Gun-specific accuracy tuning process (FX Impact or Impact X).

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Chuck26287

Chuck26287

Member
Jul 29, 2018
256
19
Indiana, United States
Continuing a discussion on the valve stem adjuster (VSA) from another thread that I was inadvertently taking off-topic (https://www.airgunnation.com/topic/regulator-break-in/), I wanted to expand it into a guideline for true gun-specific custom tuning for accuracy. This is in reference to an FX Impact (X) using Regulator Pressure (RP), Hammer Spring Tension (HST), and the Valve Stem Adjuster (VSA) settings.

I'm new to airguns, so feel free to point out where I am off-base in my understanding of things... that's the purpose of this open discussion. I've got lots to learn.

Basically, I want to tune for long-distance accuracy. My goal is not a specific FPS velocity until I determine what FPS velocity yields the best accuracy in the specific gun being tuned. Based on this, I understand the process I need to follow to be the following:

1) Start with a full 250 bar tank fill onboard the gun.

2) Completely remove the VSA knob assembly.

3) Ensure HST dial is set to MAX, and the slider has no play back and forth.

4) Set the RP to the low-end of your desired testing range. Follow all degassing procedures required to lower RP to prevent regulator damage. I'll probably start around 110 bar on my tests just to see the data, even though I'm pretty sure that's way too low for a .30 cal.

5) Setup a target for shooting groups on at a specific distance. I'm thinking 40-50 yards. This needs to be close enough to not be overly influenced by wind (try for the calmest day possible), yet far enough to allow the groups to show themselves opening and closing as RP is changed.

6) Select a pellet to tune. If you are inclined to wash, lube and weigh them, do so for the tuning so it is representative of your shooting. I will sort to 0.1 gr, and shoot all the same weight for a given RP group series.

7) Shoot and chronograph a group at your starting RP. I like 10-shot groups, but 5-shot groups may be sufficient.

8) Increase RP then shoot and chronograph another group. I will probably use a 5 bar increase increment.

9) Continue incrementing RP and shooting and chronographing groups until you have reached the max RP recommended (150 bar).

10) Evaluate groups for the smallest group size, identifying their velocity. This is where you also may want to shoot some sub-groups at intermediate RP settings based on what your groups look like at your particular RP test interval. The goal is just to identify the RP that produces the best group for that pellet, and the velocity produced for that group. This should be the aim velocity for producing your best accuracy.

11) Set the RP to a little above the pressure which produced the best group (I'll try 3-4 bar above). Follow any degassing procedures again if decreasing RP. This step will produce a slightly higher velocity than your aim velocity, but it will be tuned back to aim velocity in the next steps.

12) Adjust the HST Dial to the lowest setting that still produces a velocity slightly above your aim velocity. May only be one click, if even that.

13) Screw the VSA back on and use it adjust the velocity back down the last little bit to re-achieve the aim velocity that produced your best group for that pellet. If velocity was adjusted close with the HST Dial, you should achieve your aim velocity very shortly after detecting velocity drop with the VSA.

This is my game plan for my first attempt at accuracy tuning the FX Impact X. Any input and corrections to my understanding of the process would be very much welcome!
 
That procedure is close to what I did, save for adjusting the hammer spring preload screw so that the slider has no slop when the wheel is on MAX.

Due to conversations in the referred to conversation, I am rethinking my proceedure.

The high-low vs low-high pressure operation tuning is really intriguing.

I was/am under the (mis)apprehension that one would have better accuracy & consistancy with the lowest reg pressure possible for the fps you wanted to attain.

I'm thinking now that I need to revisit that. The numbers (spread, ES) I'm getting are not what I want.

Im also starting to suspect that the cam on the back of the power wheel has some mysteries. I'm not entirely sure that the HS reacts the same way when the wheel is set to high vs tighter preload screw & wheel on 3. It absolutely should, but something is bothering me. Hopefully the rain will let off some today so I can test it while this thread is current.

So, low reg preasure & high hammer preload or high reg preasure & low hammer preload.

How to proceed.






 
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Is the valve adjuster essentially a bumper? I would open it fully not to use it at all...what if it falls apart when hammered again and again.

Are you saying the major tuning guide videos is still not good enough?

both the Low reg pressure and high hst and high reg pressure and low hst are the avoid scenarios. I would rather start in the middle.


