Featured 80 fps loss in Cyma MP5k

Discussion in 'Gun Building, Modifications & Repairs' started by plasticnoob, Nov 10, 2020.

  1. plasticnoob

    plasticnoob Active Member

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    My cyma mp5k has been shooting at 340-350 for a year now. I decided to buy a bore up kit, lonex mp5k set. The stock compression was trash, new kit installed fps dropped to 260.

    Things I have tried and combinations of switching stock cylinder head to ZCI red v2/v3 head, didn't work.
    Using the lonex v3 cylinder head which didn't fit due to the over lap on the head. I took off the o ring and placed it on the stock cylinder head which did nothing either.
    I tried switching lonex cylinder with stock cylinder also cross with zci red head, and stock cylinder head. None of this has worked.

    During all of this I have replaced the piston with a shs full metal teeth piston and the lonex POM head that came with the kit. This has a good compression with these parts installed.

    I tried changing the spring to an m100 the fps didn't change either. Never touched the hop up it has remained the same.

    I do notice one thing, the CYMA mp5k cylinder head is taller than the zci piston head, no feeding issues I have test fried it.
    My last options is to return all to factory specs, and try changing each part out at a time.


    Any suggestions? was the piston head and piston rack the main cause?
     
  2. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    Stock MP5K with their short barrels rarely hit that high of a FPS mark with .20g bbs. And since you say stock fcompression was trash, I think you had chronoing issues.

    LEFSE can comment here.

    Also...most parts are "not" upgrades as you have experienced directly now. And Lonex parts are not 100% compatible. It is more like 92% compatible...again as you have experience.

    What is the barrel length on your MP5K?
     
    Last edited: Nov 10, 2020

  3. aotsukisho

    aotsukisho Well-Known Member

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    260fps on an MP5K is impressive. My old K got 200fps stock with 2.4% full auto fps variance, and with an SP120 gets 225fps with 2.4% full auto fps variance. I still run it with the stock 110x6.08mm inner barrel.

    Marui stock cylinder (50%)
    SHS cylinder head with Air-Pad, teflon tape
    GMT piston
    GMT aluminum piston head
    Marui stock tappet plate/nozzle
    Guarder SP120
    hop unit everything stock Marui

    I find it highly unlikely your Cyma K did 300+ out of the box especially if the compression was bad.
     
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  4. Lefse

    Lefse Well-Known Member Supporting Member

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    Ah yes, the notorious "compression gnome" that live in MP5K's and eating FPS. CYMA AEG's actually often have surprisingly good compression and consistency out of the box, I'm guessing it's because of the relatively heavy piston and the piston head and o-ring that fit juust loosely enough to minimize friction while still having a somewhat seal. I've also noticed that they use relatively short and stiff springs, possibly leading to a sharper pressure spike in the cylinder.

    My CYMA MP5K actually had similar FPS out of the box, haven't been able to "replicate" the stock compression after I replaced the stock internals. I've found AEG's with barrels shorter than 150mm to be extremely challenging in the compression department.

    I have thought about testing a "theory" of mine with my MP5K, guess I should get around to it once my tech corner is up and running in my new apartment. I'm thinking about trying to experiment with "porting" the nozzle to creat a controlled leak. I doubt it'll benefit the compression efficiency, but will be interesting to test anyway since I'm a curious guy.
     
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  5. plasticnoob

    plasticnoob Active Member

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    Impressive? lol When I got it the fps was around 340, with a new barrel and the cyma threaded adaptor it was around 341-345/350. Heck last time I played (2 weeks ago) it was again around 340 range. My chrono was the same readings.


    Prometheus 110mm mp5k barrel
     
  6. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    The only way that fps can get so high for a tiny barrel is the impulse wave is very high and very short...some worked out the magic in China.
     
  7. plasticnoob

    plasticnoob Active Member

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    It is listed online as 350 to 370 fps
     
  8. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    Listing at that speed and what you can physically do from a 110mm barrel is a feat that not many can replicate.

    Yours isn't the PDW is it?
     
  9. OPairoft

    OPairoft Active Member

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    Interesting fps discussion about short barrels, not to sidetrack, but this has helped me mentally with my Kriss Vector DSG, with a sp150 getting 310 fps with zero hop but with hop up to 320. Even with a sp170 it hasn’t changed.
     
  10. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    Short stroked, right?

    With a 170 and Short stroking...the creates a very small but powerful pressure wave...which could be what is in the CYMA MP5K.

    OP...do you have original piston and spring still?
     
  11. plasticnoob

    plasticnoob Active Member

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    Yes its the mp5k, not sure how hard its to believe its shooting 340 fps lol. Post from last year in my Facebook time line.


    Yes I stil have all the orignal parts. I am going to reinstall them.

    The factory spring is shorter than my prometheus m100 oddly enough.


    No it is not short stroked, it was a factory cyma mp5k I had bought, only changed the air compression parts l, piston and piston head.
     

    Attached Files:

  12. -Spitfire-

    -Spitfire- Well-Known Member Supporting Member

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    I'd try changing each of the parts, one at a time, and see which one (or two) makes the biggest difference... I doubt a cylinder head will make a difference for example. There's actually a few threads on this already, usually after swapping compression parts or correcting AOE. Personally, I think it's a combination of cylinder porting/piston weight. In all practicality, these issues make the mp5k the perfect candidate for severe SS-ing or DSG-ing.
     
  13. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    What I would like to see is the piston and the teeth...mainly how many it has.
     
  14. Splode

    Splode Well-Known Member

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    Just out of curiosity, have you checked if it holds compression on a finger test? I know that's basic but a lot of people just dont.
     
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  15. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    And the finger test does "NOT" involve the nozzle in the test.
     
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  16. plasticnoob

    plasticnoob Active Member

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    Yes I just posted this video on my airsoft page on fb.


    Put all oem parts in and cleaned up a little dirty. New fps was 329. Installed my asg infinity 18k motor got 340 fps again. ‍♂️ Photos and videos on my fb page.

    Stupid theory but, if the compression is so weak, air is just rushing out everywhere with such a small barrel couldn't it raise fps?


    When I bought the cyma mp5k it was advertised as 330-370 fps online ad.

    I'm not messing with it anymore than adding thr m110 spring to get it over 350 fps. The new motor really did what I wanted and boosted the trigger response with my 7.4 titan battery.
     

    Attached Files:

  17. plasticnoob

    plasticnoob Active Member

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    I might later, it took all week to finish this I was so busy with life and the mp5k is a hassle to take apart lol.
     
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  18. Guges Mk3

    Guges Mk3 Well-Known Member Supporting Member Lifetime Supporter

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    I would replace that "inferior" Titan battery and with a Good LIPO and your Trigger response would improve further.
     
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  19. JHat

    JHat Active Member

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    How well do the cylinder head and cylinder fit, like how snug? Did you teflon tape the cylinder head if it wasn't snug? Idk, if these are dumb questions, but you never know.

    My MP5K had the OEM inner barrel, TK 256mm, ZCI 300mm and back to OEM inner (150mm?). Seemed to have no problems with a SHS cylinder head and Lonex POM MP5K nozzle. Just needed teflon tape on the cylinder head.
     
    Last edited: Nov 19, 2020
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