Question about transfer port, dwell time and air use

[JPSAXNC]

I have a PCP pistol with a 9" barrel. Would a small transfer port with a longer dwell time have the same air use efficiency as a larger transfer port with a shorter dwell time at a given velocity?

[nervoustrigger]

Is it regulated?  If so, to maximize efficiency, you want large porting and short valve dwell.  This combination delivers a short burst of air and then quickly closes the valve before the projectile has moved very far, allowing this finely metered dose to expand as the projectile accelerates toward the muzzle. 

If a conventional PCP, large porting has the potential to produce a short string of excessively high velocity.

[JPSAXNC]

Hi Jason, The gun is not regulated, right now I'm using a small transfer port and enough dwell time to get the velocity I want. But air use efficiency is low, so I'm wondering if I use a bigger transfer port and adjust the dwell time to get the same velocity, will air use efficiency improve or stay the same?

[nervoustrigger]

Technically larger porting is more efficient for an unregulated PCP as well.  But I think the question would be, is that the goal?  Meaning, are you going for high efficiency (fpe/ci) or high shot count?  To some extent, these are competing goals.
 
For example one way to maximize efficiency is to use only the leading portion of the bell curve, where pressure is highest and lift and dwell are lowest.  However that seems like more of an academic pursuit rather than a practical one.

[JPSAXNC]

The gun I'm working on is a Jet One pistol the reservoir is only 40cc's. I'de like to get better air use efficiency, but I don't know how to go about it, or if that is even possible?

[avator]

James...
You might some interesting things in this build I did some time back.
I'll link you to where the good stuff begins. From there you may need to read the following few pages to get it all.

https://www.gatewaytoairguns.org/GTA/index.php?topic=182687.msg156082734#msg156082734

[JPSAXNC]

Bill, Thanks for the link.

[avator]

Welcome James.. I hope it helps.
I was fortunate to have the guys with experience help out in that thread. Pellet Junkie (Frank) was awesome making and sending me those bits and wouldn't take a dime for them.
One thing I wanted to point out if you didn't catch it... the conical transfer port. That tube is so small and trying to get the most efficiency and flat bell managed was a lesson. To be honest, some of it was luck.
The limited volume of the Jet you are working with just made me think of it.

[JPSAXNC]

Well, I decided to open up my transfer port and see what happens, My fear was that air use efficiency might drop further, but it might help also. Bill, about the tapered transfer ports I have used them in some builds with good results.

[rsterne]

Shorter dwell generally improves efficiency.... Larger ports tend to be like using a racing camshaft in an engine.... more power over a narrower (pressure) range.... Smaller sips of higher pressure air are more efficient than big gulps at lower pressure.... My bet would be larger ports and shorter dwell is more efficient, but unregulated you may not get a higher shot count because the ES will increase when you are not in the sweet spot....

Bob

[JPSAXNC]

Thanks Bob, I'm not planning a drastic change, just enough to see if it makes a difference.