http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

then read

http://www.prime-mover.org/Engines/GArticles/bsfc.html

somewhere i read a link about a company in Florida that runs classes for gasoline engine private airplane owners about how to set cruise at the 200 degrees F past the lean side of peak EGT in order to get to best BSFC...but I can't find the link in my bookmarks

someone ought to dig out this article and post it online:

http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=933033

because I seem to remember it had a bsfc graph that would tell us the typical behavior of a Magnum engine like cylinder, but I am not certain of what is actually in the article because I looked at it so long ago