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Thread: Airflow: Is more ever too much?

  1. #21
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    Maybe OT, but I did some messing around with my cocker today....ID V2 valve (11/16") and valve spring, ID hammer and hammer spring in my Resurrection FLE. I gauged it and it was sitting at about 160 psi, which was shooting about 280 fps with the IVG flush with the back of the body for me last weekend. Efficiency was not up to par.....I left the IVG alone and increased the HPR pressure. Max velocity was achieved around 190 psi (320-330 fps) and as I increased past that, it started to drop again. At about 220 psi, I was back in the 280 fps range. With the extra pressure closing the valve faster, I'm thinking efficiency should improve. This was done with a questionable gauge, so pressure error could be +/- 10 psi or so.
    OlllllllO

  2. #22
    I don't think that's off-topic in the slightest.

    Indeed, that puts a very interesting color on the matter. I'd be very interested to learn what you find in testing.

  3. #23
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    The 'bell curve' relationship wasn't surprising to me, but I've never seen it quite so defined as I did today. It should be quite efficient now, but I'll report back when I can get a good test in. That could be a while.
    OlllllllO

  4. #24
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    I'm not aware of explicit efficiency tests being done at different points on the curve. Would be interesting

  5. #25
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    Doable, but would take a fair bit of paint. If I'm able to get it done, I'll report.
    OlllllllO

  6. #26
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    No need really, if it's very efficient above the sweet spot there's your answer

  7. #27
    I too will mess about with my Ressie, as it is nowhere near as efficient. Maybe 1500 on the best of days, and I'm not even sure about that.

    I'll express tentative early skepticism, as it seems that giving up 40-50 fps velocity available at the sweet spot (which could be brought into line with different springing) is still potentially giving up efficiency, since the system is now expending its own energy to limit velocity, and (to me) is now working against itself.

    That said, few things in life are more fun than being wrong, so here's to great fun, and much learning! :-)

    Ryan,

    Don't know if you caught my question about your note on throttling. Can you clarify?

    Thanks very kindly again, both of you.

  8. #28
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    Oh, right.

    2 holes will just ave more losses even at the same sectional area because reasons, where reasons includes entrance and frictional effects (more perimeter per area). So, it can be useful to use multiple holes where you're after a more controlled release of air ("softer shot") but it IS lossier compared to a single hole.

    I don't think it's conceptually correct to think of being above the sweet spot as "expending it's own energy to limit velocity". I mean, yes, there is a piston work associated with the force x valve travel, but it's miniscule - I calculate at 200 psi on the order of 0.16 Joule - A paintball takes about 12 Joule to be accelerated to field velocity. So, you're only at 1% loss to crack the valve open.

    More likely in my mind is that the system is dwell limited, such that you're just operating on the overall valve energy function:

    Energy to ball = Flow Path efficiency * Valve flow constant* valve time open * pressure drop across valve

    (or something like that)

    and within reason we think that a high valve flow and a low dwell time, and a high dP across a valve seems to be best for efficiency.
    "So you've done this before?"
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  9. #29
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    Hammer spring is matched to the valve spring. So both are super light.
    Let me guess - you have to cock the gun before you air it up or else it leaks?

    Matched sets normally are not very good - no matter what the seller is stating. The light valve spring is a problem. A significant one actually. So is the light hammer spring. Being the US Tech behind Racegun for a while I had access and tuned a lot of guns, right down the to millisecond. There were a lot of things that I learned doing that.

    Many of the early shops in an attempt to catch AKA's low pressure used a light valve spring and made the system run as a dump chamber. The hammer would hit the valve and it would stay open. This resulted in 'Bolt Stick' from them dumping the LPR also if not timed right (in an interesting side bit, this can also result in some guns cycling once, then not cycling, then cycling on the next shot.) But hey - low pressure!

    To fix this the maker would set the timing really tight, so the 3-way would be engaged before the hammer would hit the valve - often timed to lift the hammer off of the valve. Shocktech, Belsales, almost every one did this. In the Racegun timing with too light of a valve spring you needed to run it at a 5ms operation instead of 7-8ms (7-8ms would create suction on a well sprung setup.) Efficiency would change dramatically between this 2-3ms window, from 800 to 1200 shots on a 68/3000 fill. (this was a bit ago - ha!)

    A heavier valve spring will result in a faster closing valve and more efficiency, but also a lower sweet spot pressure point on the Bell Curve. That was part of the reason AKA could get low - the bell curve is based partially on back pressure on the valve, the spring tension, and flow. AKA had a heavy valve spring. Put in a heavier valve spring and your sweet spot will drop. More on that in a bit.

    Also look at hammer spring length. That can dramatically affect efficiency. If you drop the hammer and it holds the valve open (say on airing it up, or just by feel when you have it unaired up) then it naturally wants to hold the valve open. You don't want that of course. A problem in the hammer spring that is light is that to make it heavy enough to break the back pressure of the valve spring, you needed to compress it more - resulting in a spring that would still be adding pressure to the valve when it was forward.

