Check out the video:
http://www.archive.org/details/Portafil ... ionProject
Here is an excerpt from a thread on the Counter Culture boardroom forum from David Lamont:
"SO, here’s the bottom line; the fact that the portafilter's only opening to the outside world is the hole at the bottom keeps the espresso from flowing out of the portafilter completely as it is extracted--it creates a vacuum..."
Basically the experiment in the video is a straight up comparison of shots through a stock LM portafilter and the modified Faema. There are certain things that became evident during the experiment and will be obvious by watching the video. There is one thing that you can't see in the video that is very important. The position of the holes in this Faema portafilter are not the best. While they function as predicted most of the time, they do become partially obscured by the over-hang of the bottom edge of the grouphead. There were a few shots were they became clogged or blocked, and they subsequently had none of the desired effect. I did not include these shots in the video. The most practical position for the holes is just behind the ear flanges. The holes will not be blocked if they come to rest at a position that is aligned with the slot intended for the ear flange insertion.
What you will notice is:
1- As soon as the flow of crema is full enough reach the top of the exit chute, the air in the peripheral portafilter chamber begins to oscillate in and out because of the air tight seal (in effect, respirating). This is the gurgle effect.
2- Fresher espresso that is gaseous will gurgle a lot, and a ventilated portafilter virtually eliminates the churning of the stream of coffee caused by the respiration of the portafilter.
3- The effect is much more subtle, if noticeable at all, in well aged espresso that has become very stable. I suspect that the ventilation holes in the last shot in the video my have clogged, as you can see a couple of gulps in the stream.
The benefit is that a good Barista can read information from the stream that indicates the quality of the extraction. It is a lot like reading the quality of extraction by watching a bottomless portafilter, only it is reading in a different language. Striping, blonding, curling of stripes, homogeneous creama color, bubble texture, flow rate etc... these things indicate something about the extraction. If this information is obscured by the gurgle effect, where the air is repeatedly sucked into and spit out of the exit chute, the Barista is receiving less feedback than they could be getting from a ventilated portafilter.
I don't think that ventilating the portafilter makes the coffee taste better. It does allow the Barista to monitor the quality a little more efficiently. They have the opportunity to toss out a questionable shot, increasing their percentage of good shots served.