Thread: pcp theory
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Old 21st January 2016, 10:00 PM
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Tench Tench is offline
WHFTA World Champion 2016.
Join Date: Sep 2009
Location: derby
Posts: 1,413

You have picked a funny rifle to play with in the HFT500, the 400 type AA action uses pneumatic forces to close the very heavy valve as a spring just couldn't do it fast enough with that valve weight. For example putting a softer valve spring in that action reduces power because the valve head gets knocked further back into the pot quicker sealing the pot off so the res pressure fires the valve closed sooner. Most other pcp's work quite differently with a softer valve spring slowing the valve down and increasing the air delivery duration.

Well, that is how they used to work until AA increased the pot dia to stop the valve spring rubbing brass particles off the pot which could cause a firing valve leak, presumably for reliability and financial reasons to reduce warrantee returns for valve leaks? but slowing the valve down and increasing air consumption in the process. The HFT has a massive air tube compared to the 400k yet barely beats it on shot count, having said that it still shoots very well. The BSA GS gets the same shot count from half the air volume of the HFT and from a shorter barrel.

In general though better air flow and a shorter valve duration will make a gun shoot better and use less air with an higher bc and better accuracy, or less pellet fussy! but may increase muzzle flip due to a strong short air blast against a gentler longer one. You can go too big on the porting too, it is about finding the best balance of many variables for each particular rifle design with port length and volume playing a part too. Long ports are not good for high flow rates without having a larger dia, this creates a high volume which causes a slow pressure rise behind the pellet. it is all a compromise.

My preference would be for the shortest lowest volume air path with nicely shaped and flowed porting with no step dia changes. Lightest smallest area valve possible, Lightest hammer with zero preload on the hammer spring when forward so the valve can close fast without having to push the hammer back against the spring. (HFT has 23mm of spring preload!!!) With a NF on top!

Most valve designs are also used in fac versions so have greater area than needed which requires an heavier hammer and spring. We are using rifles with compromised designs to standardise on parts across a rifles model range, yet they will bs us with stories of it being designed for our purpose rather than the real reason of their economics. Compromised by accountants! not developed to be the best they could be!

There is no right setup with the rest being wrong, the different design and architecture of each rifle dictates a different approach.
HFT, for people with only 2 buttucks!

VooDoo TwoTwo The next chapter.

Last edited by Tench; 21st January 2016 at 10:15 PM.
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