Higher Shot Count with Higher Velocity?

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Higher Shot Count with Higher Velocity?

Postby Bullfrog » Fri Sep 21, 2012 12:58 pm

A while ago I was perplexed as to how I was getting a higher velocity with a more restricted velocity port setting than with a wide-open setting. It was explained to me that having the velocity port screw set too open can cause dead space by which air is pushed into the dead space intead of behind the pellet. By constricting the screw down some, it will close that dead space and force all of the air behind the pellet. Ok, that makes sense. So riddle me this...

I had to replace my valve body for the second time a few months ago because I yet again stripped my velocity screw head. I received an entirely new valve body assembly from Crosman and installed it. I found that unlike my previous valve, which gave me wide-open settings at 2 1/2 turns open on the velocity screw, this valve doesn't give me wide-open settings unless I have it 6 turns open. Here's where it gets really wierd. If I constrict the valve down below 6 turns open, it turns my velocity down AND my shot count. If I turn it at 6 or slightly above, my velocity goes way up AND I get extra shots.

Why or how can I get more shots AND more velocity at the same time by opening things up more?

My fill pressure is 3200psi. I am using the 10# spring with pre-tension set all of the way up, hammer set 1/2 turn from full stroke. If I adjust my velocity screw down to get 800 fps with Barracudas or up to 850 fps with Barracudas, I only get about 10 shots either way. If I open it on up to get 870+fps with the cudas, I can get 12-14 shots depending on where I cut off my bell curve. What gives?

Although I'm glad my gun is pushing high power so well, I'm concerned that I may not be able to turn it down for a better shot count if I want to use the gun for squirrels and whatnot. I'm wondering if I have an odd valve.
Bullfrog
 
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Re: Higher Shot Count with Higher Velocity?

Postby louigi600 » Sat Sep 22, 2012 10:28 am

The volumetric difference that one turn of the transfer port make is about 1 cubic millimeter approximately nothing. I don't think that can be the cause.
I'd say is has something to do with the valve counter pressure that is higher when the transfer port is more occluded and thus in some circumstances can cause the port to stay open a bit longer and let a bit more air out. Maybe something else .... but surely not the volumetric loss of having next to no volume difference.
Regards
David

Benjamin Marauder .177 cal.
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Re: Higher Shot Count with Higher Velocity?

Postby Bill G » Sun Sep 23, 2012 10:47 am

You'd be surprised.
example
my rifle stock had a .140 TP with a throat of .204 and an inlet of .240. With the hammer and sroke settings I had, I got a valve open travel of .1143".
The inlet and forces of the hammer dictate this. A .240 inlet has 144.76lbs of force on it. This stroke will allow .212ci of air to flow for this shot. Lets say that you have an inlet that is .002" smaller than mine(.238"). That will have 142.36lbs of force holding it closed. This will allow a travel of approx .1162" thus flowing .220ci of air. My resulting velocity is 883fps. Your resulting velocity would be 900fps. That is 17fps gain for .008ci of air at the same psi fill and the same hammer strike. The duration that the valve is open is less than 1milisecond and the projectile is engaged with the bore for about 2.2ms. As the psi drops the valve opens further and more volume is used thus efficiency goes down. This is why mechanical valves have the curve. A valve will not flow any higher rate once the opening stroke exceedes 1/4 of the smallest passage diameter. For me that would have been .035". Ideally, to open the valve .035 and hold it open long enough to flow precisely .212ci of air would have given the best efficiency, rendering the most consistency and the highest shot count. As the psi drops in the reservoir, the duration would have to increase to accomodate the volume requirement to achieve desired velocity. This is how electonic valves work. Mechanical valves don't have this luxury of modulating duration. Instead, duration is a function of stroke, so to get longer open time, we open further than the requirement for max flow. With the above example, you can see how very minor deviations can affect the valve of a PCP. Minor manufacturing variations are what give our rifles the since of individuality.

