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Vacuum and pressure in the block really doesn't affect the pistons' travel at all, actually...
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What about a single?
Actually there has been quite a bit of work done on trying to reduce pumping losses on a single in the motorcycle world.
Also, there is a fairly marked variance in pressure in the crankcase at higher rpms. Enough so that one or even two quarts of oil are actively drawn into the vortex created by the pressure differential. There are also serious differences in the paths the pumped air needs to take. Compare the pattern of a 180 degree straight four to a 120 degree straight six. Probably the boxer configuration is best to minimize pumping losses. Look at some of the designs with knife and fork connecting rods and disk bearings (fairly low rpm engines).