    A medium weight spring that was backed out would result in a spring that wasn't under tension when the hammer hits the valve. And it would close faster, significantly so. Making your marker more efficient. My tuning setup was to take stock cocker springs, stretch the valve spring out just a bit, maybe add 1/4" to length. Turn the IVG flush, and tune in the sweetspot. Then add IVG to increase velocity. For a while my write up on how to sweetspot your gun was the go to copy and paste. I would consistently get a 1800 to a case on that setup, all stock components. I didn't tell but a couple people that if adjusted the reg to a point just over the sweetspot pressure when you are cycling at high speed, because the reg couldn't keep up, the velocity will spike. Simple cheat that nobody seemed to have caught, which I am glad of.

    So, those basics aside, the 'How I See It' follows Ryan's calculation, but goes a bit like this (and a lot of this is in my head, but I may not get it out fully, so feel free to ask about the SWAG this is):

    The valve, all the points after the cupseal really, have a flow co-efficient. More total flow and volume generally meant the more air that could be transferred to the ball for the same input and pressure (with an exception below). It also means that at a lower pressure a system with a bit more volume or a better coefficient could transfer more energy before a valve closed. So, a better flowing setup, for the most part equals a higher velocity shot for the same pressure. With lower pressures (150psi to 300 or so) the flow of the valve doesn't impart too much of a restriction. Remember the cocker body is only storing so much potential energy. The part fighting that would be the back pressure on the cupseal, in a perfectly sprung setup. At 400 to 600 psi this tends to lower velocity because the back pressure matched with the valve flow limits the amount of energy that can transfer, but at you get to 800psi and up the energy transfer seems to exceed the restriction. It is a bit above my education as the why that happens. Ryan/Steve?

    The cupseal nominally (I am just going to throw this number out there from dusty memory- I don't want to take about one of mine apart to check) has about a .35" sized hole. That gives you a .096" area, or .1" to round it a bit - and to make the math easy.

    At 200psi, you get 20lbs of pressure on the valve seat. 300psi lands you 30lbs. That is what you are fighting against at the moment of impact from your hammer. Now, the moment you do crack it open, and the valve starts dumping, you get some backpressure on the cup seal, dependent on the amount of restriction in your system, and back pressure from the ball sealing. This is all, well, there are a lot of variables. Let's just say here we enter guess work, to some extent. So a spring that has 1oz of break away, vs one that has a couple of pounds doesn't do too much right at this point. So light valve springs are just a small factor right at this point.

    It is the closing that matters. And that is where we get some more interesting dynamics with the light valve spring. It doesn't have the energy to push the hammer back in most cases. So the valve stays open longer than it needs to.

    But the heavier spring also changes the sweet spot, and lowers it. Why? Same reason higher pressure does - it acts as if you are running a higher pressure, and hence the sweet spot acts as if you bumped up the pressure 10lbs for every pound of spring tension. AKA ran rather heavy valve springs. Yet they were able to run down to 150-200psi because they sweet spotted the system. Generally the older cockers didn't have the available volume and energy potential at that pressure.

    The sweetspot is a balance between flow of the valve system and back pressure of the valve - if you hit that you require the lightest hammer tension, and hence, your hammer spring is lighter, and the valve can close faster, making it more efficient, while still allowing enough energy to escape to fire a ball at decent velocity.

    In addition, the AKA (and Inception actually) valve help close the valve by putting 'fins' in the system to help the valve close against the hammer - the movement of air helps close the valve.

    Now, the exception is a weird one. I remember some saying the FreeFlow bolt added the most velocity. This is the older 6 or 8 venturie bolt, with the big lower port. Its flow was sad though (especially when using the name Freeflow). I thought about that and realized the FF bolt would cause higher back pressure - resulting in the cupseal having more balancing pressure on that setup. The reason for the large cut working in their bolt is it helped retain more air between the cupseal point and the bolt, and you only had pressure from the spring till you hit a certain point. If you look at the LV1, and Ryan/Jack could speak at length about that, they had a larger balancing side on the valve stem to counter that.

    So there are a lot of little, not quite able to figure variables in there - and most of that is from my experience tuning cockers in the early 2000's - 15 years ago. But the basics for how to setup springs still hold. Heavier valve spring, medium to light hammer spring that isn't tight when your hammer is down, and a sweet spot. You should be good from there.
    Last edited by pbjosh; 11-19-2018 at 02:01 PM.
    Josh Coray
    J4 Paintball
    Lead Design
    www.j4paintball.com

  10. #30
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    One point on the above - the force to return the assembly decays as a function of the valve chamber size.

    So, the more volume you have, the higher return forces you have to snap the valve shut, and you avoid "long tails".

    I know John Chambers has given this a lot of thought in terms of tuning the valve return profile (and shoot, Colin from DW for that matter). There's a lot of ways to skin this particular valve closing cat, but running above the sweetspot will definitely sharpen the closing forces.

    That Racegun timing point is insane - that can't have been terribly consistent, to have the backblock pluck the hammer off the active valve. Cockers are such a glorious mess.
    "So you've done this before?"
    "Oh, hell no. But I think it's gonna work."

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