We're not sure, you decide for yourself:
Working on an engine project some years ago, we made an important discovery. At the time, we were using a 2-ring (1 compression/1 oil ring) piston. We decided to try a 3-ring (2 compression/1 oil control) piston design.
After the change, the engine was running very lean so we increased fuel flow several jet sizes and the mixture was back to normal. Looking for answers, we turned to Kurt at Aerodyne Research, Inc. who was monitoring the project. He set up a test cell it in their lab, and we were amazed at what he found.
What he told us, of the total increase in airflow, 60% of that increase was due to the increased vacuum during the intake stroke.
Intrigued, we asked for an explanation. He said the design of the second ring, having a angled face, was much more efficient at pulling air in during the intake stroke than the barrel-faced top ring design. He found that one, the angled shape of the ring face created the best vacuum on the downstroke and two, the cylinder oil film gently "lifted" the ring face off the bore surface on the upstroke. Further, his data showed "no measurable increase in friction during the upward movement of the piston" and three, the 3-ring piston showed much improved compression sealing properties.
Kurt asked us if the 2-ring design was common, which we said yes. Asking why, we answered that leaving one ring off reduced the amount of sliding friction, which he found puzzling. Why we asked? He replied, “First off, during the downstroke, that second ring is responsible for 60% of the total increased airflow and what they don’t realize is that during the upstroke, the angled face picks up oil and lifts it off the bore, so there’s virtually no friction increase at all".
Our take on this popular design:
1) Understanding the benefits of a 3-ring design, there's a logical argument that the second ring is the most important of the three. So why it eliminate it?
2) Regardless of the gas port design, on the downstroke, the ring moves up in the ring land, opening the gas ports and actually decreases the total vacuum which pulls in the air/fuel mixture.
The purpose of gas ports is to improve ring seal:
Isn't that like using a hand grenade to get ring seal? Sure, great force is exerted on the rings at the beginning of the downstroke, but 1) doesn't that pressure allow the ring face to push through the oil film, resulting in hard contact and increasing ring face wear? 2) isn't that outward force quickly dissipated to the point where there is little or no outward pressure during the lower part of the stroke and 3) doesn't it make sense for the rings to capture all of the combustion gas pressure possible?