Piston rings are responsible for most of engine friction, so let’s compare the inches of
piston-ring seal (the sum of
piston circumferences). For the NC, it’s 18 inches, but for our hypothetical 669cc sportbike Four, it would be 33.6 inches, or 87 percent more. Bearing loads are also proportional to the weights of reciprocating parts such as pistons and connecting rods. So, these are scaled to the NC’s lower revs. It all works together to cut fuel consumption.