Seems like you got one that is closer to the magic combination of inlet, throat and TP than the rest of us. Don't strip out the choke screw again. It seems by your discription and the problem you are having with the stipping of these adjustments that you utilize the screw in a big way to get the velocity you want. I would suggest that you try and hit the start and end velocity that you want with the port wide open, perhaps just slightly high, using only spring tension and stroke. once you achieve the desired output, then use the TP choke to flatten the curve and very fine tune the velocity. I think that you'll find a bit better efficiency this way. You are correct in the assumption that the psi drop accross the valve it altered with the change in TP choke, thus altering volumetric flow. The spring tension and stroke are much more fine than the choke screw. This is because we are altering both energy to open the valve and the momentum that dictates the duration of the valve which are the root of the volumetric flow. The choke is simply a throttle. If your cars fuel pump did'nt put forth enough fuel to move it at the speed you wanted, it wouldn't matter that you had the peddle on the floor. On the other hand, if you had no throttle and you car ran on what ever the pump put out, you'd be going 170mph every where you went and the idle would be annoying to say the least. The idea is to have a enough fuel and a pump to deliver it at a rate that is very close to the throttle position that you want to operate. tuning a PCP is like walking a razor blade.
It is a dangerous proposition to believe that you can reason with evil.
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Re: Higher Shot Count with Higher Velocity?

Postby louigi600 » Sun Sep 23, 2012 12:52 pm

If you calculate that pellet has to accelerate down the barrel and will not instantly start moving at 900fps it's more likely it will stay in the barrel 4ms. To estimate this I assumed that the force on the pellet would be constant and all the way down the barrel but what's happening in the barrel is much more complicated then that and there are a lot of things that affect the pressure that will actually push the pellet down the barrel during it's travel down the barrel itself.

Bill G: how can you measure within 1/1000" how much your valve opens ? Is it calculated based on all forces that act on the valve ?
Regards
David

Benjamin Marauder .177 cal.
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Re: Higher Shot Count with Higher Velocity?

Postby Bill G » Tue Sep 25, 2012 1:52 pm

Yes it is calculated but backed up by quantitative tests.
It is a dangerous proposition to believe that you can reason with evil.
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Re: Higher Shot Count with Higher Velocity?

Postby Bill G » Tue Sep 25, 2012 1:52 pm

Yes it is calculated but backed up by quantitative tests.
It is a dangerous proposition to believe that you can reason with evil.
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Re: Higher Shot Count with Higher Velocity?

Postby Bullfrog » Tue Sep 25, 2012 6:59 pm

I appreciate the info Bill. You are correct, I relied heavily on my velocity port screw in my previous tunes. Not so much for velocity as to save air. However, on my current valve I don't seem to save air no matter where I have the VP screw set so there's no reason for me to adjust it any tighter than wide open.

I regulary adjust my hammer but I always leave my spring on full pre-tension. Or so I thought. Today I tried to lower my pre-tension to save some air without losing velocity and it didn't work. Even less than half a turn knocked my velocity down 10fps. However, I actually didn't have the spring on full pre-tension to begin with. I had it on half-a turn from full. Reason for this is that when I am turning the spring clockwise I turn it until it clicks and then go back half a turn to make sure its threaded. Today I just turned until it clicked and left it. When I did this, I gained an additional 10fps, pushing my max power with the gun to 880fps with 31 grain 'cudas. I also upped the hammer stroke up a slight bit. At 3200psi I got a shot at 880 and they dropped into the 870s after that. On the bottom end of the curve. Filled to 3300psi and got this:

31 grain 'cudas
3300psi
872.4
877.8
878.3
882.8
877.3
877.4
870.8
867.4
2800psi
15.31 ES
4.89 SD

That's a solid medium-game hunting tune for 1 clip. I feel like I'm at the best my gun can do. Which I thought that before and I keep getting better results the more I try different tunes. Of course I've also lost great tunes because of curiosity.
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Re: Higher Shot Count with Higher Velocity?

Postby Bill G » Wed Sep 26, 2012 8:34 am

That is a great tune with the Barracuda's. That would be nearing 950 with the JSB's, I'd bet. 935 is the best string I've ever gotten using JSB's and that only got me 10 shots. With good shot placement out to 30yrds, that's yotes and smaller pigs.
It is a dangerous proposition to believe that you can reason with evil.
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Re: Higher Shot Count with Higher Velocity?

Postby PakProtector » Thu Oct 04, 2012 4:03 pm

hey-Hey!!!,
Now with one of your spare valve bodies, take out all the bits and drill the vertical transfer port out to say .148, and do the transfer sleeve too. With the drill as a guide, put a good velocity screw back in 'til it hits the drill shank and you will know then that it is all the way clear. If you'd be willing to part with one of your valve bodies with the wrecked screw let me know I am looking to try some modifications on both the body and the screw-in cover. This will be using a .25's valve in a .22 ov course.
cheers,
Douglas
1990 D250, NV5600, Luk 05-101, 16cm housing, TST Kit KDP tab, 366 spring, Powr-Lok'd 3.07, HX35...IC soon
She wasn't built to travel at the speed a rumour flies
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