FX PCP Airgun Tuning - Why would the FPS go down when increasing the Hammer Spring at a constant reg pressure.

[SOTL]

Hi All I have a very early FX Crown (in the first 500 made) It has the 8mm reg piston - I only mention that to date it.

Now I have been out of shooting it for a while and it used to be an good gun and Shot JSB King Heavy MKII’s best at about 800fps and 120bar reg, this was how it came from the factory with a 600mm barrel.

I wanted to experiment with slugs. I watched all the usual Youtube suspects re tuning and got to work practicing on pellets first. In the end I got to about 155 bar reg pressure and had effectively maxed out the HST, on the MAX setting. it was shooting the MKII’s at about 930 fps but it sounded terrible, it wasn’t consistent, or accurate with the extra distance I could now achieve and I realised somewhere down the road I’d turned into a blind alley and this was just not right…

So Ive left the HST on almost Max when it was on MAX on the dial. I then reduced the Reg pressure to 100 bar, to start again. I had a tin of the lighter JSB Kings and started using them to experiment with. With my reg at 100 bar Im getting about 910-920fps with the HST at the “MIN” setting on the wheel.

Now here is what I really cant get my head around, and is making me question what I think I know…

When I keep the reg the same and increase the HST the FPS falls, and it does so linearly in line with the increase in HST so at the “MAX” position on the wheel I get 720ish FPS. Also the sound of the gun changes and becomes quieter and more “dull” less of a pop or crack and more of a hollow “thud”. what is more this is very consistent, if I turn the HST back down I get 920, back up I get 720. its not just all over the place

I thought that HST would increase FPS if every thing remains the same, or at very least it would not increase FPS, but to significantly reduce the FPS??? this makes no sense and I cant fathom how the mechanics of this would play out. I get that increased reg Pressure provides more resistance to HST so you get a shorter open duration on the valve and results in a decrease in FPS, but Im not touching the reg…so what gives

Can someone help me understand why this is? I thought I understood this, well until today!

Regards
SOTL

 

[stovepipe]

[edit after some sleep] - All I can think of is that the hammer hit is so hard it hits the valve and the valve then travels inwards allows air out then bounces off the return spring so fast that the effective dwell (open time) is less with a smaller “puff” in total hence lower FPS or some issue with the gun ( which I doubt).

You answered your question right here. This is correct. Nothing is wrong with the gun.

stovepipe

 

[Caliber 22]

The answer to your question, my friend, is only one, namely: Excess air, which leads to the slowed down speed.

Everybody is missing the linear part.

 

[JungleShooter]

SOTL,

your post was intriguing, because the phenomenon was counterintuitive. Cool.

I like your desire to understand what's happening, not just to fix it and get on with shooting.

Then, like you said: Once you understand, you can tune any gun.



⭓ Unrelated question: Did you intend your screen name, SOTL, to stand for Scholarship of Teaching and Learning?

Matthias

 

[AlanMcD]

Everybody is missing the linear part.

No, we are not - when over driving the valve with the striker bottoming out on the valve body, as the hammer force goes up the hammer will rebound faster , closing the valve faster, and reducing the airflow thus slowing the projectile down. It is not at all surprising that this is happening.

I will say that there is a small coincidence going on here, in that the minimum hammer setting at the low regulator setting is either already at maximum speed, or is already overdriving the valve. We don't know what might happen if the hammer energy could be lowered further, as we really don't know what state things are in - it is possible (if not likely) that the pellet might speed up a bit and then slow down as the energy drops further.

I'm not about to get into the math on this - I've got better things to do - but we've explained what is happening here and why it is occurring . . .

 

[SOTL]

SOTL,

your post was intriguing, because the phenomenon was counterintuitive. Cool.

I like your desire to understand what's happening, not just to fix it and get on with shooting.

Then, like you said: Once you understand, you can tune any gun.



⭓ Unrelated question: Did you intend your screen name, SOTL, to stand for Scholarship of Teaching and Learning?

Matthias

Everybody is missing the linear part.

Yes I don't understand that 100% either. However I have watched some disassembly Videos and seen some diagrams and I hadn’t realised that the crown’s valve is a fully captive shuttle, with a return spring. I think that as others pointed out that the 100bar was just too easy for the higher HST to overcome and when it was hit with an excess of force that was then stored in the return spring and the valve was snapped backwards to shut again, the harder it was hit the faster it snapped back, like the ball analogy.

 

[SOTL]

No, we are not - when over driving the valve with the striker bottoming out on the valve body, as the hammer force goes up the hammer will rebound faster , closing the valve faster, and reducing the airflow thus slowing the projectile down. It is not at all surprising that this is happening.

I will say that there is a small coincidence going on here, in that the minimum hammer setting at the low regulator setting is either already at maximum speed, or is already overdriving the valve. We don't know what might happen if the hammer energy could be lowered further, as we really don't know what state things are in - it is possible (if not likely) that the pellet might speed up a bit and then slow down as the energy drops further.

I'm not about to get into the math on this - I've got better things to do - but we've explained what is happening here and why it is occurring . . .

As this is .25, Higher power (FAC) and Im looking to get heavies and slugs as fast as I can manage with the stock system, I set the HST Maximum to be as high as it would go at the “MAX” on the Dial.

As an aside - I do see that the back end of the shuttle is accessible from outwith the gun and it would be possible to measure the force that the hammer hits with which may be interesting to note.

Thanks for your help.