From: "Gary Casey" <glcasey@adelphia.net>

<<Also, I am interested to know about how much
 manifold pressure is lost using a good air filter system versus an unrestricted fresh air system (no filters). I am building an alternate air system for my IO550G that will flow unfiltered air and I would like
to know what improvement, if any, I can expect in the numbers.>>

I will offer some comments, not from experience, and partly to attract other more comments on the subject.  I'm not yet to the point of thinking about the inlet system on my ES.

1.  The actual pressure drop across an adequately-sized air filter isn't much - a few inches of water maybe.  I did a very crude  test on my IO-360, doing a full-throttle run-up on the ground with and without the filter element.  I couldn't detect a difference in the manifold pressure reading. You've got to be looking at less than 3 inches of water (13 inches of water is about 1 inch mercury).  While that is small it is still real.  1 inch mercury increase equates to about 3% more power.

2.  The normal location for  the inlet in the top plenum is probably not the optimum, mostly because, in my experience, there is always  a lot of turbulence and air mixing in the plenum.  some of that air has already been heated, so you're likely going to get air that is a little warmer than ambient.  For every 5 degrees of heating you are going to lose about 1% power.  The heating is probably less than 5 degrees, but still real. 3.  The pressure in the upper cowl is probably less than  the total (ram) pressure.  I measured the cowl pressure in my C177 and got mixed results -  it was quite a bit less than total pressure, though.  I forgot off-hand what the ram air pressure is at different speeds, but as I  recall at least 2 inches mercury is available at your airspeeds.  In my mind the alternate air source should have its own dedicated inlet to capture as much as possible of the ram air pressure.

In summary, the combined effect of all three factors would be very significant if everything were taken advantage of.  Just by-passing the filter is probably the least advantageous of the three. This means that a dedicated inlet has to be used and the valve has to shut off the air cleaner path and provide a restriction-free path from the air inlet. I remember the old Mooneys that had a "ram air" inlet, but all it really did was by-pass the air filter.  With it on (so I was told) air would come in the inlet and was free to flow back out the air filter.  Look at the recent change to the PT-6 air inlets - they were moved from a "styled" inlet back about a foot from the prop to a location right at the prop - significant performance increases resulted.  A few 320, Glasair and RV builders have now started to take advantage of the idea and moved their inlets forward.

                 Gary Casey
                 ES project