Winnipeg, Manitoba
You should get well Into the 700's

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PostPosted: Wed Apr 01, 2015 10:55 am 
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Thanks, that's sort of what I was thinking. Need a lot of volume to get high FPS I imagine, so not very efficient and probably pretty loud.

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


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PostPosted: Wed Apr 01, 2015 11:18 am 
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Joined: Sat Nov 03, 2007 4:19 pm
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Location: Coalmont BC
When you are working on it, do any port work first, and then keep reducing the hammer spring preload/length until the velocity just drops off.... That is the point where you have the best balance between efficiency and power.... Adding hammer strike beyond that just wastes air.... You should get wayyyyyyy more that 40 shots, over 100 would be more likely on just 850 psi.... The pressure is the biggest limiting factor....

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 Apr 05, 2015 3:00 pm 
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Joined: Thu Sep 11, 2014 10:59 am
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Location: West of Ottawa
Thanks Bob. Will do.
I did the porting today and with stock hammer spring and no preload I am getting 770 Fps average on .177 rws meisterkugeln 8.2 grain pellets. At 850 psi I think that's pretty good.

Bob, do you have a guess on how I should mod the spring/preload? It's a bit of a pain opening the breech multiple times snipping a bit at a time. It doesn't have to be perfectly optimal.
Thanks,

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


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PostPosted: Sun Apr 05, 2015 3:51 pm 
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Location: Coalmont BC
On a QB, snipping the spring is the simplest way.... Keep cutting off 1/4 coil at a time until the velocity drops a bit and quit there....

RVAs are possible on a QB, but not simple to make.... You are looking for this type of performance as you reduce the preload....

Image

Notice that above a certain preload, the velocity doesn't increase, you just use more air.... You want to operate on the "knee" of the curve, before the velocity starts it's rapid decline.... On the above gun, I tuned it to 4.5 turns out....

Bob

_________________
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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