Removed the brass firing pot.

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tonyc
Posts: 254
Joined: Mon May 29, 2017 10:55 am

Re: Removed the brass firing pot.

Tue Sep 10, 2019 11:14 am

RichieRich612 wrote:
Mon Sep 09, 2019 9:09 pm
I think reducing or increasing the hole makes no difference at all.
The pressure within the cylinder at full charge is immense and would easily fill the plenum volume instantaneously regardless of hole diameter. We're talking tons of pressure here at 190 bar....

Another factor to take into account is that the plenum is just a way Air Arms have simply designed a way to adjust compression on the valve spring.
Therefore it's purpose is only to adjust the valve opening.
You've missed the point of the design. The hole just needs to be more restrictive than the transfer port to start to make a difference. Let's keep it simple.....say the hole is the same size as the transfer port cylinder (more precisely, offering the same resistance to flow at a given pressure differential). Then, for the flow into the pot to be the same as the flow out of the pot, the differential between cylinder pressure and pot pressure has to be greater than the differential between pot pressure and barrel pressure. If the resistance to flow into the barrel is less than resistance to flow into the pot then the air pressure in the pot (and at the firing valve) will drop, restricting the flow out. Without the pot the pressure at firing valve remains (near enough) at cylinder pressure.

As the cylinder pressure drops the reduced pressure at the firing valve allows the valve to remain open longer, so the pot is counteracting this by reducing the pressure at the valve when cylinder pressure is high.

The overall effects are (a) to reduce air flow as the barrel pressure drops, reducing air wastage and improving air efficiency,) and (b) making air flow less dependent on cylinder pressure so the power curve is flatter.

You don't need a plenum "just to adjust compression on the valve spring". The pot assembly has two purposes: plenum and valve pre-load adjustment.

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RichieRich612
Posts: 125
Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Tue Sep 10, 2019 11:27 am

Can I send you an image of a 100 shot string I did using the carbine cylinder on my Mpr-ft.
The results speak for themselves.
Rich

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tonyc
Posts: 254
Joined: Mon May 29, 2017 10:55 am

Re: Removed the brass firing pot.

Tue Sep 10, 2019 8:24 pm

If you have two sets of data, one full string with the pot and one full string without the pot, post it. The only explanation I can come up with for no-pot setup as good as with-pot is if the pellet hasn't left the barrel before the firing valve closes and the pot vent hole is too large. That may be the case with the 6ft lb MPR (I wouldn't know) but it's not so with the 12ft lb S4/5xx series. My S400 is definitely worse without the pot. In any case, I can't see any reason why removing the pot would cause an improvement.

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RichieRich612
Posts: 125
Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Tue Sep 10, 2019 9:45 pm

tonyc wrote:
Tue Sep 10, 2019 8:24 pm
If you have two sets of data, one full string with the pot and one full string without the pot, post it. The only explanation I can come up with for no-pot setup as good as with-pot is if the pellet hasn't left the barrel before the firing valve closes and the pot vent hole is too large. That may be the case with the 6ft lb MPR (I wouldn't know) but it's not so with the 12ft lb S4/5xx series. My S400 is definitely worse without the pot. In any case, I can't see any reason why removing the pot would cause an improvement.
The Mpr-ft is 12ft lb just like the s400 series just a heavier more accurate barrel, they're not the cheap s400 series barrels. I'd post a pic on here but it's a primitive forum which doesn't enable pics to be posted :roll:

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RichieRich612
Posts: 125
Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Tue Sep 10, 2019 10:20 pm

Plus there's more chance of the valve still being open before the pellet exits with the plenum attached. With no plenum there's more force on the valve from the cylinder so it closes a lot quicker enabling a quicker lock time.

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