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A few days I got an idea on how to improve engine balance when converting från a straight-four to paralell twin: Use a 360 degree configuration and leave the unemployed pistons in place! To reduce friction losses the pistons should be modified by cutting holes in the piston crown and removing the rings. The pumping losses are at a maximum about "halfway" between full throttle with normal valve lift and no valve lift at all. When introducing holes in the piston crown this will also introduce internal pumping losses! The holes must therefore be as large as possible. Perhaps the piston skirt should also be modified to reduce friction even more.
I have seen single cylinder motorcycle engines with a balancing device working as at counterweight to the piston, reducing the vibrations in the same way as a "perfectly symmetric" 2-cyl boxer would do. My passive modified pistons should act in a similar way. The big challenge may be how to cut the pistons for minimum pumping losses and friction while keeping the maximum weight.
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