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
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.
Severe engine damage can result from
operation above 75% power with an excessively lean mixture. At a pressure altitude of 7000
feet, the engine produces only about 75% power at full throttle and can tolerate leaner
mixtures. Consult the engine manufacturer's operating manual for proper leaning procedures
for fuel injected engines.
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
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.