Ring seal probably, maybe a little bit from reduced windage. This engine has an 043 top ring with lateral gas ports so pan vacuum probably helps the ring seal and maybe reduces flutter. Can't say I know for sure but more vacuum does make more power.
Pumping loss work should be lower with a vacuum in the crankcase, that is likely where the power is coming from. It is probably making about the same indicated power, but the brake horsepower is up from the lower pumping losses.
'18 Ford Raptor, random motorcycles, 1968 Plymouth Fury III - 11.37 @ 118
