Does bigger valve seat give more fps on impact?

Forums PCP Airguns Does bigger valve seat give more fps on impact?

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    PigeonMan
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    I see some people are replacing the standard impact vs to 6.5. How much more fps would you get from it? Would the 6.5 valve seat fit into the stock impact valve housing? I liked to get more power from my impact. I'm maxing at 900 fps with jsb heavy mk2 with my 600mm 25 cal impact. I already elongate the brass transfer port on the brass connecting to the barrel. My stock gun was shooting max 850 to 860 fps with heavy. After I drilled the transfer port I'm getting 900 fps. I plan to use slug and want to shoot them at least 950 out of a 600mm. Slug I plan fo buy will be 34 to 36gr.

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    Fire_at_will
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    I would go for more plenum space first. You can order larger one from FX. Or go the route of adding space to the regulator gauge.

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    PigeonMan
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    Fire_at_will

    I would go for more plenum space first. You can order larger one from FX. Or go the route of adding space to the regulator gauge.

    Bigger plenum give more power than bigger VS? I heard fx isnt selling the bigger plenum use on the mk2. I'm not into those big external plenum. They look ugly imo. 

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    Fire_at_will
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    Yes, plenum will give more power than valve seat.

     here is the problem, you can't just change out the valve seat. The new style valves are milled into the valve tube, and are not removable or changeable. 

    From my experience, there is not a big difference in power based on the valve seat alone. 

     

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    Fire_at_will
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    One thing I don't understand is why your set at 34-36 grn slugs at 950fps when you haven't even tested anything out yet.

    Your gun might be plenty accurate at 880 or 915 or 930 who knows? You need trial and error first,  not just what someone else got good results with. Sorry to be blunt, but it's simple fact.

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    heavy-impact
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    The larger valve seat performs better as the caliber increases and the same is true for a larger plenum. Brass pellet inlet and pellet probe porting perform better as caliber decreases.

     

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    Airgun-hobbyist
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    There's a few reasons why you are not getting the air flow/power you are seeking on the top end.  I'll tell you what I did to my .30 caliber to run 980 FPS @ 170 bar, and you can do the same with your .25 caliber and probably be in the 1000fps range.  Port and polish the housing, port the block, port the transfer port, open up the pellet probe hole to match the other three port sizes, add a brass hammer weight, open up the valve seat between 6-6.5mm, and turn up the regulator.  The long story:

    To get the "total package" out of opening up the brass transfer port you also need to open up the block's port, as well as the valve housing.  All three ports should be matching when assembled so there is not and steps or ledges for the air to flow around.  Also, you need to be sure that the pellet probe is in the correct spot when pushed into the transfer port.  (Your goal is  smto nehave one smooth transition of air from the valve seat out of the barrel.). Check out this thread I did on porting the valve housing.  See the nice raduised corners and how I ramped the back of the housing to smooth the airflow?  

    https://www.airgunnation.com/topic/fx-impact-valve-housing-modifacations/

    The  seat stock valve seat can be opened up to close to 6.5mm, but it makes the back of it weak and may cause the thing to break and leak air.  Then you'll have to just buy another one, or make one.  I modified the stock seat by putting it in a drill and filed the center out slowly with a small round file untill it was as big as I dared go (6.5mm) without cutting into the o-ring groove.  Then I polished the seat and rounded the leading edge of the seat to the air moves in around a nice raduised corner once the valve seal is opened.

    Also, get a brass hammer spring weight as well.  When using the higher regulator pressure and to open up the valve stem quickly and to hold it open a touch longer, you need more weight.

    There's a kit available for the Impact's on eBay made by forum member Edosan that has the bigger valve seat, hammer weight, and a delrin C3 bumper.  This kit and the porting job will get you all the power the gun can handle.  Shot count will go down considerably though untill you add plenum tube space.

    I did not make much more plenum space, aside from modifying the valve housing to remove excess material.  The plenum  space seems to only allow for more air for higher shot count per fill.  It may add little bit of power on the top end if you're running a .30 caliber with a regulator pressure at or above 170 bar, but I will only be able to confirm that for sure when I add more space and I obviously don't need it to push these 50.15 grain JSB's at 980fps!

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    cfs916
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    Bigger valve needs hammer to open 

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    PigeonMan
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    Fire_at_will

    One thing I don't understand is why your set at 34-36 grn slugs at 950fps when you haven't even tested anything out yet.

    Your gun might be plenty accurate at 880 or 915 or 930 who knows? You need trial and error first,  not just what someone else got good results with. Sorry to be blunt, but it's simple fact.

