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Ram Air a Benefit?

Posted By:
Bruce Hinds
#1 Posted: 10/12/2010 19:22:54 Modified: 10/12/2010 22:32:09


My intake gets no ram effect on our Republic Seabee as it sits on the cabin roof just inside the cowling.  I've noticed many other aircraft have the carb airbox mounted to get a ram effect.  Is there any way to know if it would help to change the angle of the airbox or put a scoop on it?  I've calculated the velocity of the airflow at cruise power to be about 45 mph. 
Would any pressure be gained at 100 mph to increase Manifold Pressure?

I'm not looking for much of a boost, but 2-3 inches of MP would make a big difference for cruise at 5-9000 feet



Bruce Hinds
Ron Blum
Young Eagles Pilot or VolunteerHomebuilder or CraftsmanAirVenture Volunteer
#2 Posted: 10/14/2010 21:56:01

If the carb air box is inside the cowling and the cowling is a plenum chamber, it is getting ram effect as the plenum chamber is at total (pitot) pressure and not static (altitude) pressure.:\)

Thom Riddle
#3 Posted: 10/15/2010 06:45:23

Back in the late 60s I flew Mooneys fairly regularly. At that time, Mooneys had what some called a poor-man's supercharger. At altitude where the air was cleaner, the pilot could bypass the normal air filter and get direct un-filtered ram air. If I remember correctly, at typical Mooney airspeeds, it was worth about 1" Hg in manifold pressure. At slower speeds the ram air effect would be somewhat less.

Sonja Englert
Homebuilder or Craftsman
#4 Posted: 10/15/2010 18:58:34

The ram air effect to increase manifold pressure is useful only at speeds of >180 kts. The Mooney manifold pressure "increase" is actually the removal of the loss from the airfilter, which is about 1 inHg. Without the airfilter, the pressure is back to where it should be and does not change much at Mooney speeds.