Looks like everyone has things pretty well covered.
I installed the Moroso Crankcase Evacuation Kit around 2002 on my Charger. It pulls a fair vacuum at idle and improves with increased RPM. I mounted Baffle Plates inside the valve covers beneath the breathers to reduce/eliminate oil loss.
http://www.moroso.com/eb/web/instructions/25900_inst.pdfLiked the end results so much, I made my own kit from hardware store 1/2-inch pipe, Smog Pump Check Valves, vinyl tubing, breathers and grommets, mounted to the headers, for use on the V8 engine in my CJ5 Jeep.
As suggested, people think about the air/fuel mixture above the piston, but may forget about the air below the piston in the crankcase / cylinder block. That air has to move around in the crankcase and energy is wasted in the process and it adds to crankcase windage.
Someone figured this out many years back, maybe it was a NASCAR engine builder, at least that was what I had heard back in the day, but maybe it was some amateur racer? By connecting a vacuum pump and creating a partial vacuum in the crankcase, horsepower increased.
Bracket racers used to convert Smog Pumps into vacuum pumps for a similar purpose.
In any event, there are benefits to creating a partial vacuum in the crankcase.
