Tuning FX Impact Mk2/M3 - slower first shot - no reg creep - quasi-scientific analysis

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Usually u here about happing over night not in a hour or 2 I bird hunt and if I'm worried about the first shot I will dry fire it with bolt closed but I haven't really experienced it my regs are all in 125 to 115 range but if that doesn't work I know others have stated it can be tuned out of it u have a peek valve seat and is still happening I would do search and find what others suggested for tuning it out of the equation my bird hunting is usually pretty steady as far shooting goes but it depends on the day to
 
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I have a peek valve poppet and a very weak valve return spring. They did not help.

I am pretty sure the issue is with a PUR90 oring under pressure. It does not have a groove - it seals against a cone. This is why it bites the valve rod more and more the longer it stays untouched.

Do you think I can replace PUR90 with a pure PTFE oring? That should minimize the stiction.


 
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I would be tempted to try it but mentally prepared for it to fail. My understanding is that PTFE O-rings are only used for static seals. It does not conform to surfaces well enough to seal reliably in a dynamic application. Perhaps if the mating surfaces are impeccably uniform and smooth...like "I build my own telescope mirrors in my basement" good.

It is also notorious for cold flow (creep) so it might extrude itself out over time. There are filled PTFE compounds that are designed to combat it so that might be something to look into.
 
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When I got my mk2 impact it was tuned to 140 bar reg. 
I also noticed the first shot being off consistently.

I wrote it off as reg creep, and rebuilt my reg. 
however, later on I retuned the gun to shoot on the knee of the reg pressure. That appeared to help the most. 


mk2 impact valve is design to perform best, between 110-120 bar…

Click to expand...

Why is it that it's designed to perform that way?
 
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I'm not finding much on the coefficient of friction for perfluoroelastomers (FFKM) except that PTFE is used in its compounding so it inherits some of its qualities (lubricity, chemical resistance, temperature resistance) [1]. Intuitively, anything elastic enough to be useful as a dynamic seal will necessarily trade off some lubricity relative to a harder material like PTFE or UHMW. But all of engineering is a balance of compromises so it may be that this material is the best choice, and almost certainly better than either polyurethane or PTFE.

Starting at $30 per O-ring, they sure are proud of them: https://www.mcmaster.com/o-rings/material~kalrez-rubber/



[1] https://www.marcorubber.com/blog/3-benefits-of-perfluoroelastomer-ffkm-seals/
 
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When I got my mk2 impact it was tuned to 140 bar reg. 
I also noticed the first shot being off consistently.

I wrote it off as reg creep, and rebuilt my reg. 
however, later on I retuned the gun to shoot on the knee of the reg pressure. That appeared to help the most. 


mk2 impact valve is design to perform best, between 110-120 bar…

Click to expand...

Why is it that it's designed to perform that way?
Click to expand...

At the time, I spoke to FX regarding issues I was having with tuning the gun around 130-140bar.

this was the tech’s information that it had to do with the airflow and back pressure generated when the valve is hit. 
told me the best ES will be around 110-120 bar plenum. 
 
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It was easier to make an oring groove in gen1 because the plenum was symmetric there. Here the oring axis is off the plenum axis. It makes it more tricky to make such a groove. I guess that's why they decided to use the cone.

However, they should be aware of the consequences I guess.

Anyway, I will try PTFE oring to check if it solves the stiction problem and if it seals properly.

I am totally shocked that noone really noticed this issue with Impact in the past.
 
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I'm not finding much on the coefficient of friction for perfluoroelastomers (FFKM) except that PTFE is used in its compounding so it inherits some of its qualities (lubricity, chemical resistance, temperature resistance) [1]. Intuitively, anything elastic enough to be useful as a dynamic seal will necessarily trade off some lubricity relative to a harder material like PTFE or UHMW. But all of engineering is a balance of compromises so it may be that this material is the best choice, and almost certainly better than either polyurethane or PTFE.

Starting at $30 per O-ring, they sure are proud of them: https://www.mcmaster.com/o-rings/material~kalrez-rubber/



[1] https://www.marcorubber.com/blog/3-benefits-of-perfluoroelastomer-ffkm-seals/
Click to expand...

I was thinking about Kalrez 7075: https://www.allorings.com/kalrez-o-rings
 
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The 7075 looks like a good choice.

$50 for one O-ring to try it :) Looks like that's the minimum order from allorings or the price of a single at McMaster. 

BTW if you haven't seen it already, Parker's O-ring handbook has a section discussing friction, both break-out (stiction) and running friction, and techniques for minimizing it.
https://www.parker.com/Literature/O-Ring%20Division%20Literature/ORD%205700.pdf
see page 113
 
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They used PUR oring which is even worse than NBR when it comes to stiction. If I leave Impact for about a day then the coefficient of friction between the PUR oring and titanium rod will be about three times as high as after a shot or two. We need to note the oring is squished by the pressure in the plenum. It does not really seem to be properly thought through. I think the undesired effect of stiction can be reduced by the hammer spring adjustment but it's not what it's all about in a 2k$+ gun, right?


 
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When I got my mk2 impact it was tuned to 140 bar reg. 
I also noticed the first shot being off consistently.

I wrote it off as reg creep, and rebuilt my reg. 
however, later on I retuned the gun to shoot on the knee of the reg pressure. That appeared to help the most. 


mk2 impact valve is design to perform best, between 110-120 bar…

Click to expand...

Why is it that it's designed to perform that way?
Click to expand...

At the time, I spoke to FX regarding issues I was having with tuning the gun around 130-140bar.

this was the tech’s information that it had to do with the airflow and back pressure generated when the valve is hit. 
told me the best ES will be around 110-120 bar plenum.
Click to expand...

Thanks for the information. That's the upper limit of where I want to be with the reg/tune on my.25cal MKII.
 
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