20+ fps increase after upgrade

[Preston]

I've put in shs 13:1 gears and a shs high torque motor into my g&g gr4 g26. Before this it was firing at approx 335. Now it's firing at 355+-. The spring and piston is still stock. Will it be a case of changing to a lower spring or will I need to change any other parts?

Thanks

[Airsoft_Mr B]

Stick an M100 in there. Some sites might allow 351-352 but I doubt they'd allow 355.

[Preston]

Thanks. I'll get hold of a m100 and a m 90 incase. I wouldn't even try and get this past a chrono. Just not cricket.

[Sitting Duck]

wow just from gears ???

dunno coz its got nowt to do with pressure I would of thought

I mean yeah the piston is coming back quicker than stock 18:1 say

but no weight added/lessened on the piston release

(so same piston is releasing at same speed etc...)

 

daft question - but did you fit bearing spring guide or piston spacer

(coz that would add about 20fps to compress spring a little more)

 

sounds to me at first noobish glance a better seal on piston or something

coz at present can't quite get my head around gear ratio making such an increase

 

unless some other chaps gunsmith guru can put me straight

 

I think I would wait n see or leave spring compressed a bit say overnight

then re-chrono after a few thousand rounds first, before changing spring

coz it kinda sounds to me like it went back together a bit better than before

(normally I bollox it more but fair do's if ya that good)

 

give it a chance to bed in a little and all fps drops a little after a while

 

did you remove piston o-ring - sometimes you can get a slightly better seal on piston/cylinder

if you remove o-ring, turn o-ring 180 and refit, sounds really daft but have found it can make a slight difference

though some could say the o-ring was stretched slightly when remove/refit

or

the o-ring surface may just work/seal that tiny bit better on the "other way round"

(not turn o-ring inside out but just put rear of o-ring at front so to speak)

 

some of these things may only be a temporary thing - like stretching o-ring

(bad idea - fit a new one if possible)

but these qwirky findings is why I say about maybe wait n see perhaps

 

and if you redid AOE (which should of been checked) with sorbo then you probably would lose fps

yeah - bit of a puzzler if you left everything else as it was - unless a better seal like I said

[Preston]

Well other than gears and motor and the shims for the gears, I only greased and lubed it all. The only thing I can think is that this gun has a pneumatic blow back. Maybe that's not functioning properly and the extra air is going out of the barrel?

[Sitting Duck]

dunno m8 - only blowback I have is L85 or rather R85

might be this or that or some grease on o-ring/cylinder giving it that tiny bit better seal - FOR NOW

 

deffo put it through some paces - see how it performs after a while when its bedded in

[Preston]

I'll give it a good run through and see if it drops any. If not then I will drop in a new spring. Hopefully that will sort it. Cheers guys.

[Samurai]

Most probably it's because of the better airseal you got with cleaning/greasing it fresh. Or it can be simply because of a different setting of the hopup.

 

Edit: or maybe you used different chronos for the 2 measurements.

[Lozart]

I only greased and lubed it all

 

This makes more of a difference than many people realise.

 

Also - looking at the places you play at I'd go with the M90 spring.

[dex]

Did you do both chromo measurements with the hop fully off?

[Preston]

Just checked with hop fully off and averages about 348 over 10 shots. Hop fully on and 355 over 10 shots. If I short stroke the piston by a couple of teeth will this lower the fps enough?

[Lozart]

Just checked with hop fully off and averages about 348 over 10 shots. Hop fully on and 355 over 10 shots. If I short stroke the piston by a couple of teeth will this lower the fps enough?

No it'll increase the rate of fire and introduce a whole host of other issues. Better to either cut down your spring a bit or change it for a lower powered one.

[Preston]

Okay. I'll just have to wait for the new springs to turn up. Rather annoying. Running it for about 9 months, as stock. Clean it all up for my first skirmish of the year and I get this. Oh well.

[Shizbazki]

Hi mate as a fellow G&G gun owner with two guns that have G&Gs pneumatic blowback system namely the two linked below:

MP5A5

http://www.guay2.com/web/gt_product/tgm_a5_rtb.php?lang=en

and

HK 418 in Tan

http://www.guay2.com/web/gt_product/TR418_Light.php?lang=en

 

I thought i would put in my two cents.

 

TL:DR version

Its to do with the Pneumatic Blow Back System, if you drop a M100 spring in your gun you will probably go down to about 300FPS, open your gun, lube the green rubber seal between the main cylinder and blowback cylinder, take out the stock spring, using a pair of wire cutters and snip about 2 or 3 coils from the piston side of the spring (not the spring guide side), re install, re chrono if its still too high snip some more of the spring. I had to do this for my G&G TR4-18.

 

LONG VERSION:

When getting these G&G type guns and wanting to chrono them its best to ensure the following before chrono. Do use a decent BB brand at .20g and Ensure hop up is fully off.

