PDI VC or HD

[Spooros]

I have been looking at some PDI parts on their site, they advertise all parts in either a VC model or HD model. The English isn't great on their site so they have confused me somewhat, could anyone explain?

Oli

[Spooros]

Re: PDI VC or HD

For those that need it, I have found the answer but will post here for others.

<QUOTE> HD vs VC in terms of design differences: An HD ("hard") piston is like and standard airsoft piston in that it keeps the cylinder-barrel system completely sealed at all times, while the VC ("vacuum") piston has a 1-way valve in the head that is sealed when the cylinder is compressing air, but open when there is a vacuum. The idea behind it is that when using barrels significantly longer than stock, the cylinder does not displace enough air to push the BB all the way to the end of the barrel, and thus the piston's stroke ends before the BB has left the barrel. At this point, the BB essentially creates a vacuum in the sealed system because the quantity of air behind it is not changing despite the ever-increasing volume. The VC system corrects this by allowing the 1-way valve to open at the end of the piston's stroke, which allows the system to become open, and thus extra air can be sucked into it and cancel the vacuum effect previously referred to. PDI claims this reduction of vacuum in the barrel contributes to more stable BB flight, along with the obvious gains in velocicy when compared to a sealed system experiencing so-called "barrel suck".

There was a test a while back someone posted up that compared a HD and VC when using a longer-than-stock barrel, y'all might want to search it - not sure what it was called, but it shouldn't be too hard to find. IIRC, the results were negligible between the two. If you're the mathamatizing sort, there was another post a few months ago someone drew up covering cylinder vs. barrel volume calculations, so you can calculate how much barrel is "too much".

I run a PDI VC acceleration piston with my 650mm barrel, just to be safe. In terms of operation, it does what it says it does very well. As far as how that translates to trajectory, accuracy, and so on... I don't know. I couldn't imagine it would be much different unless you're making a huge increase over stock barrel size. Also consider: The VC hardware adds a few grams of weight to the piston, so a VC piston will be ever-so-slightly heavier than a comparable HD. This is why I went with the lightened "acceleration" version of the VC. And finally, you can't run a 7mm spring in a VC piston because only a 9mm ID spring will fit around the valve housed in the backside of the piston-head. </QUOTE>