Larger plenum test on Impact X .30 700mm ... Test No Good See Edit

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heavy-impact

heavy-impact

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Sep 13, 2018
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I got an idea after seeing Ernest Rowe add a larger plenum to his Impact test gun via the regulator gauge hole. He says "larger lung = faster dart". So I was going to machine one like Mr Rowe had until I noticed something on my bench that would work as is. Anyone with a Yong Heng compressor will recognize it as the little inline filter housing. The threads are M10x1 which is extremely close to 1/8" BSP. I ordered an M10x1 to 1/8" BSP male male coupler from Ebay to mate to the rifle and added 3 turns of heavy duty aluminum foil around the threads of my OEM FX manometer and used a steel washer with oring to be sure it sealed. The fit was very good with no leaks at all. The end result was a good bit more velocity but terribly inconsistent. Unmodified velocity is 887. With the 45g JSP Some shots were 840 up to 915 and everything in between. I plan to install a larger valve seat from Mr Rowe and make a twin transfer port and probe. I've taken my toys apart and modified them since I was a little kid, it's what makes me tick. What do you think caused the extreme spread?

EDIT: I didn't notice at the time of trying this that the valve housing lower bridge blocks the rear gauge hole. So my test was useless. Ernest Rowe mills the lower bridge off for his power tune.

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Pretty cool. Nice impromptu utilization of that filter housing as a plenum tube.

You may have it regulated too high now. The extra plenum changes things quite a bit. Maybe lower the reg pressure and then adjust HST to find the highest fps with the lower reg pressure. Test and adjust until you find your bearings. 

I’ve found that best efficiency comes when you find the highest fps with the HST at a given reg pressure and then dial it back by 2% +/- 0.2%. If max is 900 fps then back it to 880-885 fps. 

Let us see how it turns out. 
 
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I've had some experience with various plenum sizes. I went from 20, to 40, to 53 cc's of plenum. All sizes were tunable with LOW extreme spreads, obviously reducing pressure drop between shots shouldn't change the variance MUCH unless there are OTHER problems present causing variations, such as poor tune or inconsistent hammer strikes OR inconsistent reg psi output.



The power gains were really, really surprisingly low, considering how much volume was added. 20cc was around 52.5~ fpe max, 40cc was 58.5 fpe , 53cc was 60 fpe. All regulated @ 2030 psi. All .25 cal



You will only notice big changes in plenum when your original was way undersized for the power you're trying to accomplish, such like me going from 20 cc's to 53 in the end, nearly 200% the plenum yet only 16% more fpe.



I HIGHLY advise AGAINST two transfer ports, its ALWAYS better to have one full bore port than 2 ports of half size, or of the like...I won't begin to explain the air flow dynamics of it all because I wouldn't do a good job...but TRUST me. You don't go from a closed system with 1 exhaust, to 2 exhausts (tps), back down to 1 exhaust (barrel)...makes NO sense and guns that have had dual TP's in the past are historically known to be modified to have only 1...it also creates more WASTED volume which WASTED volume (volume that is not barrel lining for pellet to go vroom) = BAD. SO PLEASE FOR THE LOVE OF FX DON'T DUAL TP THE IMPACT.



Regulated ES should NEVER exceed 4% IMO and should IDEALLY (when tuned correctly) be 1-2%.





840-915 = 75 es, that is atrocious (8%) if all shots were ON regulation...I get less extreme spread tuned @ 500-600 fps which high power rifles tuned at lower fps are prone to higher extreme spreads, for example I get 4-5% ES at 600~ fps, but only 2% es @ 850.

If perhaps you have reg creep and the 841 shot is from your first shot, that would help explain that, but if its jumping from 841-915 mid string you have something weird going on with that kind of deviation...



I suspect if you increase hammer strike some or lower the reg psi 1 bar at a time you'd eventually get a much tighter spread. JMO GL. zx10wall was spot on with his advice, although I use 3-5% instead of 2%.
 
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I didn't state what I did for arguments sake. Larger plenums are always a plus, if I could have an infinite one I would...if two ports were better than one, then 3 are better than 2 and so forth, why not just say infinite transfer ports would be best...wait...no...they wouldn't be...you see where I am going here? maybe so maybe not...I'll let you ponder that.



I'll take the following arrangement for max power...



Infinite reservoir, Infinite pressure, Infinite barrel length...but it would use ONE transfer port..
 
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I can never understand Ed so I won't watch his videos, but are the single ports equal in combined Cross-Sectional Area to the two ports? There is tons of empirical evidence to support the use of only 1 transfer port is superior...the individual who holds the record for fastest obtained velocity from an airgun utilized one transfer port...hmm



I am not sure you understand the importance of transfer port volume...please refer to https://www.researchgate.net/publication/326191398_Internal_Ballistics_of_PCP_Airguns and note in the section about transfer plenum volume...the effect is quite dramatic...as I said the evidence is quite empirical.
 
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The Taipan Veteran has dual ports. The thing with the Veteran and EdGuns, they don’t flow from the firing valve through the multi TP and then through a single port, oval or otherwise into the barrel. These barrels thread into the action and have no TP. From the multi port right down the bore. They are great guns and I really like their design. 
 
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I would certainly agree that you can't have too large of a regulated chamber, though of course there are diminishing returns as you get larger. If the regulator gauge doesn't move when you shoot, it's big enough :) 

As for transfer ports on the Impact, it's certainly less than ideal for the air to have to make that sharp 90 degree turn to flow down the barrel. We can't really change that, so all we can do is vary the TP area. I would bet that we need more TP area than the cross sectional area of the barrel, in order to overcome some of the flow restriction of the sharp 90 degree turn. If we're physically limited to an insufficient area with one TP hole, then I'd bet two will be better, and I'm not sure having too much TP area would be bad in any way. I do agree that the milled area around the brass barrel inlet is going to waste a bit of air, but I'm not sure how significant that is.

