Inlet Airbox

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 airflow 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. 

In general, the efficiency of the induction air inlet can be judged by engine smoothness at full throttle and the extent to which the engine can be leaned at cruise power. An inlet with good flow characteristics will allow an engine equipped with a fixed pitch propeller to run smoothly with the mixture leaned 200 RPM below peak power when operating at or below 75% power. An engine equipped with a constant speed propeller should demonstrate smooth operation when leaned to peak exhaust gas temperature while operating at or below 75% power.

Good Inlet Configurations

Fig. 6-1 through 6-3, illustrate good inlet configurations. These promote excellent cylinder to cylinder fuel distribution because air enters the Throttle Body Injector inlet uniformly from 360 degrees around the inlet centerline.

Good Inlet Designs

Inlet Configurations to be Avoided

Inlet configurations such as shown in Fig. 6-4 and 6-5, require intake air to undergo a sharp 90 degree bend while entering the Throttle Body Injector, causing some of the metered fuel to be deflected against the throat wall. Full throttle operation will be rough due to poor fuel distribution, and the engine will have little tolerance for operation on lean mixtures at cruise power settings.

Bad Inlet Designs

Some configurations which do allow 360 degree air delivery like the one shown in Fig. 6-6, will experience problems at full throttle due to the short vertical distance between the Throttle Body injector and the opposite air filter flange. This configuration promotes the formation of a standing vortex in the inlet bell mouth, reducing the airflow capacity of the Throttle Body Injector with resulting full throttle roughness and loss of power.

Improving The Performance of 90 Degree Inlets

The performance of engines with bending inlet flowpaths can be improved by increasing the bend radius or by providing a straight section of duct between the Throttle Body Injector and the bend.  Alternatively, a 90 degree change in airflow direction can be accommodated by feeding the Throttle Body Injector from a relatively large volume plenum chamber as shown in Fig. 6-7.  Dimensions shown in this illustration should be considered minimum. Increasing any of the dimensions will result in improved fuel distribution.

Large Volume Plenum Chamber

If a Throttle Body Injector is to be installed with existing intake ducting, then ground tests should be conducted to determine whether any performance deficiencies exist. If any adverse symptoms are noted then the information contained herein should be used as the basis for designing a new inlet configuration.

Optional Flow Grid

In some installations with confined spaces or poorly configured inlet airbox arrangements, and where unstable operation is encountered at higher power settings, it may be possible to correct these problems with the addition of an airflow straightening grid.  These may be obtained from Aircraft Spruce and Specialty, see  http://www.aircraftspruce.com/catalog/eppages/carburetorheatbox.php.