What effect does closing down the transfer port have on the dwell of the valve?

It will actually dwell a little more or stay the same, but not by much. It’s the same effect as using heavier lead.

I did that yesterday. Switched from slugs to pellets and had to get the velocity down to reasonable level. I changed the TP wheel to medium and reduced the hammer spring to No. 3. Now shooting JSB 18.1s at 934 fps. Deadly accuracte.

Airgun-Revisions said:

It will actually dwell a little more or stay the same, but not by much. It’s the same effect as using heavier lead.

Click to expand...

“It will actually dwell a little more or stay the same ...”

By reducing the transfer port???

My thoughts on this - the main closing force acting on a conventional airgun valve is the pressure x stem-cross-section-area. So, reducing the transfer port diameter has the potential to decrease the dwell duration. But only when the smaller port causes the pressure on the valve side of the stem to remain higher and/or of longer duration. That can happen under certain conditions. True that significant changes to dwell duration are not common via the transfer port.

But the condition could be if the transfer port is made so small such that flow through the port becomes choked early on, pressure on the valve side of the stem is held higher, and the valve closes sooner. More so with small plenum volumes.

I can't think of an instance where a smaller port would cause a longer dwell in a conventional valve. But I'd like to learn of how that could happen.

To the op, it muddled the waters so to speak by giving the example of going from slugs to pellets. If you have a heavy slug that maintains a high pressure on the conventional type VALVE STEM during the shot cycle, it puts more closing force on the stem than if you loaded a light pellet.

Scott explains the key to think about here. Pressure at the valve stem. Pinching the system downstream from that area, reduces valve dwell by increasing closing force.

Dave

MEC17670 said:

Hey Folks,
This question has me thinking.
Let's say I'm shooting slugs out of my Crown mk2 and decided to shoot pellets. Without changing the reg pressure , or power wheel adjustment but only close down transfer port from high to medium or low. What effect does the excess air have on the dwell time of the valve or does it? Does the back pressure increase the dwell time or just what happens to all that extra air. It's in a sealed chamber, does it work against the return spring for the valve?
Any thoughts?
Thanks,
Mike

Click to expand...

Mike, I believe you are on the right tract. It is a balancing act between high and low pressure. This is tuning 202. You can increase or decrease the values on either side of the equation to alter the outcome. Your original question was pertaining to the low pressure side. My reasoning is that if you decrease the low pressure porting at any point it will take longer for the valve spring to close due to the slower pressure/spring/valve balancing , therefore a longer dwell time, and by then, the pellet/slug is further down the barrel before the valve closes. I'm sure I'm missing something and can learn from the best here .
Patrick

MEC17670 said:

Thanks for the reply.
All a matter of back pressure then, whether through air transfer, or heavier projectile.
By putting in a lighter return spring for the valve, could the same velocities be achieved at a lower reg pressure? This would be done in conjunction with all other tuning factors?
I guess I'm asking if it's worth installing a light return spring?

Click to expand...

When you say a lighter return spring for the valve, are you talking about the hammer spring, or something else. Call me ignorant... (smile)

Scotchmo said:

“It will actually dwell a little more or stay the same ...”

By reducing the transfer port???

My thoughts on this - the main closing force acting on a conventional airgun valve is the pressure x stem-cross-section-area. So, reducing the transfer port diameter has the potential to decrease the dwell duration. But only when the smaller port causes the pressure on the valve side of the stem to remain higher and/or of longer duration. That can happen under certain conditions. True that significant changes to dwell duration are not common via the transfer port.

But the condition could be if the transfer port is made so small such that flow through the port becomes choked early on, pressure on the valve side of the stem is held higher, and the valve closes sooner. More so with small plenum volumes.

I can't think of an instance where a smaller port would cause a longer dwell in a conventional valve. But I'd like to learn of how that could happen.

Click to expand...

It will dwell slightly longer but it’s nothing drastic because you’re starting to create more back pressure. Any restriction from a point will just START the dwelling process.

Can you explain how a restriction STARTS a dwelling process.

Dave

My take on this having done massive amounts of Valve design prototyping & mods ...

In a conventional valve asking if or not a smaller or larger transfer path changes dwell, it is an issue of Pressure differential. That being the pressure within valve before opening and the brief moment poppet opens pressurizing the throat and transfer path.

Within the valve lets say we have 2000 psi that is going to exit into throat and then threw transfer path & off to launch the pellet.

WHEN TRANSFER PATH is a lot smaller than the throat of valve this 2000 psi may very well for a moment be at 2K on either side of the poppet head when open leaving the primary closing force that of the poppet return spring and some stem area effect. Once the pellet starts moving the pressure within throat starts to drop and the valve will react to the pressure difference and want to close it self. SMALLER the transfer size LONGER will the dwell be simply due the poppet reacting slower because the pressure on both sides of poppet stayed near equal for a longer period. .... * Now read below

A LARGER transfer path with all else being equal ( As above ) Has the timing of valve lift to pressure collapse within the valves throat happening faster and valve will want to shut faster creating LESS dwell.


Whats odd tho explainable, Is why when we use guns with Adjustable Strangle knobs of transfer path to control output power we don't save as much air as you would think it should ?

This because when we strangle / choke the flow from the valve to pellet we have chopped the pressure pulse Strength or Amplitude from a sudden and strong SNAP of air to a lower flow. The pressure is the same, but without the high and free flow of a big path to flow threw, once restricted the flows from valve is slower and lasts longer passing into and out of valves throat & barrel. Tho per shot almost nearing the same air use ( Tho there is some air savings to be had. )

The VALVES DWELL tho generally will be longer when flow is restricted at transfer port, tho we save air simply because the air cannot get out as quickly when choked down in flow.


Hope this helps those seeking to understand.

Eloquently said Motorhead.

Patrick

After reading through some of these posts, most make arguments based on certain assumptions (me included).

I, for instance, assume a heavy tune because we are coming from shooting slugs. As such, I assume a large lift of the poppet such that a significant amount of the dwell time is spent at full flow (where poppet lift creates a curtain area that is equal or larger than throat flow area). Once this amount of lift is reached, pressure differential of the poppet face vs plenum pressure is 0. This making the significant closing force the pressure at the stem. For example, at 2000psi, a .125” stem exerts close to 25# force to close the poppet.

But I can see that different initial tunes can change when/where the most significant differential is created. Therefore changing the effect choking at the tp has on dwell.

Dave

This to follow up motorhead. I made an adjustable transfer port knob for a .30 uragan. With that I made a removable valve from stainless and a peek poppet with no return spring. When the poppet diameter was too close to the throat diameter crazy things happened with the transfer port adjusting. Each click I made to close it, the valve dwell got longer. By the second to the last click, it would dump the whole plenum ( what a recoil it was lol). By closing the throat some, and increasing the poppet diameter it worked correctly. 1080, 1010, 930, 850, 710, 580, 440 fps. At 580 fps the shot count is very very high. So, it WILL save air when made correctly!

Excellent info ...