I have an IO-550 N that I am getting ready to install in my Legacy.  It was
built with GAMI injectors and I am planning to use the Lancair Ram Air.

It has been suggested by a knowlegeable source that I will need to
pressurize my injectors (like a turbo) because of the difference between the
induction air pressure and ambient air pressure (about 1.5 inches) or else
two things will happen:

1)  There will be some fuel leakage and staining around the injectors
2)  The "atomization" of the fuel will be affected and the engine will have
trouble running smoothly lean of peak.

Mike Mahar has RAM air pressurizing rails to deal with this issue.

On the other hand, another knowledgeable source agrees with number 1 above,
but disputes number 2.

Since I hope to run my engine lean of peak smoothly, do I need to pressurize
my injectors to have them properly "atomize" fuel?  Or is the issue of
pressurization merely one related to a small amount of fuel leakage and
staining?

Doug,

 

In a NA injected engine standard installation, induction air may be drawn from cooling air above the engine or from some other non-efficient opening and always filtered.  In such cases at WOT, the induction air pressure (MAP) seldom exceeds the pressure available to the air hole in normal fuel injectors.  Of course at reduced throttle, such as idle where the MAP may be 10-12" Hg while the ambient pressure in the cooling compartment may be 29" Hg, there is a dramatic pressure difference.  In other words, normally the injector air hole uses air at the same or higher pressure than that in the induction system. 

 

A turbo charged engine can produce MAPs much higher than that available to the injector - so high that fuel can be forced out of the hole.  This problem is solved by using shrouded injectors fed by "deck" pressure - i.e. the injector see air pressure at least as high as the induction system.

 

Thus, a 200 Knot Lancair using a non-filtered efficient ram air system at WOT might see a MAP with as much as a 2" rise over the ambient pressure (at 240 Kts, the rise is 2.9" Hg or 1.4 psi).  Of course, air available in the cooling plenum is not at ambient pressure, but somewhat higher although certainly less than that seen in the induction system.  This condition does not help atomization.  Can the fuel run back out?  I never saw any in my 320, but you are going to be flying a much faster Legacy.

 

There is another at least as important benefit.  Consider that GAMI injectors control fuel flow to allow matching the power produced in each cylinder, especially for smooth operation LOP.  I believe that such injectors compensate for induction air differences and possible imbalances introduced by differing air pressures available at the different injector locations.  If air is sent equally to each injector, part of the problem is solved.  I have used such a system to help equalize the power developed in each cylinder.  Furthermore, it doesn't matter what else I do in the cooling plenum because any change to airflow or pressure there will not affect the injectors.  Others that have tried this simple change have noted that the cylinder head temps are closer together without any other change.  If I could further benefit from tailored GAMI injectors, I would seek them.  I currently see no more than .2 -.3 gph difference between cylinders after performing the GAMI Lean Test (note that my engine monitor doesn't have the fineness to know more precisely).

 

Although tapping the ram air plenum for use by the shrouded injectors is desirable, it was not so for me.  Thus, I used a separate "pitot" to feed a small plenum (remember that little air is actually used by the injectors):

(the pitot can be seen sticking out from the right air inlet and the plenum is in the middle of the pic)