Screaming
along at 2900 MSL (25.5" MAP, 2550 RPM, 11 C) and with the
cockpit vents closed, I was showing 186 KIAS.
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....
Scott Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)
PS Piston powered Lawn Darts can be dangerous if you are
stuck on stupid.
PPS My firewall has been sealed from day one. In
addition to structural tie-wraps, red silly-cone has been liberally applied to
cracks and crevices.