Posted for "Metcalfe, Lee, AIR" <Lee.Metcalfe@jocoks.com>:

Duane:

Scott Krueger and I have very similarly configured IO-320s in our LNC2s
and there are no significant differences in our airframes, except that
Scott has the forward-mounted fuel servo with a straight-in ram tube
from the chin scoop (a beautiful piece of work, I might add), where I
have the updraft servo with ram into an airbox.  Neither of us were
running air filters.  Last weekend we flew together and at wide-open
throttle at 7500' he was able to pull about .6 or .7 more MAP than me at
about the same airspeed (he was actually pulling away from me with a 2
or 3 knot advantage).  With MAPs matched at 24.5 and RPM at 2500 we were
the same speed.  As a result of this comparison, I am seriously thinking
about switching to the forward-facing servo with direct ram, using a 90*
elbow from the servo to the sump flange.  I don't know how this compares
to the set-up where the servo mounts to the front of the sump, but Larry
Henney is of the opinion that the air flow characteristics of the
front-mount sump may not be as good as the updraft sump, thus detracting
to some degree from the advantage of the direct ram into the servo.  I
read somewhere that there is another advantage to the forward-facing
servo with direct ram.  With the air flowing directly into the servo on
the same axis as the servo you get more even pressure on the four sensor
tubes that project into the airflow and thus better functioning of the
servo in fuel metering.  Using the updraft servo with an airbox, the
incoming air has to make a 90* turn from aft-flowing to upward-flowing
in a very confined area (about 1" of vertical space in my case), thus
creating a lot of turbulence at the inlet to the servo, right where the
sensor tubes are.  The article said the solution to this problem is to
use a large airbox with an automotive-type round filter to settle the
air down before it enters the servo (my paraphrasing).  I don't remember
where I read this, but when I get some time today I'll try to find it.
It was a pretty good article, as I recall.

Lee "Moondog" Metcalfe
N320WH - LNC2 - IO-320 - 492 hrs. - Kansas City (IXD)