    True I havent even test in the 800s. My gun right now max at 900 with jsb heavy so I'm guessing slug would be around 850 ish? The reason I want to push 950 to even 1000fps is people are saying slug are not accurate at low speed and need to be over 900 to near 1000 fps to be stable in flight and be most accurate.  I do not have any slugs or slug liner to test right now. I'm trying to up my power first then purchase the slug liners and slugs.

    Fire_at_will

    Yes, plenum will give more power than valve seat.

     here is the problem, you can't just change out the valve seat. The new style valves are milled into the valve tube, and are not removable or changeable. 

    From my experience, there is not a big difference in power based on the valve seat alone. 

     

    I havent taken my impact apart but I watch earnest video on the valve seat and it's a small ring looking thing with a o ring on it. Its located inside the valve housing and he pushed it out with a rod. My impact I mk1 gen 3, so I'm guessing I have the same style valve seat?

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    cfs916
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    Only the recent mk2 impacts came with the integrated valve seat. You should have a 5.5mm valve seat. FX sells 6.35mm titanium valve seat

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    biohazardman
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    Maybe, maybe not it depends on where your choke point or smallest port is.  If it is your VS then you will get a gain if it is elsewhere you may get no gain at all. I got a gain when I ported my .25 barrel and probe slightly also when I added a smallish plenum with a small port to the valve.  But I got no gain at all when I drilled my VS from 5.5 to 6.1. Drilling the VS simply required me to hit the hammer harder to get the same velocity I was getting before the drilling.  If my barrel port were a bit larger, it is my smallest port, I would have expected to see a gain in velocity when I drilled the VS.  As explained above all ports must match to get the most from the tune.  From what I have read mostly here on the forum the early MK2's have a replaceable VS later ones have the VS integrated into the valve body.

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    PigeonMan
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    cfs916

    Only the recent mk2 impacts came with the integrated valve seat. You should have a 5.5mm valve seat. FX sells 6.35mm titanium valve seat

    Ok. Do you know if it will fit in impact x gen 3? Also how much more fps would I get from it. Example 30fps more?

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    cfs916
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    Yes it’ll fit. Not sure on how many FPS you would gain but it should help. My 25cal 700mm Impact X is getting 970fps at 145bar with 32.8gr VK slugs. Mods are 7gram hammer weight, 7.1mm valve seat, and elongated TPs. 

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    edosan
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    There is a reason why latest Impact MkII has change the VS from 5.5mm to 6.35mm integrated now (Much better IMO), but that is the ONLY impacts with VS integrated (june 2019 mk2 —> ) all the rest of the universe of impacts are VS changeable.

    6.5mm bore VS is as big you can go with the stock delrin valve seal. Can you go bigger? yes you can, but you need to change the rod seal, and that might be tricky.

    I have worked on many, many impacts, and all does benefit from a larger VS bore (more fps), BUT it will depend on the cal and weight of the ammo, larger VS on 0.22 18gn or less + porting, will take the speed up way too much, taking out spring setup and only been able to take the speed down using the Valve Knob.

    Is all about air flow, restrictors and air volume … once you get that is easy to understand the function of each part.

    • This reply was modified 10 months ago by edosan.
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    PigeonMan
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    Well called fx usa and was told they no longer sell larger vs or any power mod. A bummer.

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    Airgun-hobbyist
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    PigeonMan

    Well called fx usa and was told they no longer sell larger vs or any power mod. A bummer.

    Message member Edosan for the kit, or periodically check eBay for the Impact's power mod kit.

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    sgtBunny
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    Edosan is an awesome guy.  You can also find him on facebook with his latest adventures in tuning his own equipment.

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    heavy-impact
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    PigeonMan

    Well called fx usa and was told they no longer sell larger vs or any power mod. A bummer.

    You can drill the original valve seat to 1/4" (6.35mm)

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    RabbitBlaster
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    Is there anyone in here who has an FX Crown? I have a Crown Continuum .25 cal and I have done the port, probe and hammer. I have ordered but not yet received a 6.35mm valve seat. I am wondering how much improvement stands to be gained by going from the 5.5 to 6.35 in .25 cal? I have brought my FPE from 55 to 69 so far (at 160 bar), and I am hoping to hit 80 FPE…if I can keep the reg in the same place that would be good because I will not need to replace the spring washers for stiffer ones.

    What do folks reckon are my chances of hitting 80 FPE by replacing the last airflow constraint that remains in my system?

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    cfs916
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    You’d probably gain 40fps at the same setting by switching valve seats to a larger bore. To get pass 150bar reg u need a heavier hammer. From 150-160bar is minimal changes without adding hammer. 

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