SENSOR LIFE AND CALIBRATION
When used in a racing application with leaded
petrol, sensor life will probably be less than 10 hours.
Free air calibration should be performed on a regular
basis, such as before the start of every test session or
race event. If free air calibration fails, the sensor should
be replaced. Free air calibration must be performed in
an environment free of hydrocarbon vapors. Typical race
shop environments may prove to be too contaminated.
Even outdoors, free air calibration can fail if a carburetor
bowl has recently been removed or another vehicle is
running nearby. In general, sensors that are at the end of
their useful life will fail free air calibration.
OPERATION
The Control Unit has red status LEDs for each
channel. When power is turned on, the LEDs blink at a
slow rate until the corresponding sensor has reached
normal operating temperature.
After installation, the Control Unit requires free air
calibration. This should be done with the sensors dangling
in free air. The environment must be free of hydrocarbon
vapors. We suggest that you perform the free air
calibration outdoors. Turn the free air calibration trimpots
on the Control Unit full counterclockwise. Turn on power
and wait for 60 seconds so the system can fully stabilize.
Then slowly turn each free air calibration trimpot clockwise
until the corresponding LED starts flashing at a rapid rate.
Try to set each trimpot at the point where its LED just
starts to flash.
The free air calibration procedure should be
performed at reasonable intervals (every 250-500 hours if
using unleaded petrol or every 2-5 hours if using leaded
racing petrol) or whenever a sensor is replaced. If you
cannot get an LED to flash when its trimpot is turned full
clockwise, you either have a damaged sensor or very high
hydrocarbon levels in your environment.
The Control Unit includes internal diagnostics for
abnormal battery voltage (less than 11 volts or greater
than 16.5 volts), sensor open circuit, and sensor short
circuit conditions. A fault condition causes the status LEDs
to blink at the slow rate.
EXHAUST CONSIDERATIONS
The Control Unit system may give inaccurate
results in certain situations:
Excessive exhaust back pressure.
Wide-band
sensors are affected by back pressure. Excessive back
CAUTION: Racing petrol containing lead
will quickly degrade the sensors. Under
these conditions, expected sensor life is
less than 10 hours. There is no warranty
on sensors.
pressure causes exaggerated AFR indications under
rich and lean conditions, but has little effect at 14.7 AFR
(stoichiometric). Race vehicle exhaust systems are free
flowing and problems with exhaust back pressure are not
likely.
Exhaust reversion.
Reversion is the term for a
negative pressure wave that can suck ambient air back into
the exhaust and cause an erroneous lean AFR indication.
Open “drag pipes” usually suffer from reversion effects and
may not be suitable for use with the this system except at
or near wide open throttle. Reversion effects will be most
noticeable at idle, part throttle low RPM, and deceleration.
Excessive scavenging.
Tuned exhausts in
combination with a high overlap camshaft profile can pull
unburned air and fuel mixture through the cylinder into the
exhaust and cause an erroneous rich AFR indication. The
same effect can occur with high boost turbo/supercharger
applications.
Misfiring.
If the AFR is so rich that the engine
misfires, high levels of oxygen will remain in the exhaust
gas and result in an erroneous lean indication.
ENGINE TUNING GUIDELINES
Higher AFR values correspond to a leaner (less fuel)
condition. The practical operating range for most engines
using gasoline fuel is from approximately 11.5 to 14.7 AFR.
Combustion of a stoichiometric mixture (exactly enough
air to burn all the fuel) results in 14.7 AFR indication.
Automotive engines with catalytic converters operate near
14.7 AFR during cruise and idle. Race engines usually
require a richer mixture to limit cylinder head temperature
and prevent detonation. The table below lists reasonable
AFR values for race engines without emission controls.
Operating Mode
Recommended AFR
Cold Start (first 30 sec) 11.5-12.5
Idle
12.8-13.5
Part Throttle Cruise
13.0-14.0
Wide Open Throttle
12.5-12.8 (values down to
11.5 may be used to
reduce detonation)