Why Can't I Get the FPS Out of My 30 cal Impact

What hammer spring are you using? Old or new?

Set of calipers and this website would tell you the difference.

https://www.acxesspring.com/compression-spring-rate.html

From what I have read you need to use the old spring, longer one, for max power.

Couple of things to look at, first of all, the new spring with pp will push the 44 jsb 920 no problem, (assuming 700 mm)second, check the probe spacing, some people mess that up and your restricted, all it takes is the bolt linkage to turn one time when disassembled, next, do not jump from 135 to 140, that’s a big jump and I’m guessing you were already over, slow down and go 1 bar at a time, also, 4-5 shots after reg adjust to settle before you do anything, last, be careful about thinking your gauge is accurate and you can copy what other people are doing and telling you, I have a lot of Fx guns and I would be crazy if I told you half of the gauges were accurate, several are 20 bar off, no big deal, they are consistently 20 bar off, roll with it, they are for reference only, the chrony data and your target tell all! Good luck

I don't have experience with the new power plenums but I heard you can obtain faster speeds without high reg pressure. I don't think at 140bar you'll even notice the need for my hammer weight or different spring. Have you tried higher reg pressures like 160bar?

Also whats the length of your barrel?

This topic was touched upon in my hunting post about shooting snapping turtles with the purchase of my new 30 cal barrel kit. I stated that I was shooting the JSB 44.75 grain at 840 fps but that was on power wheel setting #5 but on Max I do get about 870 fps and on max pw get about 814 fps for the 50 VK's. Posters commented this was pretty slow for the set-up I have. This initial tune however was more for shooting the JSB's at about 840 fps for accuracy. I do want to get the VK 50's going at about 900 fps which I am pretty sure will require more reg . pressure so I went out this afternoon to try to get a that tune for the VK 50's. I set the valve on line 4 which is 10.85 mm, the reg was on 130 bar and the hammer spacing was at 15.30 mm and the power wheel on max.. I then shot three Vk's to get a base line and got about 840 fps. ( for each setting I was shooting three to five slugs across the FX Radar which was missing a lot of reading today grrr. ) I raise the reg to 135 and the velocity dropped to 820 so I knew I needed more hammer pre-load and adjusted the hammer space to 14.40 mm and that got me to 840 fps. I then raise the reg pressure to 140 bar and got 833 fps. I then turned up the hammer pre-load which maxed out at 14.22 mm and fps did not improve. So my question is how are you guys getting these high fps with your rigs. I installed the power plenum kit with the supplied parts. Do I need a heavier hammer and or go back to the older springs. I would appreciated some advice and ideas. Bill

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Are you shooting at high altitude by any chance?

Faulty Chronograph? Got a buddy with another one to test it?

50gr at 840fps is 78fpe. Thats the quoted value according to FX Website for an Impact Mk2 (w/ power plenum).

Are you using a 700mm barrel?

I would also make sure that your reg gauge is accurate. You should also be able to gauge velocity by zeroing at some predefined distance and then checking the drop at 100.

Bill, could the FX chronograph be giving some faulty readings if the dopler was being interfered with the other chrony in front of it? 

jking

@AiirgunBill your starting with the valve adjustment knob on line 4 right? You mentioned the FPS doesn't increase after 130bar, sounds like its stuck in efficiency mode.

LOL, or you messed something up when you installed the PP.

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I think there is a small change that FX made that is getting overlooked by most of us. There are two general types of Impacts with Power Plenums out there... those where the PP was installed by FX (either bought new with the power plenum installed when the gun was originally manufactured at the factory, or those that were sent back to FX to have the PP upgrade installed), and those that were upgraded to the PP by the owners who just purchased the PP kit and installed it themselves.

A common opinion is that when you install the PP yourself, you need to use the original 57mm hammer spring to avoid the lower velocities that most people are reporting after installing the power plenum with the 52mm hammer spring that comes with the PP kit. This is a perfectly viable solution to the condition of running out of hammer spring tension preload at the hammer spring tension adjuster. After all, if you cannot add any more hammer spring preload tension, and the hammer is not opening valve enough to obtain the higher velocities, you have to increase the hammer striking energy another way... and the easiest way is to use the original spring which had more power when an equal amount of preload was applied by the hammer spring tension adjuster.

Using the 57mm hammer spring is one solution to the problem, but I think FX implemented another method, but it was not part of the PP kit. Generally speaking (I'm sure it's possible some of the very first ones out the door from FX may not have had the change, so I'm speaking "generally" here), the Impacts with PPs coming from FX (factory builds or FX upgrades) are coming to customers/owners with a 16mm Hammer Spring Tension Adjuster (HSTA) slider. This was changed from the original 18mm slider that pretty much all guns in the field had when the PPs started shipping for the DIY owner installed upgrades. So, if your PP was installed in the field after the PP was announced and started shipping from FX, it was installed on a gun having an 18mm HSTA slider vs the 16mm slider the FX installed PP guns were shipping with. The 16mm slider gives you an additional 2mm of HSTA preload adjustment before the HSTA maxes out, achieving a lot of what you achieve by using the 57mm hammer spring... more energy from the hammer strike.

