Plenum Box

Airflow measurement and fuel scheduling in the Ellison Throttle Body Injector (TBI) are accomplished by sensing both the direction and velocity of air flowing past the metering tube.  This means that engine performance can be adversely affected by air which enters the TBI inlet with substantial directional bias.  Such directional bias can be caused by any inlet configuration which forces induction air to undergo a sharp turn as it enters the TBI inlet.  Subtle roughness at wide open throttle is the result of such an inlet condition.  When replacing the Marvel Schebler carburetor with the Ellison TBI, it is sometimes necessary to re-design the air inlet box to encourage smooth air delivery.  On some tightly cowled aircraft, clearance problems may be relieved by installing a plenum box between the sump and TBI.

To gain much needed cowl clearance, Ellison Fluid Systems, Inc. has designed a plenum box (click to see a dimensioned sketch of the box) which mounts to the standard Lycoming updraft sump and allows the TBI to be mounted in a horizontal draft orientation. The plenum box gives about 3 inches more cowl clearance than a standard TBI/airbox combination and allows a straight in ram duct from the front of the cowl.

There are however, disadvantages to using the plenum box.  The acceleration performance is somewhat degraded, meaning that rapid opening of the throttle is more likely to cause the engine to stumble, especially in cold weather. Also, the TBI / Plenum box is very susceptible to ice formation. An intake air heat system capable of a 90 deg. F. temperature rise is mandatory! There has been one fatality resulting from engine failure due to intake system icing. This engine was equipped with a similar plenum box without a functioning intake air heat system.  Possible plenum box configurations are shown below.

Here are some sketches of possible plenum box configurations.

Click for plenum box dimensions.