what is BMEP
 

and how does it affect performance and all that shiz

Brake Mean Effective Pressure. I dunno how it translates into performance.

If you use desk top dyno, that's all the program really does is figure BMEP and IMEP (Indicated Mean Effective Pressure).

Basicly if you spin a motor without it running and measured
the air pressure in the cylinders at different rpms thru the usable rpm range and graphed the results, the shape of the curve
would closely follow the torque curve of the running motor.

Up to the octane limit of the fuel used, the more air you can trap in the motor, the more power it will make. If you know the pressure (BMEP) and temperture of the air, you can calculate the
volume and density and predict power output.

Yes, BMEP means brake mean effective pressure. Its formula is

BMEP = (HorsepowerX 792000) / (Engine displacement X RPM)

It has nothing to do with the average cylinder pressure within the cylinder. Rather, it provides method, numericaly, to describe & compare the specific power output of an engine at given displacement and engine speed. These comparisons can be done either with a given engine displacement along its operating speed range, or by comparing 2 different engine displacements at a constant or variable speed condition.

For example:
300 BHP @ 5000 rpm 350 CID = 135 BMEP

300 BHP @ 5000 rpm 400 CID = 118 BMEP

The 400 CID engine has lower bmep, indicating the stress levels all the compoents ( block, head , pistons, etc) see are lower relative to the 350CID engine, most other things being held constant. On the other hand, you could say the 350 is running at a higher potential performance level than the 400.

Try different examples of bhp, cid, & rpm and it allows you to understand performace potential & relative stress levels of different combinations. Allows engine designers to understand where they were and where they are going, & the price they are going to pay for a different design.
Regards, FJK