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FA Forums::News Discussions::Volumetric Ratio Chart
SGT DT (FAST)       Posted: 12:33:00 on 02-19-2010
This message edited by DT (FAST). 2010-02-19 13:34:00
Here's a quick chart for those who were not aware of volumetric efficiency for airsoft AEG's (Fireman :P). It is always "ideal" to produce a 2:1 compression ratio for spring rates m100-m120. Some feel 2.5:1 for m120 rates.

Here is a quick chart with rifle references.



C - 4/5 PORT CYLINDER * 400 - 301 MM BARREL * M4 RIFLE


E - 3/5 PORT CYLINDER * 250 - 201 MM BARREL * MP5 SMG

F - 1/2 PORT CYLINDER * 200 - 110 MM BARREL * MP5K SMG


Type C,D,E are substituted with what is commonly referred to as a 3/4 PORT cylinder which is the most common in AEG's. FULL cylinders being the next most common used in A & B type AEG's.


CPT FA-Ivan       Posted: 12:58:00 on 02-19-2010
Moved to Airsoft Guns and made sticky...

Good post Luis.


LCpl GEEBS_NRG       Posted: 13:04:00 on 02-19-2010


Prv 2&chest1&head       Posted: 10:09:00 on 08-24-2010
This message edited by 2&chest1&head. 2010-08-24 10:14:00
love the chart,need a little help. can you point me in the direction to study in full ,the data in this chart, and how it relates to my AEGs and future mods.I'm FNG ignorant,but a quick study. Thanks in advance.There may not be one answer to my question,but I would sure appreciate "ANY" links,or refrances you might provide.I'm assuming that the chart is referring to the size of each port and not position on cylinder with exception to the 3/4 C,D,E statement.


Prv 2&chest1&head       Posted: 15:55:00 on 09-05-2010
Maybe my question was a little to broad,how do you figure the compression ratio? Whats the laymen term formula, if there is one?

I can tell in the chart that the barrel length is part of the equasion for optimum ratio. I,m trying to understand the peramiters of the chart and how the ratios can be manipulated from factory ratio to optimum ratio in differant scenerios of machanics.

The fractions you listed are where I'm sort of lost, are they referring to position of the port on the cylinder,size of port,the ID of cylinder,etc....

And why is the spring rate(M100-M120) important to compression ratio as compared to higher spring rates? Is there something spicific about the m100 and m120s,or are they just more common springs to the majority of players?

Not trying to be a pain in the donkey, really, just trying to be a better player!!!


Prv Drake       Posted: 19:01:00 on 09-05-2010
I got a question, What would you figure to be the optimal length of a PSG-1 cylinder. They're rated for 590-650mm but I wonder if it can handle more? Also how do you compensate for tighter bores, I'm sure this chart was meant more for 6.08 type barrels. What about 6.05, .04, .03, .02, .01? Surely the amount of air produce extends the lenght of the barrel that you can use depending on its bore.


2nd Lt. Rooster       Posted: 21:22:00 on 09-05-2010
This message edited by Rooster. 2010-09-05 21:51:00
@ Jamie: You're over-thinking it. There isn't really a magic number or formula for the quote unquote best compression ratio. Basically to sum the whole concept up in quick, simple layman's terms, is that the amount (or volume) of air that occupies your inner barrel should match the amount of air in your cylinder. You don't need to worry about this unless you're changing the length of your inner barrel, specifically when you're ADDING length. IME, stepping down in length doesn't affect performance. Performance starts to become negatively affected when you take a normally short-length stock gun (specifically, the cylinder, which is ported in this case), and throw in a longer inner barrel. Normally what happens is the air that's available in the cylinder to push the BB is equal to the amount of air in the inner barrel, and it "cancels out" when the BB is fired. When you increase the inner barrel length but don't compensate for the added volume with a different ported (all the way up to non-ported) cylinder, you begin to create a negative pressure situation when a BB is fired, effectively acting like a small vacuum and robbing you of FPS. Vented piston heads help out with this "sucking" effect, but if you don't change out the port location on the cylinder, you still won't have enough air to properly propel the BB out of the gun

Really the only time you worry about this is when you add length to your inner barrel. Stock guns are already matched properly. If you change the inner barrel length, just do the basic math to figure out which cylinder you need by comparing the volumetric ratios, hence the chart above.

@ Michael: I think you meant to ask about the optimal barrel length, not cylinder length. That length can't change. However, to answer your question, what you CAN change about the cylinder is its inner bore. If you want to push past the 650mm threshold, you'll want to get a bore-up cylinder, cylinder head, piston head, and air nozzle. Good luck trying to find an inner barrel longer than 650mm though...It took me forever to find my 705mm that I use in my AUG.

Also, the bore of the barrel does *technically* come into play, but the differences in volume between say a 650mm, stock 6.08mm barrel and an upgraded tightbore 6.01mm barrel is only 142.942 cubic millimeters, which is only a change in volumetrics of 1.151% - Clearly negligible for this application.


CPT FA-Fireman       Posted: 23:32:00 on 09-05-2010
"Here's a quick chart for those who were not aware of volumetric efficiency for airsoft AEG's (Fireman :P)."

Yeah, yeah, yeah...

I LOOK at the cylinder in the gun, IF I replace it, I replace it with a cylinder with the SAME hole...

Can't go wrong with that! At least not unless the wrong cylinder was put in at the production level.


Prv 2&chest1&head       Posted: 12:38:00 on 09-06-2010
Ooooooooooh,I WAS over thinking it. Thankyou Sean. I get it , the chart is referring to volume of cylinder vs volume of inner barrel hence the barrel lengths in the chart.

If you install a longer barrel in a stock "short" barrel gun ,the VOLUMETRIC RATIO changes

And the fractions are referring to port position on cylinder in length from cylinder head.

The piston stroke length/piston hight would also be part of any changes involving port position to try and correct any losses in ratio,which makes putting a sniper barrel on a CQB gun expensive and not feasable for performance because of the changes in VOLUMETRIC RATIO!

Small increases in barrel length can be made without noticable changes in volume efficiancy,but DEFINATELY is a factor when large increases are made......thanks dude!;)


PV poorboy13       Posted: 06:26:00 on 01-11-2012
This message edited by poorboy13. 2012-01-17 06:28:00
Just wanted to say great thread! However if you really feel like doing some math, one thing I do when adding mock silencers/barrel attachments I have noticed it effects accuracy some, so to counter this I have figured chamber air in mock add on against my inner barrel (on a % basis) then used heavy fiber tape to block that % of port on cylender, seems to help some.
For example if I have a 300mm inner barrel, and I add a 100mm mock surpressor, I then cover 1/3 of my port. Idk if that's. On par with being right, but then again I was never good at figuring volume. ;) I will be building up a variant of a MK12 mod 0 spr (mod 0 front, stag receivers, crane stock) in the next couple weeks and ima try to do the volume correct & see where I wind up. Here's the start.......



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