Hopup Compression Loss
 

Hi guys
I finally got the motivation to reopen my gearbox for my KA M4 that had FPS issues since the beginning of times.

Long story short, I have real bad FPS out of my KA, even after having upgraded "almost everything" I could.

Current Setup:

Stock Gearbox shell (King Arms 7mm)
Stock Tappet Plate
Stock Brass Cylinder, unported
Stock King Arms 1 piece metal hopup unit

SHS Brass Nozzle with 1 oRing
Mad Bull Ultimate Bearing Spring Guide - V2
ARS MASK 3G Force DUO for V2 Gearbox (Cylinder Head and Piston Head)
Modify Stainless Steel Shims
StormTech MAX Impact Polycarbonate Piston
Prometheus 300mm 6.03 SS Barrel

Shimmed by gundock, who also changed my gearset for I don't remember the brand but nothing fancy.



Now to the interesting part:

Compression test of piston, cylinder, cylinder head as A1;
When I add the nozzle on the piston head, there's a slight loss between the nozzle and the cyl. head. So I added some teflon tape on the cyl. head. It helped.

Barrel has brand new Guarder Black bucking and nub, teflon taped to barrel
Hopup unit slid on top, teflon taped where it meet the barrel (where the C Clip is)

I put the barrel-hopup assembly on the piston-cylinder-nozzle assembly and tried the compression test again. There's loss, pretty important from the look of it. I added some telfon tape on the outside if the nozzle (I have WAY too much of that stuff :D), seemed to have helped a bit, but no miracle here.

From the look, and sound of it, it seems like the hiss is around the hopup arm :confused: As if the hopup unit was leaking air. I'm not sure how/if that is possible at all.

I've been thinking of changing the whole hopup unit for maybe a prommy (Neo Strike) or madbull (Ultimate) one... any recommendations in that regard is more than welcome, but I'm not sure this is really worth it.

1. What do you guys think my problem might be, with these infos
2. Should I change the hopup unit and cross my fingers?
3. ????
4. Profit


Thanks

shouldn't that cylinder be ported for a 300mm IB?

what spring and what were your FPS readings before fixing and with your current setup?

Ported Cyl: That's what I thought, but that's the stock cylinder KA decided to put in there ... O_o Original IB length was 315mm. I doubt 15mm would make a diff there but...
Spring: forgot to mention, I changed the spring but don't exactly recall what it is, but I would guess SHS M110

FPS:
That's a hard question, as I always had issues... Factory statement and witnesses from other KA buyers: 380fps
What I got when I first Chronoed it at a game: 365fps
Second Game: (from memory) about 330fps
Third Game: maybe 315fps
Fourth Game: less than 300fps
(you understand I don't have access to a chrono at home)

That's when I bought the listed internals, along with an ASCU2 unit and gave that to a gundoc (Ninja_En_Short on ASC) to install, shim, etc all this.
He told me one of my gear (don't recall the name, the flat bottom one I think) was slightly crooked, barely noticeable, but it was affecting performance. So he changed the gearset.

That was last season. This season I played a single game , and it was reading something under 300fps still. So yeah I think I got THE lemon of the batch... :'(

Once I'll have put enough money in it I guess it should be fine and all, just need to pinpoint the "real" problem... :/

After re-assembling the mechbox, and the nozzle is in its most forward position, place the complete hop-up unit with barrel against the mechbox.

Shine a light through the feed hole

Peep down the barrel from the business end, do you see ANY light? If you do, then you have a bad nozzle to hop rubber pairing.

Nice. Just added that to my little bag of tricks. Thanks, Stealth.

Will try this out, thanks

Regardless of what KA put in there, you need a ported cylinder in there for that barrel length period, breaking point where a full cylinder can work is about over 400mm I recall (somehow 470mm sits in my mind). With that length barrel and a full cylinder, you are already 30fps lower than a ported one would give you.

Not saying it's your issue, just saying it's an issue to deal with. Maybe it was put in to get the fps down cheaply?

Point taken - Wil get a new cylinder then

Make sure the hop-up unit also have the little spring that pushes the unit against the gearbox, otherwise it may cause the nozzle not to reach the rubber and it wouldn't seal perfectly.

You should also make sure that the SHS nozzle has the same lenght has the original nozzle. A shorter nozzle would cause the same thing as the unit not seating againts the gearbox properly.

I see
The spring is there but doesn't look like it's doing it's job well

Thanks for all the infos

One more thing, your FPS loss might be due to a compressed spring for a long time. If you don't shoot the gun in semi after each use, the piston spring might be compressed in a half-cycle, and this is causing the spring to lose power over a long time.

That's a misconception, I've left a spring fully compressed for 7 months and it only lost 8fps

Well I did some testing with a couple spare springs and I did see a noticeable decrease in FPS. There's more than just the compression factor in this equation. The spring's material can actually change the results, some alloys are not affected by long term compression, while it does alter other alloys.

The spring in this discussion might just have a bad alloy that doesn't keep his strenght simply by behind installed in the gearbox.

That tiny spring at the top of the hop unit isn't very effective. You're better off with using some o-rings on the inner barrel.

Hell, even G&P ships their guns with a spring on the inner barrel these days.

Depends on the steel used and the way it was manufactured. I have a Modify 90 in my MP5A5 that shot 330fps, and a few months later it was down to 300fps. Some spring steels are better than others for long term compression.

oh rry?
Then I can attest that prometheus and G&P springs don't lose power

Ya, that's the first time I saw it happen that made an impact, remember chronying it a month of so later and saw it had dropped to about 285fps..... and this was always after cycling in semi end of game day. But was also the only time I used a Modify 90, the other springs I had previous were Modify 100s and were fine. Plus using various other springs, and installing all kinds of springs into dozens of guns in my tech days, never heard anyone complain of spring power loss, just that one fucking Modify 90! Lol

And that is why we should all stay away from modify. lol

^^ +1 :P Had bad experiences with those springs, they were the worst over my compression tests...

I also had a Modify S120 that started at 425fps consistently and is now down to 390 consistently. Similarly a Modify S110 in a different gun started at 430fps and went down to 410fps.

Regardless of what KA put in there, you need a ported cylinder in there for that barrel length period, breaking point where a full cylinder can work is about over 400mm I recall (somehow 470mm sits in my mind). With that length barrel and a full cylinder, you are already 30fps lower than a ported one would give you.


That statement is wrong. Actually it doesn't matter. Dont waste your money on a new cylinder. A port will give you less air, less fps. If it has a non-ported cylinder that is fine, especially if you put in a shorter barrel. Non ported is good for barrels up to like 400mm or something so if you put in a 300mm you are definitely fine. Sounds like your just loosing air between the air nozzle and hop-up chamber.

bumping a year old thread?

Anyway, to somewhat debunk your advice while not entirely wrong, it's not entirely accurate (or complete)...

There's truth in properly matching air volume of your cylinder to your barrel in a ratio, while it may not have an effect on fps with one BB weight, overvoluming like you are suggesting creates higher than expected muzzle energy when using higher weight BBs on the same setup. In perfect lab conditions, you would expect muzzle energy of a .2 with X spring to be the same as a .3 with the same spring, minus the difference in velocity. When you fuck with the cylinder to barrel air ratio, the projected outcome starts to change. Overvoluming can net you higher velocity and muzzle energy with higher weight ammo. It's called Joule creep or energy creep. It's a common effect on GBB guns.

Yes you can use a non ported cylinder on a super short barrel. A ported cylinder will not necessarily give you less fps unless you drop below the magic ratio of cylinder volume to barrel volume to the point that you are undervoluming.