 
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I was/am under the (mis)apprehension that one would have better accuracy & consistancy with the lowest reg pressure possible for the fps you wanted to attain.
Click to expand...

Well I suppose I'm responsible for causing you to doubt your understanding so please allow me to elaborate. Your initial understanding is generally true. Compare it to the extreme scenario where you have a high regulator setpoint such that you need a very short lift and dwell just to keep the pellet from going too fast. That's not good for consistency, it's essentially operating in a state of partial valve lock. An example you may be able to relate to...if you overfill an unregulated rifle past its sweet spot--not so much that you reach full valve lock but just enough that the velocities are low--and then you run it over the chronograph, you'll see the velocities jumping wildly up and down. That's because the valve is barely opening (very short lift and dwell) and that causes slight variations in hammer friction, poppet stiction, etc. to be amplified. So it is indeed advantageous to keep the regulator setpoint low enough that this doesn't happen.

However if you have a transfer port metering screw (or install a smaller transfer port), you can keep the high regulator setpoint, dial up the hammer to give the valve a good whack, and just restrict the airflow to bring the velocity down to a reasonable level. Based on my understanding of the FX's valve adjuster, it would have a similar effect as a transfer port metering screw. That's what I had in mind when I replied in the previous thread with a few virtues of using a high pressure/low volume approach.

So, low reg pressure & high hammer preload or high reg pressure & low hammer preload.
Click to expand...

Sorry to nitpick but this association isn't quite right. All other things being equal, low reg pressure needs less hammer spring preload to knock the valve open. High reg pressure needs more hammer strike. 
 
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Is the valve adjuster essentially a bumper? I would open it fully not to use it at all...what if it falls apart when hammered again and again.

Are you saying the major tuning guide videos is still not good enough?

both the Low reg pressure and high hst and high reg pressure and low hst are the avoid scenarios. I would rather start in the middle.

Click to expand...

Yes, there is a rubber ball in the VSA knob that acts as the bumper.

Yes, once it is opened or backed off enough that the valve rod is not going to travel far enough to hit it, it is out of play entirely. Same as completely removing the knob.

The rubber ball can deteriorate. I've seen people say it can get a bit of a flat spot on it after so much use. The ball is easily replaced, or it can even be simply rotated to move the flat spot to the back, exposing new round rubber ball surface to extend the use of the rubber ball.

Not saying any video is not good enough. I've watched many different videos to come to my understanding that I've outlined. Sometimes I just like a procedure reduced to a checklist format vs a video.

Why do you say "low reg pressure and high hst and high reg pressure and low hst are the avoid scenarios"? Not disputing it, just wondering what your actual rationale is.
 
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i wish i had an impact to experiment with 😁

it is generic pcp tuning rule still applies though.

with low reg pressure high hammer will consume far too much air to make it to the desired fps if you make it at all. plus it will make the gun loud





the high reg pressure low hammer on the other hand contrary what has been written previously by someone else up here -not necessary will tesult in wild fspeedvariation. not at all. just low fps. but you will see a new phenomenon along with it if it is far too unbalanced, output will trend so gradually drifting up fps. 

if you have to use the valve adjuster you are out of optimum. avoid using it if youcan

surely the bare existence of an end stop a bumper is against the what the designer has reportedly said cited elsewhere that there is no fight going on in this pcp.



what about the nonsense also people keep posting that 10fps up adjustment completely stops flyiers or 10fps down? 

i also seriously doubt that this is true. if it is it is a big handicap not a tuning advantage if fx barrels need a very specific speed ONLY with certain pellet to perform. slightly off and the group falls apart, cant be true! come on.



i am not here for popularity either👍just need to see clearly. with all this said nothing can replace real exprimentation so i would spend time tuning, if you think there is one knob way too many, dont use it. easy
 
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Continuing a discussion on the valve stem adjuster (VSA) from another thread that I was inadvertently taking off-topic (https://www.airgunnation.com/topic/regulator-break-in/), I wanted to expand it into a guideline for true gun-specific custom tuning for accuracy. This is in reference to an FX Impact (X) using Regulator Pressure (RP), Hammer Spring Tension (HST), and the Valve Stem Adjuster (VSA) settings.

