Tuning Should FX recommand not to install the Slug Power Kit on the Impact M3?

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MajorSylvester

MajorSylvester

Member
Jun 2, 2021
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France, France
Hello,

Maybe I'm wrong but I think the slug power kit just do not work on the M3, except maybe the pin probe. Here is my logic.

1- The rubber ball is reduntant with the already existing 2 captives rubber balls in the M3, adding a third will reduce power, not increase

2- The valve spring is not designed for the M3, it is better to keep the original M3 spring

3- I do not understand the need for the 2 spacers. With the M3 you can already get the maximum spring compression (ie the spires are so compressed that they are in contact and cannot store any more potential energy). This is usually achieved one or 2 macro whel clicks after the 5th graduation on the ladder. How do you know you have achieved maximum compression? Because you can't cock the lever anymore. Adding the spacers just enable to get the maximum spring compression for a lower value of the macro weel but it will not add any more potential energy stored in the spring. No added potential energy in the spring, means no added kinetic energy in the hammer therefore no added maximum velocity for your slug.

4- Installing the kit needs messing up with the internals of this great rifle. I did it. Poorly. I messed it up and it was a pain in the neck to return to the original rifle performance. I invented a lot of creative and very rude swears in the process.

Yes the pin probe is probably a nice to have feature but is that worth the purchase of the power slug kit for the M3? Anyone with a different experience or opinion?
 
I got like 10 fps with hammer weights.
Click to expand...

Yes it will increase your speed for a given wheels setting. I think you should get approximately the same speed by adding one macro wheel click or 8 micro wheel clicks without the spacers. But in any case, I think the maximum speed at the maximum wheels settings you can get for a given pellet or slug should be the same with or without spacers. Was the 10 fps increase for your maximum speed setting?
 
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It's because of two things, on some there is a lighter valve spring on the m3 and all have the larger plenum. As the plenum is increased in size, the pressure needed to get the desired speed lowers. Thus, the square inch of pressure on the back side of the valve also lowers. Hence, it takes less hammer weight to open the valve. Resulting in no need for added weight. Adding weight, would cause valve lock and bounce, causing unpredictable bounce back. Making your standard deviation get crazy. It would require less spring pressure to bring the standard deviation back down. but then your dwell would be off the chart's causing your gun to waste air and get loud and puffy sounding. In note, adding weight would be pointless to add weight in the first place, for no final gain and all loss.

Other then, when...

If your going to push a large slug with a .30-35 at 1000fps. I would guess your going to be running your regulator very high and you may need to add weight to your hammer. 

or if your gun has the stiffer valve spring that hasn't been cut
 
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The two spacers seem to have a significant inertia effect in addition to preloading the spring. Consider the following .30 700mm M3 tunes. Right when the M3 came out, 68 whiskey posted a video with their two .30 M3s. The guns were identical, with the exception that Keith had installed the two spring spacers on his. Both were shooting FX 44.5gr Hybrids



Keith's gun with two spacers installed:

Reg 125b, speed ~970fps



Norm's gun bone stock:

Reg 155b, speed ~970fps



My M3, stock like Norm's:

Reg 150b, speed ~960fps



I can confirm at least on mine, that the tune is ~96% of max at that reg pressure. I think that it is safe to assume that Keith and Norm are very likely tuned similarly and that in three examples, the valve is closing just before the slug exits the barrel. I am having trouble wrapping my head around what is going on such that the 125b tune can push slugs at the same speed as near maxed out 150b tune. If the additional weight of the two spacers is keeping the valve open longer, then I would expect it would enter 'unusable/wasteful' territory and the valve would be open beyond point of slug exit. Is the 125b tune acceleration profile that much slower at the start, but then speeds up drastically at the end of the slug travel??



Another related data point for Keith's 125b tune with spacers, is the following:

When he switched from slugs at ~970fps to pellets at ~890fps, he not only had to drop his power wheel from 16 down to 1, he also had to turn the valve adjuster WAY in. On my gun I can do a similar slug to pellet switch, but I only need to drop the power wheel from 16 to 13. No valve adjustments needed. 

What exactly is going on here?
 
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I Just watched the Whiskey 68 video. It’s very interesting. The spacers seems to have some effect on pressure optimization. Yes the spacers do add mass to the hammer BUT since the maximum energy released by the hammer spring is still the same than the energy released without spacers, it means the hammer will hit the valve with a lower speed. Applying the kinetic energy formula, the hammer speed will decrease in proportion to the square root of the hammer mass increase. All things being equal I still do not understand why it would have an influence on the amount of air released at a given reg pressure.

For me 2 questions remain to be answered:

  1. Can the addition of spacers improve the maximum speed we can get from the rifle and a given slug?
  2. Why do the addition of spacers helps decreasing the reg pressure for the same slug velocity?
    [/LIST=1]

    Any air gun expert offering a detailed technical answer to those questions will benefit for my eternal gratitude and constant worship. And, for good measure, a nice bottle of champagne during his next visit to Paris.

    Pascal.
 
