A bit of research has taught me that
Brake Mean Effective Pressure is the ratio of an engines Torque to it's displacement, whereas the
Brake Specific Fuel Consumption is the ratio of fuel consumption to the power produced.
In other words,
BMEP will tell you how much power you can get from your engine at a given designed displacement, whereas
BSFC will tell you how efficient that engine will be at a given power output. The two measurements are related but compete with each other.
In a design environment, especially with modern CAFE standards, I would start with the question of "What is my target fuel efficiency?". I would then ask myself "What is the minimum acceptable average power that my user expects in order to have a reasonable driving experience?". The answers to these question will give me a rough target for
BSFC. Do note, though, that
BSFC will change at different power output - I can design for a specific
BSFC at a typical "standard" highway speed, or I can design for a
BSFC at a typical 0-60 acceleration rate, or (more likely) try to do both.
Once I have my target
BSFC, I can use that to help me design my engine. I'll use other factors (input from management, target demographic, etc) to determine whether I want a four-cylinder, v-6, v-8 etc. Then, I can play with various displacements and use the
BMEP to figure out whether or not the theoretical engine will produce enough torque to meet my power requirements. I'll want to keep the
BMEP high enough to produce enough torque so that the user's driving experience is not muted, but also low enough such that I do not miss the mark on my
I'd like to stress that this is a pretty simple overview, but hopefully it does give a bit of insight into how a typical design process may go.