If you really want to know the actual CFMs, contact an HVAC company. They can measure using a Volumeter made for balancing systems.
You should be exhausting from down low and makeup air should come in high.
Here is an explanation of fan performance... ftp://www.nyb.com/Letters/EL-03.pdf
As far as what should the CFM be... if your engine compartment is 10 cubic feet and you are able to move 10 CFM, you would exchange all the air in that compartment in 1 minute. Running that fan for 5 minutes would exchange that air 5 times.