I really look forward to doing more testing on this eventually.

Rusty


 
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The Taipan Veteran has duel ports. The thing with the Veteran and EdGuns, they don’t flow from the firing valve through the multi TP and then through a single port, oval or otherwise into the barrel. These barrels thread into the action and have no TP. From the multi port right down the bore. They are great guns and I really like their design.
Click to expand...

Yeah these are awesome guns! But like said the dual transfer port is in a brass thimble that the barrel butts up against. Superior design to a single transfer port
 
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I've had some experience with various plenum sizes. I went from 20, to 40, to 53 cc's of plenum. All sizes were tunable with LOW extreme spreads, obviously reducing pressure drop between shots shouldn't change the variance MUCH unless there are OTHER problems present causing variations, such as poor tune or inconsistent hammer strikes OR inconsistent reg psi output.



The power gains were really, really surprisingly low, considering how much volume was added. 20cc was around 52.5~ fpe max, 40cc was 58.5 fpe , 53cc was 60 fpe. All regulated @ 2030 psi. All .25 cal



You will only notice big changes in plenum when your original was way undersized for the power you're trying to accomplish, such like me going from 20 cc's to 53 in the end, nearly 200% the plenum yet only 16% more fpe.



I HIGHLY advise AGAINST two transfer ports, its ALWAYS better to have one full bore port than 2 ports of half size, or of the like...I won't begin to explain the air flow dynamics of it all because I wouldn't do a good job...but TRUST me. You don't go from a closed system with 1 exhaust, to 2 exhausts (tps), back down to 1 exhaust (barrel)...makes NO sense and guns that have had dual TP's in the past are historically known to be modified to have only 1...it also creates more WASTED volume which WASTED volume (volume that is not barrel lining for pellet to go vroom) = BAD. SO PLEASE FOR THE LOVE OF FX DON'T DUAL TP THE IMPACT.



Regulated ES should NEVER exceed 4% IMO and should IDEALLY (when tuned correctly) be 1-2%.





840-915 = 75 es, that is atrocious (8%) if all shots were ON regulation...I get less extreme spread tuned @ 500-600 fps which high power rifles tuned at lower fps are prone to higher extreme spreads, for example I get 4-5% ES at 600~ fps, but only 2% es @ 850.

If perhaps you have reg creep and the 841 shot is from your first shot, that would help explain that, but if its jumping from 841-915 mid string you have something weird going on with that kind of deviation...



I suspect if you increase hammer strike some or lower the reg psi 1 bar at a time you'd eventually get a much tighter spread. JMO GL. zx10wall was spot on with his advice, although I use 3-5% instead of 2%.
Click to expand...

+1 to you, I totally agree with you regarding the dual TP. The air flow will be "fighting itself" to escape out of the barrel, resulting in waste.

Drilling the probe with a radius end mill instead of a normal drill bit in axial direction will result in a smoother airflow towards the barrel.

Regarding plenum size if the impact, I got 975fps with king heavy .25's with stock plenum and 600mm barrel (69+ ft/lbs) only using a brass hammer weight (heavier than steel) and a increased regulator pressure. Looks like the plenum is big enough already :)

Only disadvantage is that I can't turn the power down significantly by just using the power wheel. I need to use the valve stop too for that. Still need to sort that out, but didn't have the time. I think I'm gonna start by making the hammer weight a bit lighter.

Cheers,



Gijs
 
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I value everyone's input here. In another thread Earnest Rowe told a guy that it's better to elongate the transfer port than to dill it larger. Then he said he would rather have a dual transfer port than an elongated one. What's the easiest way to add hammer weight? Wouldn't a stronger spring be better than weight? Mr Rowe said he was pushing the heavy .25s to 1025 fps.

No need to get excited. Parts can be changed back to original at any time.
 
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Best I can tell, aftermarket parts are in really short supply for these things. I haven't found a source for anything other than stock springs and hammers. Ernest did post a very interesting mod that's being tested, and as far as I can tell, not available for sale. It's clever in that it can increase spring pre-load, and add hammer weight with one fairly easy parts change. 

https://www.airgunnation.com/topic/heavier-hammer-or-stronger-spring-for-impact/#post-365388

Rusty
 
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With all this talk of larger regulated air chambers, I finally got around to testing something that's bothered me for a couple week. I had done some testing at various regulator pressures, and made notes of all the readings. Later, I took the gun apart and replaced the smashed C3 bumper, and screwed the hammer stop back in place (it had unscrewed about 1mm). I fully expected to see higher velocities, but instead saw about 25 fps lower. A few days ago, it dawned on me that I was using my digital gauge on the earlier test, and that requires some adapters and a short tee to allow me to connect the stock gauge for verification of it's accuracy. These fittings don't seem like a lot of volume, but it's clearly enough to make a difference. I just ran the test again, and got 804 fps with the normal gauge, and 829 fps with the digital gauge and fittings. Amazing that it makes that much difference. 

The pic below shows the rig, obviously on the main pressure port, not the regulator, but you get the idea of how little volume was added.

Rusty

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I few things to point out here. 

Between your external plenum and internal plenum is the valve housing. For the air to pass from one to the other in the original mk1 Impact is a very small pin hole sized hole. Later impacts had a milled housing but the slots milled into the housing don’t line up with the gauge hole. They have a tiny groove around the outside of the housing for air to get to the gauge. 

Ernest drills out and retaps the original hole and increases it diameter. He also aligns the slot in the milled housing

Michael
 
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