Carb vs. EFI: How It Works

EFI Sensors Explained
A powerful EFI setup needs a mix of sensors to monitor engine conditions and deliver the right amount of fuel and optimal ignition timing. Engine rpm can be read from a factory-style distributor or some type of crank trigger. Other sensors are required to keep tabs on the engine, so we compiled a list of those you're most likely to encounter.

ATS: Air Temperature Sensor. The ATS is calibrated to deliver the temperature of the air entering the engine. It's usually plumbed as close to the throttle body as possible (i.e., at the base of the air cleaner) to give the most accurate temperature reading.

CTS: Coolant Temperature Sensor. The computer has to know the coolant temperature to trim the cold-start and warmup circuits to maintain all-around driveability. The CTS is plumbed into a coolant passage (usually somewhere on the intake manifold in OEM applications).

Knock Sensor: Often mounted directly to the engine block, the knock sensor senses detonation in OEM engines and adjusts ignition timing to stop it. Radical, noisy cam profiles and loud exhaust can "sound" like detonation to a knock sensor, so for that reason, it's usually not present on highly modified engines.

MAF: Mass Air Flow sensor. Plumbed into the intake tract, the MAF sensor tells the computer how much air is entering the engine so it can supply the right amount of fuel. However, most aftermarket EFI systems are speed-density, which means there's no MAF sensor to directly measure the volume of air entering the engine. Instead, the computer uses signals from the TPS and MAP sensor, and knowing the engine rpm, calculates how much air is fed into the motor and supplies it with the right amount of fuel.

MAP: Manifold Absolute Pressure sensor. This sensor is plumbed to a manifold vacuum port and measures the difference between the ambient air pressure and the pressure (or vacuum) inside the intake manifold. A one-bar MAP sensor is designed for normally aspirated engines, while forced-induction motors require either a two-bar (1 to 20-psi max boost) or three-bar (21 to 30-psi max boost) sensor.

O2 Sensor: Oxygen Sensor. Simply put, the O2 sensor compares the amount of oxygen in the exhaust stream with the air outside the engine. It outputs a voltage to the computer--a feedback signal--that the computer uses to trim the amount of fuel depending upon whether the exhaust indicates it's too lean or too rich. A narrow-band oxygen sensor outputs any voltage from 0 to 1 V--a high voltage if the air/fuel ratio is richer than 14.7:1, and a low voltage if it's leaner. It just doesn't do so very accurately or in a linear fashion. A wide-band O2 sensor (WBO2) can sense a range of air/fuel ratios, from a rich 10:1 to a lean 20:1, with reasonable accuracy. Knowing the exact air/fuel ratio makes the WBO2 an immensely useful tuning aid.

TPS: Throttle Position Sensor. Mechanically coupled to the throttle shaft, the TPS signal is critical to proper idle function and acceleration enrichment.