The dynamics may change at cruise and you have a slight positive
  pressure in the cabin,

Chuck,

 

I have yet to hear of an unpressurized high performance composite aircraft with a positive cabin pressure - even when all the cabin vents are open - even when it is rear engined.

 

One of the tests that should be done in phase 1 is opening the static to the cabin at cruise and slow flight, with and without vents open to understand the speed and altitude changes (higher altitude, faster indicated speed) you will see using such alternate static.  It should be recorded in your POH or placarded.

 

As I reported once before, at 180 KIAS, the pressure drop in my cabin is .38" Hg less or about 380 feet higher at 3000 feet.  There is only a .05" Hg increase when the vent is opened while using cabin static pressure.  At approach speed (110 KIAS), the difference was about .1" Hg. or a 100 foot higher reading.  I don't have access to my pressure/air-speed table and I can't remember the exact speed change numbers.

 

The fact that the cabin is alway less pressure than the lower cowling (it is always more than static) and static means that bad stuff can enter firewall openings (front or rear engine) or other openings that exhaust fumes pass by.

 

Scott Krueger