I think this is why we are seeing a lot of people who have owner-installed PPs using the 57mm spring, or using the 52mm spring from the kit, then commenting about unexpectedly low velocities, and inability to achieve the higher velocities. If they have an 18mm slider installed, they have 2mm less HSTA preload available to them than the factory PP models have. This is why the 52mm spring is fine for the factory install versions of the PP models, and as such, is included in the PP kit. Unfortunately, the 16mm HSTA slider is not part of the PP kit, so a field install is not the same as a factory built PP model unless you change your slider to the 16mm version as well. It would be really nice if this 16mm slider was included with the PP kit. Maybe it is not, because every field upgrade already has a 57mm hammer spring available if needed.

Just my thoughts after my experience with this issue. I have field-upgraded 4 of our 5 Impacts, and I saw the same thing the OP in this thread is experiencing. I then CNC trimmed my 18mm sliders to 16mm sliders, and now have no problems achieving higher velocities if using higher regulator pressures and going into that extra 2mm of adjustment now available to me (using the 52mm kit spring).

Does this seem to make sense?

Bill... I originally set the hammer stop to the 6mm spec. It was also not bottomed out and snugged. I ended up doing as you did, and leaving it a little less than 6mm, but snug.

My logic on this was it should not have the affect of decreasing the velocity for two reasons...

First, the hammer should be hitting the stop with a miniscule amount more energy when the stop is closer to the cocked hammer when it fires. For instance, with yours set to 5.85mm, the stop is 0.15mm closer to the cocked hammer than when it is set to 6.0mm. When the hammer fires, the hammer spring actively accelerates the hammer toward the stop. Once the hammer spring has fully expanded and is no longer actively pushing on and accelerating the hammer, it starts free-sliding down the rod. The physics are undeniable... once the hammer has no propulsive force behind it and it is free-sliding down the rod under its own inertia, it IMMEDIATELY begins to lose energy and slow down from drag. The same as how a pellet immediately starts slowing down after leaving the end of the barrel, and loosing the propulsion of the air behind it. Based on this, the less distance the hammer has to travel before it hits the stop, the more energy it will have when it hits it.

Second, positioning the hammer stop 0.15mm closer to the hammer also positions it 0.15mm farther from the Valve Stem Adjuster knob. This basically allows the valve spring to decompress by 0.15mm, which is same as backing the VSA knob back out 0.15mm.

I have no idea whether this 0.15mm difference is significant enough to cause any significant increase in velocity, but it clearly shouldn't be considered as a possible cause of lower velocities.

My guess, and I do mean guess, would be the 6.0-6.5mm spec is left over from when they wanted the end of the hammer stop a specific distance from the rubber ball of the original VSA mechanism.

For now, if I were you, I would look at your current conditions where your velocities topped out. Once thing to try is back your VSA out to where the knob just conceals the o-ring on the knob receiver. This new valve adjustment system starts applying valve-restricting force to the valve stem as soon as the valve spring starts compressing against the hammer stop. The four lines are left over from the rubber ball method, and are not necessarily an accurate representation of the effective range of adjustment with the new VSA mechanics. With the rubber ball method, placing the knob just past the fourth line usually removed all restriction from the VSA. Not so with the new spring method.

At this point, ask yourself if another two mm of HSTA preload would get you the increase in useful regulator range and velocities you are aiming for. My guess would be yes.

Okay just took my Impact apart to put back in the OEM 57 mm hammer spring. Funny how simple and less intimating taking it apart it is the next time. Anyway before taking the hammer seat off I measured how far it protruded out from the barrel support face and it was 5.20 mm. I installed the original 57 mm hammer spring in and with the valve seal hand seated without compressing the spring it now sits out 7.75 mm or 2.55 mm more of compression I assume. So at this point I am going to wait for some comments from you guys before I Locktite the hammer seat back on and put the Impact back together. Hoping this will give my the fps the Power Plenum is supposed to be capable. Bill

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If you can't modify your 18mm slider to 16mm, you are doing all you can do with what you have by using the 57mm hammer spring. Perfectly good resolution. I was primarily trying to offer an explanation of why many of us who installed our own PPs have seen lower velocities than expected with the PP kit 52mm spring. The 57mm hammer spring will get you to the velocities you want as well.

I'm confused about the new measurement of 7.75mm. If you had the hammer stop screwed back onto the rod to the same point where it bottomed out on the threads, the overall valve to hammer stop tip length of the rod should be the same as it was before. If this is the case, the valve could not have been seated when you measured 7.75mm.