 

You state that after installing new SHS 13:1 gears and motor you have increased the FPS without changing the spring from 335FPS to 355FPS and that you have owned it for 9 months and that this is the first time you have opened it up, cleaned it and re lubed it.

 

I can tell you exactly where your problem lies; it in the Pneumatic Blow Back System (herein call PBB System) and the sudden expanding of Air.

 

To go into detail, the PBB System of G&G guns work like so:

1. Piston rushes forward after last tooth in sector gear is cleared
2. Piston compresses air in front of it, BB starts to move down barrel
3. Some of that air is ported to the PBB System cylinder above the mechbox
4. That ported air pushes on a small piston in the PBB System cylinder
5. This causes the silver plate on the PBB System to pop back
6. A spring inside the PBB System cylinder the pushes the small piston forward
7. The air used in the PBB System is then exhausted back into the main cylinder before the main piston head reaches the Cylinder head completing a full shot cycle
8. At this point the BB is already about half way down the barrel

Watch this video to get a clearer picture: https://www.youtube.com/watch?v=yyeGfob0ZUI

 

In order for G&G to provide enough air for both the BB and the PBB System they upped the spring from an M100 to an M110 (or thereabouts), note that as the PBB System blows back a spring inside its cylinder brings it back forward before the main piston head hits the cylinder head. That extra air used in the PBB System as well as its small spring used to bring it forward also exerts a small amount of force on the BB travelling down the barrel even if it is small, this is important regarding why the hop affects FPS in the opposite way.

 

As we know, the general rule of thumb is MIN Hop means more FPS, MAX Hop means less FPS, we also know that a hop unit adds backspin to the BB by friction from the hop rubber.

With a PBB System and a MIN hop:

1. As the main piston travels towards the BB the BB starts to move forwards
2. Some air is ported to the PBB cylinder which pushes the blow back system back
3. The reverse then happens as the PBB cylinder's spring and some of the remaining air pressure now rushes back into the main cylinder
4. BB able to move forwards freely as there isn't any or much friction from the hop up rubber
5. Because the BB is able to move down the barrel more freely the air has more space to expand (therefore exerts less pressure on to the BB leaving the barrel), BB exits barrel at say 320FPS

 

With a PBB System and MAX Hop

1. As the main piston travels towards the BB the BB starts to move forwards
2. Some air is ported to the PBB cylinder which pushes the blow back system back
3. The reverse then happens as the PBB cylinder's spring and some of the remaining air pressure now rushes back into the main cylinder
4. BB starts to move forwards but gets caught in the friction exerted by the hop up rubber therefore slowing it down (as normal)
5. But because of this the air from the main piston and PBB have less space to expand (therefore exerts MORE pressure on the BB pushing it harder) the BB exits the barrel at say 350FPS.
6. Remember if something is compressed the it exerts higher pressure on its surroundings

 

Now if you was to disassemble the gun to the gearbox and take the PBB cylinder off you will notice that there is a small rubber air seal which fits between the cylinder and the PBB cylinder right? Mine was green like in this video:

at about the 12:08 mark. This seal which behaves similar to the piston head O-Ring in that if not lubed up it begins to shrink and therefore does not form a good seal.

 

Chances are that since owning this gun and even before that when it was sat with your retailer, this seal was lubed up at the factory and then since for the best part of probably a year was never lubed up and therefore the seal shrank and leaked a small amount of air giving you an acceptable FPS of 335.

 

Then when you dis-assembled it cleaned the mechbox, installed new gears, greased stuff back up and put it back together again you either lubed up the seal or lubed the piston head with new lube (which makes its way to the PBB seal too) creating a tighter leak free seal.Because the PBB seal is now better and air does not leak then your FPS has jumped up by 20FPS.

 

DON'T replace the stock spring with an M100 spring your FPS will suffer greatly probably going down to about 280 - 300 FPS, what you need to do instead is keep the original spring, using a pair of wire cutters snip off about 2 or 3 coils off the piston head side and bend it back flat, re install it back in the gun and test it, if its still too high snip the coils off one at a time till you reach what you want.

 

My experience in this comes mostly from my G&G TR4-18 which i got from LWA, they stated the gun was firing 355FPS using ASG Blaster .20g BBs in a MadBull Chronograph, I used the exact same BBs in an Xcoretech X3200 chrono and that returned 370FPS, my local airsoft shop confirmed the 370FPS. I dis assembled the gun, put a M100 spring in and suddenly the gun was shooting 280 - 300 FPS, so instead i cut a few coils off the stock spring, installed that and now my TR4-18 and MP5 both fire between 338FPS to 345FPS.

 

So in the end my advice to you:

1. Ensure that the PBB seal is lubed (silicone oil should do fine)
2. Use the stock spring
3. Cut a few coils off the spring on the piston end

I hope this all helps.

[Preston]

Thanks for the reply shizbazki. You was right. I cut of a coil and got 345 then one more and got 335. Working brilliant now. I put in a m100 and it dropped to 300. All sorted just in time for next game day!