I'm new to airguns, so feel free to point out where I am off-base in my understanding of things... that's the purpose of this open discussion. I've got lots to learn.

Basically, I want to tune for long-distance accuracy. My goal is not a specific FPS velocity until I determine what FPS velocity yields the best accuracy in the specific gun being tuned. Based on this, I understand the process I need to follow to be the following:

1) Start with a full 250 bar tank fill onboard the gun.

2) Completely remove the VSA knob assembly.

3) Ensure HST dial is set to MAX, and the slider has no play back and forth.

4) Set the RP to the low-end of your desired testing range. Follow all degassing procedures required to lower RP to prevent regulator damage. I'll probably start around 110 bar on my tests just to see the data, even though I'm pretty sure that's way too low for a .30 cal.

5) Setup a target for shooting groups on at a specific distance. I'm thinking 40-50 yards. This needs to be close enough to not be overly influenced by wind (try for the calmest day possible), yet far enough to allow the groups to show themselves opening and closing as RP is changed.

6) Select a pellet to tune. If you are inclined to wash, lube and weigh them, do so for the tuning so it is representative of your shooting. I will sort to 0.1 gr, and shoot all the same weight for a given RP group series.

7) Shoot and chronograph a group at your starting RP. I like 10-shot groups, but 5-shot groups may be sufficient.

8) Increase RP then shoot and chronograph another group. I will probably use a 5 bar increase increment.

9) Continue incrementing RP and shooting and chronographing groups until you have reached the max RP recommended (150 bar).

10) Evaluate groups for the smallest group size, identifying their velocity. This is where you also may want to shoot some sub-groups at intermediate RP settings based on what your groups look like at your particular RP test interval. The goal is just to identify the RP that produces the best group for that pellet, and the velocity produced for that group. This should be the aim velocity for producing your best accuracy.

11) Set the RP to a little above the pressure which produced the best group (I'll try 3-4 bar above). Follow any degassing procedures again if decreasing RP. This step will produce a slightly higher velocity than your aim velocity, but it will be tuned back to aim velocity in the next steps.

12) Adjust the HST Dial to the lowest setting that still produces a velocity slightly above your aim velocity. May only be one click, if even that.

13) Screw the VSA back on and use it adjust the velocity back down the last little bit to re-achieve the aim velocity that produced your best group for that pellet. If velocity was adjusted close with the HST Dial, you should achieve your aim velocity very shortly after detecting velocity drop with the VSA.

This is my game plan for my first attempt at accuracy tuning the FX Impact X. Any input and corrections to my understanding of the process would be very much welcome!
Click to expand...

Chuck:

I am curious to see if your set of guidelines produced your expects results regarding accuracy. Could you please update us? Thanks.
 
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I'll be glad to update this with my results, it's just going to be a while. I have one Impact X in for repairs and upgrade from the Gen2 reg to the Gen3 reg (requires trigger block upgrade for the larger diameter reg housing, and has taken some time to get the newer parts in from Sweden), and while the second Impact X is ready for testing, our weather has been either snowing/raining/freezing raining, or windy and single-digit or even sub-zero temps. Just not conducive to productive testing at all. For the time being, I'm concentrating on building a functional custom shooting/test bench to use for this testing. (Can be seen at https://www.airgunnation.com/topic/custom-shooting-test-bench/.)

Hopefully soon, but probably not until I have the first gun back and both guns ready to test. Believe me... I'm chomping at the bit.
 
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Listening to someone who knows more than me -

The VSA performs the same function as an adjustable transfer port or in powder burning firearms powder ignition rates.

A smaller port opening slows expansion of compressed air released by hammer blow on the valve (ignition).

Thus a smaller port opening (VSA screwed in) releases the same volume of air at a slower rate. So in terms of shot count, on the surface of it, it matters little. 

I did some test today before thing became too hot. Over a 5 shot group in .25, 115 bar, 720 Av fps extreme spread 24fps with preload on min. With preload at Max 784 Av fps extreme spread 25fps. Thus preload min to max fps is 64, spread is my main concern. But at 20m the FX Impact produced a 20mm ragged hole in windy conditions.

As a side issue I'm now receiving a puff of air on my cheek, this could indicate the need to replace the O rings on the chamber end of the barrel. Or the need to open the VSA to let the air escape more quickly..




 
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