How do you move the knee up or down the psi?

[Blutroop]

I got my disco/prod .172 shooting. I opened up a valve and got 740 fps with a 26 gr slug starting at 3000 and dropping. Then I opened up the ports to bore size and got 755 at 2600. Fps was still going up as psi dropped but then it burped and let out a lot of air. So I took the hint and made a ssg for it. With the sun so low throughout the day and below freezing weather I have a hard time getting numbers. As I’m learning more about tuning it got me to thinking.. is it possible to move the knee up or down on psi or do you just have to take what you get? I still don’t understand why the fps can be raising while pressure is dropping. Unless maybe to much flow?

[SwampHunter]

I will try to answer until someone more qualified comes along lol. A stronger hammer spring will open the Valve further against a set pressure, the resistance of the 3k in your reservior. As the pressure falls, it has the same effect because of less pressure pushing against the valve, so it opens further, letting more air pass by before closing.

[Taso1000]

Jeremy,

If I understand you correctly, you're getting 740 fps at 3000 psi and pellet velocities decrease with every shot, you have too much hammer spring. 

Decreasing your hammer spring tension will eventually get you a velocity curve that starts lower, peaks and then falls again.

If you want more velocity you're going to need a longer barrel and/or higher pressure.  I don't know your build details but you may already be over the guns safe working pressure using 3000 psi.

What is your barrel length?

Taso

[rsterne]

If I understood you correctly, after you opened up the ports, your velocity from a 2600 psi fill was increasing as the pressure dropped.... If you want maximum power starting at 3000 psi, you are going to need a significant increase in hammer strike.... Try to get your velocity starting at 3000 psi about 4% below the peak, which will probably be around 2600 psi.... For an unregulated PCP, you WANT a bell-curve, with the velocity first increasing and then decreasing as the pressure falls further....

Higher velocity will require larger ports, a longer barrel, or more pressure.... I crank in hammer strike until I find the plateau at the fill pressure (more hammer strike no longer increases velocity)…. and then back off about 10% to get a bell-curve....

Bob

[Blutroop]

Thanks for the responses everyone. I should clarify that I initially had a shot string decreasing from 3000 with a high of 740 Then I opened up my valve and tp to bore size. Then I shot without topping off and got a string that was increasing as pressure dropped. At 2600 I hit 755 then the next shot it started burping and sounding like full auto. I’ve since made a ssg and cut my spring so I can put more preload before coil bind. I also opened a prod valve as much as I believe possible (I used a 3/8 end mill to oval it between the screws and used a lathe to open it up behind the screws) and haven’t been able to shoot over the chrono since due to real limited sunlight always low in the sky. At 35 fpe I am a bit too powerful to shoot indoors.
Does anyone have a good source for getting some heavier hammer springs?
Taso, my barrel is 26” and tube is cothran with properly screwed prod valve so I’m confident I am within safety parameters.

[Nvreloader]

Jeremy

If you give me the following spec's for you hammer spring, I might have a possible solution for you,

Your springs spec's,
Outside diameter =
Inside diameter =
Total overall length =
Pound rating =

Tia,
Don

[Blutroop]

Hey Don, I don’t know how to give you a pound rating. I can get you all the measurements tomorrow eve though. I’m gonna try and shoot over a chrono on my lunch break. I just got done pumping it by hand.. did I mention it’s a disco double prod? I’m beginning to wonder if I’m the only one foolish or cheap/broke enough to still be pumping a big gun like this by hand?

[cobalt327]

If you can accurately measure the spring wire diameter and count the total coils and note whether the spring has closed or open ends (will be closed most likely), along with the other specs asked for above the spring rate can be determined using a calculator.

[Blutroop]

So my rifle leaked out overnight. I haven’t been able to shoot it in a while. My spring measured .3 od, 1.75 oal, .40dia wire, 20 coils and closed ground on both sides.

[Hobbyman2007]

Leaky guns are a real pain . Especially whenever hand pumping . I would think at 3000 psi , you should be making a lot more power that what you’re getting right now . Since you’re going to be going into the gun already , take as much material as possible from the poppet/ valve OD ( plastic seat ) and have enough material just to seal the valve up . That will make it easier to open . Next is the stroke , are you absolutely certain that the hammer pin is not stoping the hammer from full travel ? I’ve come across this before . Easy to check , with the valve in the tube , put the hammer against the stem . There should be at least 1/4” of slot . Next is the valve stem,. I shorten the portion sticking out of the valve to 3/16” , anything more than that is a waste. You want the most hammer stroke available with the Crosman guns . Last but not least is the pellet/ bullet probe , make sure that when a bullet is chambered that it is well clear of the transfer port . I go about 1mm past .

Hope this helps, now I’ve got to attend to a leaky disco .257

[Blutroop]

Thanks Dennis, I checked hammer stroke this morning. Plenty. My poppet is what’s leaking. I think I’m gonna try lapping it in with some mothers aluminum polish. I shortened my bolt to just past the transfer port.
I had my slugs sized right to .172 as I ordered them before reading one of Bobs comments that slugs can be as much as .002 under and still shoot good with air. I’m sure this is costing me some fps but how much?
I went ahead and ordered a spring from fastenal That was as close to what I thought I wanted (.47dia 2oal .312od) but it is alarming how little choices there appears to be with springs. Is this the weak link to this platform? Limited spring space and selection? If so seems pretty easy to make a rear cap to accommodate longer springs.

[screwwork]

I get my Prod hammer springs from McMaster Carr
They have a great assortment to from light to heavy springs 0.3 OD