I have the Airflow Performance ram air valve on my Lycoming IO360.
This valve does not close off the filter when ram air is selected.  I
have wondered how much ram air pressure I was losing back through the
air filter (and believe this has been debated on the LML).  According
to the following test by an RV builder, with a similar installation to
mine, there is no loss.  See the attached RV link and a photo of my
installation.

Clark,

 

1. MAP is a measure of the smoothed average absolute pressure in the induction system typically as taken from a primer port in the cylinder head.  Valve clatter notwithstanding.  Air is being drawn out of this system by that air pump known as the engine.

 

2. A well constructed cowling with a well sealed engine baffling system is itself a ram air chamber with air being drawn off by the engine cooling function and, perhaps, by the oil cooler and induction system, too.  For example, we expect to see 5+ inches of water as a minimal pressure difference between the upper and lower cowl for reasonable engine cooling. (5.4" H2O = .4" Hg = .2 psi).

 

3. A fair ram air system with an opening back to the upper cowl might indeed see only a small rise if the  upper cowl is closed off because when open, the upper cowl air pressure may be significant, thus preventing serious loss of pressure.  However, .4 "Hg MAP loss is approximately 1.5% loss of possible engine power.

 

4.  The recent discussion was about a filter open to the lower cowl with the combined effect of higher induction air temperature and even lower "back pressure." on the filter - yet is seemed not to have much of a loss in MAP.

 

5.  Some of us have the cowl positioned landing light and do not have room for a large air duct, let alone a filter box in that area.

 

It would be interesting if you could do some reporting on the performance of your system - assuming duct ape can really seal off the upper cowl.