Speed Density
Speed Density systems accept input from sensors that measure engine speed (in rpm) and load (manifold vacuum in kPa), then the computer calculates airflow requirements by referring to a much larger (in comparison to an N Alpha system) preprogrammed lookup table, a map of thousands of values that equates to the engines volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air density changes with air temperature, an intake manifold-mounted sensor is also used.

Production-based Speed Density computers also utilize an oxygen (O2) sensor mounted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. This helps compensate for wear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a coolant-temperature sensor, and a knock sensor as a final fail-safe that hears detonation so the computer can retard timing as needed.

GMs Tuned Port Injection (TPI) set-ups used Speed Density metering from 90-’92, as did 91-’93 LT1 engines. All 86-’87 and 88 non-California Ford 5.0L-HO engines used Speed Density metering. Most Mopar fuel- injection systems have used Speed Density too.

Because a Speed Density system still has no sensors that directly measure engine airflow, all the fuel mapping points must be preprogrammed, so any significant change to the engine that alters its VE requires reprogramming the computer.

Mass Flow
By contrast, Mass Air Flow (MAF) systems use a sensor mounted in front of the throttle body that directly measures the amount of air inducted into the engine. The most common type of mass-flow sensor is the hot wire design: Air flows past a heated wire thats part of a circuit that measures electrical current. Current flowing through the wire heats it to a temperature that is always held above the inlet air temperature by a fixed amount. Air flowing across the wire draws away some of the heat, so an increase in current flow is required to maintain its fixed temperature. The amount of current needed to heat the wire is proportional to the mass of air flowing across the wire. The mass-air meter also includes a temperature sensor that provides a correction for intake air temperature so the output signal is not affected by it.

The MAF sensors circuitry converts the current reading into a voltage signal for the computer, which in turn equates the voltage value to mass flow. Typical MAF systems also use additional sensors similar to those found in Speed Density systems. Once the electronic control module (ECM) knows the amount of air entering the engine, it looks at these other sensors to determine the engines current state of operation (idle, acceleration, cruise, deceleration, operating temperature, and so on), then refers to an electronic map to find the appropriate air/fuel ratio and select the fuel-injector pulse width required to match the input signals.

GM used MAF sensors on the turbo Buick V-6 Grand National, 85-’89 TPI, 94-’98 LT1, 96 LT4, and all LS1 engines. Ford has used MAF metering on 88 California 5.0L engines and all 89-and-later V-8 engines.

MAF systems are much more flexible in their ability to compensate for engine changes since they actually measure airflow instead of computing it based on preprogrammed assumptions. They are self-compensating for most reasonable upgrades, as well as extremely accurate under low-speed, part-throttle operation. On the other hand, the MAF meter, mounted as it is ahead of the throttle-body, can become an airflow restriction on high-horsepower engines. On nonstock engine retrofits or EFI conversions on engines never produced with fuel injection, it may be hard to package an MAF meter within the confines of the engine bay and available intake manifolding.