I confess, I have always found BSFC maps labelled like this one a little confusing. The vertical axis is labelled "engine torque" but we are discussing it as if it is "load percentage." The below image from Autospeed article ECONORAM linked in post #28 above has a map labelled "engine load" on the vertical axis. Torque and load % are not the same thing, though. And on Autospeed graph it seems like 45% to 65% load would yield the best FE.