Lower than expected hi capa FPS

[simohayha]

guys,i have a question regarding gbb fps after mods.
when i first got the gun (hi capa 4.3 marui) the fps is around 310-ish using guarder black gas (i think its green gas/propane inside).
but after i mod the gun and install all the parts, i only got 290-300 fps.
i expect to have a higher than stock fps.....anyone know why this happens? or used to experience the same problem?
my parts are as follow (the one's i think effect performance):
-airsoft surgeon metal slide
-PDI 6.01 inner barrel for 4.3
-shooters design pom cylinder set/nozzle
-nine ball dyna piston head with replacement spring for the floating valve (harder spring)

the slide movement is smooth,used 1000 grid sandpaper to smoothen all the rails.im sure the mags is not defective.and gas consumption is okay (38 shots per 4.3 mags).
thanks

[RacingManiac]

was it tested in the same temperature condition?

[simohayha]

yes,the temp is relatively the same around 26-28 celcius.
any ideas on why this happens?
what is the normal fps for the above setup? at first i expect to get around 330-340fps

[BloodSport]

You mentioned some parts you did, whats the full list of upgrades you did to the gun?

[Azuki]

Having a harder floating valve spring will DECREASE the FPS. The Spring in the floating valve (at least in WA, and I believe in TMs too) pushes the floating valve to close it after the BB is shot. The only spring that should increase FPS as it becomes stiffer is the hammer spring.

Hammer Spring Harder = Higher volume of gas released
Recoil Spring Harder = no effect on FPS, more gas required to cock
FV Spring Harder = Potentially faster closing of valve, less FPS, more blowback pressure
Magazine Valve Spring harder = Less Volume of gas released

and vice versa.

[Styrak]

So the solution is....swap the nozzle spring back in!

That does make sense. You probably have wicked kick though.

[ILLusion]

Originally Posted by Azuki

Having a harder floating valve spring will DECREASE the FPS. The Spring in the floating valve (at least in WA, and I believe in TMs too) pushes the floating valve to close it after the BB is shot. The only spring that should increase FPS as it becomes stiffer is the hammer spring.

Hammer Spring Harder = Higher volume of gas released
Recoil Spring Harder = no effect on FPS, more gas required to cock
FV Spring Harder = Potentially faster closing of valve, less FPS, more blowback pressure
Magazine Valve Spring harder = Less Volume of gas released

and vice versa.

I'd like to hear your explanation as to why the floating valve spring will decrease the velocity. All of my measurements show otherwise, and this is the theory I've surmised on it:

The stiffer spring keeps the valve open for moment longer, thus, allowing a greater amount of gas to be ejected out of the barrel and behind the projectile before the valve closes. In other words, the FV has SLOWER closing - NOT FASTER as you have indicated!

The point of the spring is to keep the valve OPEN. It doesn't push it closed - if it did, then your explanation holds water. But in this case, it doesn't.

Regarding the original post - a 10-20fps various with gas pistols is NOT uncommon. There are many variables that can affect this, such as storage temperature of your gas, volume of gas remaining in the tank, length of time between filling your mag and firing it, etc. Personally, I would not be sweating over a 10-20fps variance - I'm actually surprised you even got up to 310fps with the stock gun. Typical velocity of a stock Marui GBB running propane is only 290-300fps.

[Azuki]

For my gun the valve spring presses the valve closed. The BB pressing agasint my FV is what keeps the passage open. If the FV spring was too strong it would simple close the valve and push the BB past the hop up causing a roll out. I'm not entirely familiar with TM's design like I said, but I was going on the assumption that they used similar systems.

If the spring is located on the BB side of the valve, then yes it should keep the valve open.

:P My mistake for thinking the whole work is Magna BB

[ILLusion]

Originally Posted by Azuki

For my gun the valve spring presses the valve closed. The BB pressing agasint my FV is what keeps the passage open. If the FV spring was too strong it would simple close the valve and push the BB past the hop up causing a roll out. I'm not entirely familiar with TM's design like I said, but I was going on the assumption that they used similar systems.

If the spring is located on the BB side of the valve, then yes it should keep the valve open.

:P My mistake for thinking the whole work is Magna BB

I'm not sure how WA guns work - I'm not too familiar with them, but Hi-Capa floating valve springs force the valve OPEN to direct gas in to the barrel. Once the projectile leaves the chamber, a negative pressure exists within the cylinder, causing the floating valve to shut close from the pressure of the gas. Once closed, the gas is then redirected out the back against the pistonhead which forces the slide backwards to begin the blowback cycle.

My understanding was that almost all modern GBBs worked on this principle. I'm surprised your WA doesn't.

[m102404]

Redwolf has a "how GBBs work" write up and details both the TM and WA blowback designs. I can't remember the particulars and am no expert at this stuff by any stretch of the imagination. It's in their reviews/info section.

But by observation and to my understanding, in the TM Hicapa, the FV spring does push the valve to the rear of the unit...opening the flow of gas from the mag to the bb/barrel. A weaker spring would let the valve switch from "vent to barrel" to "vent to piston" sooner (all things such as temperature and pressure remaining the same). A heavier floating valve (made of metal) with a stock spring will also react in the same way, and so needs a stiffer spring installed to balance out the system properly.

Amazing system...in that it works as reliably as it does until you monkey with it!

Personally...I'd reseat the hopup rubber/unit and try it again.

[TriChrome]

Originally Posted by simohayha

what is the normal fps for the above setup? at first i expect to get around 330-340fps

I had those same parts installed besides the tightbore barrel. In about 22 degrees Celsius I was getting 317 FPS with .2's. I would say your number of maybe 330 FPS wouldn't be too far off.

[ichimaru gin]

Guys, I chronoed the gun with sir Simohayha and when we test it, we only filled 1/4 of the mag capacity. Maybe this is what causing lost FPS. If there is a difference in FPS when the mag is fully filled and 1/4 filled, what is the approximate difference?

[ILLusion]

Originally Posted by ichimaru gin

Guys, I chronoed the gun with sir Simohayha and when we test it, we only filled 1/4 of the mag capacity. Maybe this is what causing lost FPS. If there is a difference in FPS when the mag is fully filled and 1/4 filled, what is the approximate difference?

Yes, there is definitely a difference. I have seen variances between a full mag and 1/4 full mag of up to 50fps.

[simohayha]

maybe thats the case. i use to think that gas pressure at full or half full is the same, as long as its not near empty.ill try to chrono the gun again than.thanks

[ILLusion]

Just fill up a magazine and empty the entire thing through a chrono and watch the velocity drop as each shot goes through. That should be enough proof.