<<I have a question about the ram air design you and others
(including factory Mooney's) use.  If you pick up
sand/dirt/rocks while running filter air, won't they just
sit in front of your valve (inside bottom of the scat
hose)?  Then wouldn't it just get sucked into the engine
when you open the valve to ram air?>>

I haven't gotten to the point where I need to do something about this, but
here are some thoughts:  Bypassing the filter at altitude may be a
reasonable thing to do, but to really capture "ram air" I think you need to
do more than just pick up pressure in the cowl.  The pressure above the
engine in the cowl is probably not very close to the maximum pressure
available.  To really get ram air you probably need to provide a separate
inlet that is as close to the propeller as possible.  As in some of the
320/360's where the builder has extended the inlet forward.  Then the ram
air valve has to be made so that it shuts off the passage to the air filter
when it opens to ram air, thus avoiding the venting of the pressure through
the filter.  The Mooney's did it wrong, too, as they were built to
essentially just bypass the filter - ram air pressure wasn't captured
because the air could flow backwards through the filter.  I plan to use an
oversize filter and plumb it directly to a ram air source and not have a
valve.  Alternate air will be through a spring-load door just like on my
C177.  Simplicity.

Gary Casey