While it would be best to have no open second ring land, running an engine with this groove open certainly doesn't account for many major, easily noticeable attributes - either negatively or positively. The open groove allows the engine to favor upper revs at the expense of some bottom end grunt. Power on the pipe and through the powerband is virtually identical.
Though it seems like the loss of fifty percent of the rings would account for a fifty percent reduction of heat transferred to the cylinder wall, it is simply not the case. The lower ring does not carry away nearly as much heat as the top ring. The difference in the amount of heat transferred through the top ring and the second ring could be as great as 90% to 95% - that is, the second ring carries away almost no heat at all. Its purpose tends to be more for stability. It tends to allow the piston to align more parallel within the bore - especially near or at top dead center.
Running only one ring in the top groove of a two ring piston allows some fresh mixture to occupy the lower ring land area also cooling the piston in that area. In addition, any mixture movement within that groove will also come in to direct contact with the cylinder wall and can provide additional lubrication in some critical areas, such as the wall between the main transfer ports and the edge of the exhaust port. Any advantage here could incorrectly be thought to be completely offset by that amount of fresh charge escaping the power making process. Incorrect, that is, at least somewhat. Because the tuned pipe is so precisely timed its power pulses acting upon the mixture within the groove is (are) delivered at the wrong time (pipe timing is determined from the top edge of the piston). Any push or pull signal received there at that time is weak and diluted - at least when on the pipe due to the lower ring placement being several millimeters away from the piston crown edge and its corresponding timing (in milliseconds) being that far off. Running below or above the tuned pipe effective range changes this somewhat and can make the difference more dramatic - to the degree that it could become a problem to the fuel efficiency of the engine. Though even a strong well timed signal pulling mixture out of or pushing mixture in to this parallel groove will amount to very little mixture passed due to the small area of the groove and it becoming choked with too much lead-in area from either side.
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Will running one ring wear out the bore faster?
Not usually. It'll wear out the ring faster. In my experience running just one ring in a two stroke engine's piston gives up very little. In fact I've noticed no loss of compression on a gauge and no lack of power at low RPM. Though the high rpm power gain is slight there's an additional advantage - that being acceleration. Again, the advantage is slight but a gain is a gain. It would be best to use a piston without the second ring land so the fresh mixture doesn't bleed off through that tunnel, but if it's not available it's not available. There was much hype over a bleed-off condition similar to this within the snowmobile world this past year. I give them credit for noticing the condition and finding a band-aid for it, but I take away points for incorrectly identifying what is actually happening. In the Rotax engine (and about a thousand other engines) the auxiliary exhaust port and transfer port are connected briefly when the wrist pin and its cut out pass by. The band-aid, to weld a small area on the bottom of the piston (retarded) which closes this area or to weld the corner of the auxiliary exhaust port (the right way) cancels the affect but hurts more than it helps. The guys researching this phenomenon claim a great power gain was available if the condition was corrected. The problem was that they looked no further than the obvious. Yes a gain could be had by separating the exhaust and fresh mixture that way but a larger gain could be had if it was left as it was and a stiffer set of reeds was used. Doh!
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