pressure (filling the vacuum). A carburetor or fuel injection system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a naturally aspirated engine, output power is limited by the ambient barometric pressure. Superchargers and turbochargers boost manifold pressure above atmospheric pressure. Modern developments Modern engines use a manifold absolute pressure (abbreviated as MAP) sensor to measure air pressure in the intake manifold. Manifold absolute pressure is one of a multitude of parameters used by the engine control unit (ECU) to optimize engine operation. It is important to differentiate between absolute and gauge pressure when dealing with certain applications, particularly those that experience changes in elevation during normal operation. Motivated by government regulations mandating reduction of fuel consumption (in the USA) or reduction of carbon dioxide emissions (in Europe), passenger cars and light trucks have been fitted with a variety of technologies (downsized engines; lockup, multi-ratio and overdrive transmissions; variable valve timing, forced induction, diesel engines, et al.) which render manifold vacuum inadequate or unavailable. Electric vacuum pumps are now commonly used for powering pneumatic accessories. Manifold vacuum vs. venturi vacuum Manifold vacuum is caused by a different phenomenon than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect which, for fixed ambient conditions (air density and temperature), depends on the total mass flow through the carburetor. In engines that use carburetors, the venturi vacuum is approximately proportional to the total mass flow through the engine (and hence the total power output). As ambient pressure (altitude, weather) or temperature change, the carburetor may need to be adjusted to maintain this relationship. Manifold pressure may also be "ported". Porting is selecting a location for the pressure tap within the throttle plate's range of motion. Depending on throttle position, a ported pressure tap may be either upstream or downstream of the throttle. As the throttle position changes, a "ported" pressure tap is selectively connected to either manifold pressure or ambient pressure. Antique (pre-OBD II) engines often used ported manifold pressure taps for ignition distributors and emission-control components. Manifold vacuum in cars Most automobiles use four-stroke Otto cycle engines with multiple cylinders attached to a single inlet manifold. During the induction stroke, the piston descends