BMEP? That's a really good question. I'd like to know the answer myself. Seems like a factory engineering question, which they never answer (at least when I ask).
Pat
edit:
here's a link from an aircraft site. From our perspective, calculation of BMEP is done backwards from horsepower rating. The question as you posed it made me wonder if the factory has a test rig to evaluate average cylinder pressures.
http://www.epi-eng.com/ET-BMEP.htm