A Velocity air meter
measures the force of air rushing into an engine against a spring-loaded door in an air tunnel. A linear potentiometer
rotated along with the hinge of the air door measures the angle of the opening and sends an electrical signal to the engine control unit
(ECU) that changes as the door opens more or less. The ECU can deduce airflow into the engine based on this value after making certain corrections. The problem is that the force against the door is a function of two variables - weight of the air and speed of the air - and the ECU cannot distinguish , for a given door angle, whether the air is less dense but traveling faster or whether the air is denser but traveling more slowly. Therefore air velociy must be corrected for air density (temperature and barometric
pressure) in order to reflect air mass (actual number of air molecules entering the engine per unit of time).
Velocity air meters from Bosch
have been commonly used on systems such as older Bosch L-Jetronic and as a part of many factory systems, though they now have mostly been superseded by mass airflow sensors which have many advantages and are less damaging to engine volumetric efficiency
(VE). Velocity air meters can be rather restrictive, particularly when engines have been modified for significantly greater airflow than stock (e.g. by fitting turbochargers
). Velocity air meters have the additional disadvantage that, once the door is fully open, there is no way for the sensor to measure further increases in airflow.
Because they can be restrictive and because they rapidly run out of measurement upside on the high-end of airflow, VAF meters are virtually never used with aftermarket electronic fuel injection (EFI). They are often replaced on factory systems with speed-density or MAF conversions. Some programmable engine management systems like Motec's can be configured to access VAF as engine loading information.
With L-Jetronic type airflow meter, adjustment is accomplished by tightening the spring tension. One notch is approximately equal to a 2 percent change in fuel mixture. For example, if you change to 20 percent larger injectors, you should tighen the spring by 10 notches. Verify fuel mixture by checking with a wideband meter to verify that AFR's (air/fuel ratios) are correct under wide-open throttle and adjust if necessary.
This type of airflow sensor was used mostly on European imports equipped with Bosch L-Jetronic fuel injection, Japanese imports equipped with Nippondenso multi-port electronic fuel injection, and Ford vehicles equipped with Bosch multi-port EFI (EscortLynx, Turbo Thunderbird, Mustang with 2.3 liter turbo engines, and Ford Probe with the 2.2 liter engine) .