The air transfer port
by B.B. Pelletier
First, an announcement. The 2008 International Airgun Expo is coming up in just a few weeks (Oct. 24 & 25). I make it a point to drive out from Texas every year to attend the show. It's THAT good! Pyramyd Air will be there again this year. Like last year, they'll have loads of new and used guns, scopes, accessories, a boatload of pellets and just about anything else an airgunner would want. If you didn't go to the Pyramyd Air garage sale last month, then here's another chance to get in on some good deals. Go to the show's web page for time, location, a map and a list of hotels.
Now, on to today's blog.
There's a lot of interest in this subject. More than I would have guessed. So I'm running this second part today to give you something to talk about this weekend. Let's look at the shape of the transfer port as it relates to efficiency.
Many air transfer ports are simply straight holes bored through the end of the compression chamber. I'll discuss the size of those holes in the next and final report, but it's surprisingly similar across a wide variety of air rifles. Today, we'll look at transfer port holes that are not just straight tunnels.
Stay away from mirror-smooth
In a discussion I had with Jim Maccari, I was cautioned to not polish a transfer port to a mirror finish (assuming I could have done so). Jim told me his experience was that super-smooth transfer ports are not as effective as those that break up the airflow to some degree. In fact, he shared one of his tuning secrets - a transfer port shape he likes to use on lower-powered air rifles like the FWB 150 and 300, which are both target rifles. Both have a concentric transfer port, so this tip may work best for them and not as well for guns that have slanted transfer ports.
Jim's tip is to bore several graduated sizes of holes on the compression side of the transfer port - making a sort of bizzaro funnel, if you understand the Superman reference. A stepped funnel if you don't.
According to Jim, this makes the rifle shoot smoother. I presume it's breaking up the airflow by creating eddies and swirls at each of the corners of the steps. I have no personal experience with guns using this kind of transfer port.
The changeable transfer port
I wanted to test several theories about transfer ports, and Dennis Quackenbush was kind enough to make up several ports that I could install in an R1 compression tube that Jim Maccari donated. Ironically, Jim donated this tube because it was ruined by an airgunner who drilled out his transfer port for more power. Of course, that didn't work and his rifle was ruined, so he went to Jim for repairs.
Dennis drilled out the port even larger and made up several transfer port inserts that could be installed from the outside of the gun in less than a minute. I had an excellent testbed for testing transfer port sizes and shapes.
What about a Venturi shape?
This question always comes up because we know the Venturi shape increases the speed of the air flowing through the port. When I did the transfer port test on the R1, I asked Quackenbush to make some ports that approximate a Venturi shape. The shape he made is shown below.
I copied the shape after the port on a Webley Patriot, which is similarly beveled at both ends. I figured if it worked for the Patriot, it might also work for my R1. However, I saw no increase in speed with this shape. It may have been too large and may have reduced the compression by adding too much additional volume, like a slanted port. I don't know. I do know is that there was no increase when using the Venturi port when compared to a straight port of the same diameter.
A shape that might be worth trying
Here's a shape I never tried, but one that I think might hold some promise for airspeed improvement. The trick will be to get the angle correct, so the total volume doesn't drop compression too low and offset any potential gain.
I bet you never thought there was so much to a simple thing like a transfer port, eh? It's not just where it's located, but also how it's shaped and even how much volume it contains. Next time, we'll look at the diameter of the port, which was the question that started this report in the first place.