A hot-wire mass air-flow sensor. Notice the wire mesh?
On electronic fuel injection (EFI) cars, the air-flow sensor or the air-flow meter could be an air flow restriction, depending on which type of air-flow sensor is used. There are three common types of air-flow sensors used on car engines:
- The hot-wire mass air-flow sensor, which does not cause much air flow restriction.
- The vane-type air-flow sensor, which has a spring tensioned swinging flap.
- The Karmen vortex type air-flow sensor, which is used mainly by Mitsubishi.
Hot-Wire Mass Air-Flow Sensor
The hot-wire air-flow sensor, which is often called the mass air-flow sensor, doesn't present much restriction to the air flow as it is usually about the same size as the bore of the air intake duct. If you need to increase the bore of a mass air-flow sensor, you can swap out the mass air-flow sensor for a larger one from the same car manufacturer, but you would need to reprogram the engine control unit (ECU) so that it can accurately measure the increased air flow rate. If your stock ECU is not reprogrammable, you may need to install a programmable aftermarket ECU.
These air-flow meters usually have a fine wire mesh at either end of the bore. Removing this wire mesh will lead to a 1½ to 2% increase in power at maximum RPM.
Vane-Type Air-Flow Sensor
The vane-type air-flow sensor, with its swinging flap is more reliable than the hot-wire mass air-flow sensor but it does present some air flow restriction. You can swap out these air-flow sensors for larger units, but you would need to reprogram the ECU to ensure the correct air flow readings.
Alternatively, you could reduce the spring tension on the swinging flap. However, this will lead to the flap swinging open further, and reaching full-open before peak air-flow is reached. The ECU calculates the air-flow rate on how far and how fast the flap swings open; therefore, the ECU will need to be reprogrammed. However, the flap should never reach full open before peak air-flow is reached as the ECU would not be able to read increases in air flow once the flap is full open. No amount of reprogramming will get the ECU to increase fuel delivery if it is not able to sense an increase in the air flow rate.
Karmen Vortex Air-Flow Sensor
The Karmen vortex-type air-flow sensor that is used my Mitsubishi is an oscillatory air-flow sensor that measures the vibrations of the downstream vortexes caused by the barrier placed in the air stream. These air-flow sensors represent the greatest restriction to air flow, and they are the most difficult to adapt for better air flow. Some tuners use a bypass around the air-flow sensor, but this is terrible for modified street cars as the air flow will bypass the air-flow sensor at idle and during cruise conditions.
The best solutions on these engines is rather expensive. It entails a second air intake controlled by a secondary throttle plate, a MAP controlled ECU and secondary fuel injectors. This secondary air intake branches into the air intake duct after the air-flow sensor. Under normal conditions, air flows through the stock air intake and through the air-flow sensor; but under heavy acceleration and at high RPM, the secondary throttle plate is opened by a pressure-sensitive solenoid switch to allow air through that secondary air intake system. A MAP sensor controlled ECU is then used to deliver the extra fuel through a secondary set of fuel injectors.