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*** Warning: long and boring post***

I couldn’t help thinking of this spacer problem again and again. This morning I came up with a new explanation for the spacers. Sure, spacers don’t change the energy stored in the spring and therefore the kinetic energy of the hammer. It increase the mass of the hammer but it is at the expense of its velocity. I though the end result would be the same. Not really.

It could change the hammer momentum, let’s dig in further.

Let m1 and v1 be the hammer mass and hammer speed without the spacers

Let m2 and v2 be the hammer and hammer speed with the spacers.

The kinetic energy of the hammer being the same, we have:

1/2*m1*v1^2 = 1/2*m2*v2^2

Solving the equation for v2:

v2 = v1/SQRT (m2/m1).

Put in (approximate) English : the speed of the hammer with the spacers will decrease proportionally to the square root of its mass increase. Its just a repeat of my previous post.

One thing I have neglected up to now was the momentum of the hammer, its mass increase but its speed decrease, will the momentum be the same?

Let M1 be the momentum of the hammer without spacers and M2 the momentum with the spacers by definition of the momentum we have:

M1= m1*v1 and M2=m2*v2

Replacing v2 by its value above,

M2=m2*v1/SQRT(m2/m1) = (M1*m2/m1)/SQRT(m2/m1)=M1*SQRT(m2/m1)

Since m2 > m1, the momentum of the hammer with the spacers will be different and *** higher*** than the momentum of the hammer without the spacers. It will be higher in direct proportion to the square root of its mass increase.

What happens when the hammer bumps into the valve? My understanding is that during the shock part of the hammer kinetic energy and momentum are transferred to the valve and the plenum pressurized air is released. The whole system acts as a kind of nonlinear power amplifier or a transistor if you are in a poetic mood. By opening the valve the energy transferred by the air to the bullet will be several orders of magnitude higher than the energy spent in the hammer bumping the valve. Nothing new here, that’s just the beauty of PCP Airguns.

For the sake of simplicity, let’s assume the hammer has lost all its speed after the shock and there is no hammer rebound. Since the only mobile parts are the valve and the hammer it means 100% of the hammer momentum have been transferred to the valve. Since the momentum of the hammer with spacers is bigger than the hammer without, the valve momentum and speed after the shock will be higher, the plenum pressure will take longer to close the valve and more pressurized air will be released to push the slug.

To sum it up the new momentum generated by the hammer mass increase explains why the maximum speed of the slugs can be increased. All this in spite of the hammer speed reduction. I was wrong in my previous posts: it is not a matter of kinetic energy but a matter of momentum.

If my new explanation is correct the spacers would finally add value to the M3 after all.

It has been a nightmare for me to install and remove the power kit. I realized in another thread that I didn’t reassemble the rifle properly after installing the kit: I wasn’t careful enough and for some reasons the 2 parts of the cocking block rotated and the power plenum did the same, it changed the pellet probe position and 50% of the power was lost. I removed the kit, re-adjusted the cocking block by trial and error and the power plenum as well. I worked finally and I am now the happy owner of a fully operational vanilla M3: at Preg2 = 170 bars, the max speed of .22 Patriot 34 grains is 980/990 fps. That’ more than enough for long range shooting.

Will I be fool enough to attempt the slug power kit installation again to check by myself if my new technical explanation is correct? Will I try the experience with one spacer, 2 spacers? As muzzlebrake reminded me wisely in this thread: if it’s ain’t broken don’t fix it.

Well, I never learn … More later, when I’m back home at the end of the week.
 
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I always measure the distance between cocking block and probe block (essentially the length of exposed rod) when I disassemble. It is bound to move when shuffling the parts around. You have to pay close attention, though, because one rotation is roughly equivalent to 0.7mm. It's not much.



I didn't do all the calculations you did, but suspected there was a momentum increase behind this due to the added weight. I am still having trouble understanding how having the valve open longer still results in usable power that isn't wasted. Maybe the answer is that the valve isn't open longer at all?

These scenarios would give more 'area under the curve' (more air released and more power) even if valve open duration remained the same:

  • valve opens/closes faster while opening travel distance/depth remains the same
  • valve opens/closes at same speed, but opens more/deeper
  • valve opens/closes faster AND valve opens more/deeper

Depending on how this graphs out, I could even see the scenario where the valve opens/closes slower but the extra travel of the valve makes up for it and more.
 
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The hammer weights about 18.3g.

0.88g in total seems to be insignificant.
Click to expand...

Well, yes and no. From my calculations above, the momentum will be increase by SQRT(.8/18.3)= 2.5%. Not much for sure but assuming this increase the slug speed by the same rate you could gey 25 fps extra on an initial max velocity of 980fps.

This assumption is not really accurate since, like I said, the system acts as a *non linear* amplifier but it helps to get at least the order on magnitude
 
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These scenarios would give more 'area under the curve' (more air released and more power) even if valve open duration remained the same:

  • valve opens/closes faster while opening travel distance/depth remains the same
  • valve opens/closes at same speed, but opens more/deeper
  • valve opens/closes faster AND valve opens more/deeper.
Click to expand...

Well, in the first scenario there is less "area under the curve", not more: the valve movement distance will be the same but it will open for a shorter time.

The second scenario is not compatible with a higher valve speed as per my calculation.

In my opinion the only scenario left is the third.
 
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