Another difference between ringed and ABC/ABN/AAN/AAC non-ringed engines is that the non-ringed engines depend on temperature for a proper piston/bore fit. Prolonged idle, such as a long final, allows them to cool down to a less-than-optimum fit. Ringed engines have no such issues, therefore, all else being equal, they idle better. Plus, it is far cheaper to "freshen-up" the compression with a new ring than to buy a new P/L set. Also, for those of us that love the older O.S. SF-series engines, Frank Bowman makes rings that are at least as good as the hard to find OEM rings. Less expensive, too.