Imagine a turbocharger trying to pressurize a carburetor with its small venturis and finicky floats and you can get some idea why electronic fuel injection opened the door to big boost, big power, big reliability, big fuel economy and a cleaner burn. Some of the more prominent early beneficiaries of the marriage of turbos and EFI include the '86 and '87 Buick Grand National, the '84 to '87 Z31 300ZX Turbo, '84 to '89 Mitsubishi Starion ESI-R and '87 to '92 Mark III Supra Turbo. These 'founding fathers' laid the foundation for the prodigious power production we see in today's factory turbocharged offerings. The accompanying power-per-liter chart tells the tale:
The basic EFI set-up has remained fairly unchanged through the years. However, the precision and speed of the sensors and computers (hardware and software) have improved with the times, as has the programming prowess of the OEs and the aftermarket alike, hence the big numbers. Electronic fuel injection consists of an ECU that makes fuel and ignition tuning decisions based on input received by a variety of sensors.
MAF vs. MAP
The headline sensor in a Mass Air or Mass Flow type of EFI system is the Mass Air Flow (MAF) sensor. The other popular type of EFI system, Speed Density, features a Manifold Absolute Pressure (MAP) sensor as its frontline warrior.
A Mass Air Flow sensor is usually mounted near the air filter box or somewhere before the throttle. It meters the amount of air going into the engine. There are two different types of Mass Air Flow sensors: Hot Wire and Vane Meter.
A Hot Wire design, the predominant MAF sensor in use today, places an electrically-charged hot wire in the air stream and the sensor reads changes in the resistance of the wire brought on by the temperature/density of the air flowing past it. The sensor tries to compensate for the resistance in the hot wire and keep the resistance constant. During the act of rebalancing, the resistance is converted into a voltage signal of between 0 and 5 volts and sent to the ECU.
The Vane Meter uses a flapper door contraption that is lightly spring-loaded. As the air comes into the engine the movement of the flapper generates the 0 to 5 volt signal that is forwarded to the ECU. The drawback of this sensor is its potential as a restriction as air volume is increased, i.e. turning up the boost. This type of sensor has been out of fashion for sometime.
A Manifold Absolute Pressure sensor is an entirely different beast used in a speed density set-up. It does not meter flow; it is basically a pressure meter that converts the vacuum or pressure in the manifold into a signal that the ECU can use in accordance with other sensor inputs to make tuning adjustments.
The Supporting Cast
While all sensors perform an important job, the Throttle Position Sensor (TPS) has a profound effect on the operation of any EFI system. It's a rotary style sensor that has a voltage for idle, zero throttle and wide-open, 100-percent throttle. It forwards this voltage that represents the percentage of throttle opening to the ECU. The TPS is also able to interpolate the speed or rate in which throttle position is being changed from moment to moment, which then allows the ECU to 'predict' future fuel/ignition events.
The ever-important Oxygen Sensor provides a scorecard or report card to the ECU. It is placed on the exhaust side, usually in the header, where it measures the amount of oxygen in the exhaust gases. The data is used to determine the air/fuel ratio so the computer can lengthen the pulse duration when too much is present and more efficiently burns the air/fuel mixture. It should be noted that this occurrence is more for catalytic converter efficiency than an eye-opening power gain.
By Evan Griffey
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