Short Stroking with a Ported Cylinder

[BikerEddd]

Hi All

I'll try and keep my question short and to the point!

In an AEG the cylinder volume should match the volume of the barrel. Of course this is not always the case. In order to combat this the cylinder is ported too effectively reduce the volume of air in the cylinder making that section of the cylinder obsolete.

With this in mind will short stroking back to the start of the port cause a loss in FPS? My thinking is that past that point it is all pointless piston travel anyway as that section is ported?

This would give a quicker ROF, snappier trigger response, reduced gearbox stress all round without any drop in FPS? What am I missing surely it can't be that easy or everyone would do it?

Look forward to hearing some opinions

Cheers guys!

[colinjallen]

Re: Short Stroking with a Ported Cylinder

Depending on the position of the port, which would affect how many teeth had to be removed, synchronisation of the movement of the nozzle and the piston could become interesting. The spring could also be significantly under compressed so you could end up needing a stronger spring to get the same fps.

[Sitting Duck]

Re: Short Stroking with a Ported Cylinder

In a nutshell what he just said...

2 teeth removed would most likely need the next spring up eg: m100 then SS 2 teeth and need a m110 aprox

Other factors come into play like a small bit past the port opening to allow the piston to accelerate a little gathering momentum as it released and passes the port. Thus having enough energy to create a good seal n crap as it thunders down the cylinder etc.....

Soz, I don't have the link to open university course on airsoft short stroking malarky, but its something like that.

Plus this guesswork of expected fps generated can still throw out some weird results now n then even when you are pretty sure you done ya homework

Only way is to build same old or similar setups or just suck it n see, then adjust fps with different spring or another tooth removal if possible.
(Short stroking a 455/469 ak47 is not really an option as you need most of that full 50mm stroke BTW)

[sj_asc]

Re: Short Stroking with a Ported Cylinder

Remember that the cylinder volume is not intended to equal that of the inner barrel volume, if this were the case then you would end up with negative pressure before the bb left the barrel due to the inevitable air leaks within the system (such as the gap between the bb and bore).

In order to have optimal performance/efficiency, it is important for the BB to reach terminal velocity as it leaves the barrel. To accomplish that, the exhaust pressue propelling the BB cannot become lower than that of air resistance during any time within the barrel.

In a system with inadequate cylinder volume, the BB would reach terminal velocity before it left the barrel, the BB would then have to overcome the air resistance with its sole momentum, thus losing energy before it leaves the barrel. A typically well balanced system would run approx 1.5:1 Cylinder to barrel volume, and those with longer barrels even higher.

Excessive ratio actually reducesthe energy output with ligher spring rates (UK power levels). Remember that the Piston assembly itself has to deal with the air resistance within the cylinder. So the actual air pressure generated (with the higher ratio) would be lower than that of a smaller ratio setup with the same spring. The side effect is a louder report due to the wasted exhaust pressure.
In essence, a long barrel gun allows lower pressur but high volume exhaust to accelerate the projectile for longer duration; whereas a short barrel gun requires higher pressure but low volume exhaust (due to the shorter acceleration duration) to reach the samemuzzle energy. It just so happens that by changing the cylinder volume affects pressure as well. It's also possible to further finetune the system by changing the total piston assembly weight.

The port/s in the side of the cylinder allows for acceleration of the piston assembly as there is little to no resistance from compression before this point, which with the lighter main springs is typically why muzzle velocity is higher. If you are going to short stroke the system, even if the actual compressible cylinder volume remains the same, remember that this low resistance acceleration stage will be no longer present and that you will need to use spacers and or a higher rate spring to compensate for the loss of piston speed/muzzle energy, not just from the compression resistance relief/acceleration phase, but also the reduced spring tension caused by the shorter stroke.

As mentioned, ported cylinders allow for increased piston return speeds and thus faster total piston return time which is beneficial for over all system efficiency with high rate of fire single sector gear setups since you can get away with a lighter main spring . Typically with a non ported cylinder stup + high cycle rate you need a harder main spring to overcome the additional air resistance within the cylinder to ensure that the piston assembly returns in time.

Another advantage with ported cylinders in high speed setups is that on the intake stroke, the side port allows for a much faster intake flow than is possible with a ported piston head, so the vacuum resistance (passed on the gear train + motor+battery) is also reduced.

[BikerEddd]

Re: Short Stroking with a Ported Cylinder

Thanks guys. Exactly the kind of info I was after! Much appreciated.