The upper to lower cowl pressure differential was 9.3" H2O, certainly enough for cooling.

 

A secondary altimeter, open to the cockpit, showed 3280 MSL or a reading of 380 feet higher or 5.2" H2O less pressure than ambient.

 

The upper cowl to cockpit pressure differential was 16.5" H2O (corrected to 11.3" H2O relative to static or ambient pressure).

 

The lower cowl to cockpit differential was 7.12" H2O (corrected to 1.92" H2O to ambient).  Note that there is evil interest in fouled lower cowl air breaching the even lower pressure cockpit.

 

Upper-Lower pressure difference calculated indirectly = 9.38" H2O (close enough to that measured directly).

 

Other interesting data bits:

 

Opening the vents at 186 KIAS dropped the secondary altimeter to 3230 MSL, about 50 feet or .7" of H2O.

 

I slowed to 150 KIAS and the open vented cockpit read 3080 or 180 feet higher than actual altitude.  Then the vents were closed and the cockpit climbed to 3120 or 220 feet higher than actual with a 40 foot difference from vents open.

 

The main test certainly supports the argument of why the firewall must be sealed - In my case, the cockpit/lower cowl pressure difference of  7" H2O would definitely suck/blow in any bad engine compartment fumes, maybe even a little CO or flames - whatever is uh, appropriate....