Manifold vacuum
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.
As ambient pressure (altitude, weather) or temperature change, the carburetor may need to be adjusted to maintain this relationship.
Older (pre-OBD II) engines often used ported manifold pressure taps for ignition distributors and emission-control components.
Most automobiles use four-stroke Otto cycle engines with multiple cylinders attached to a single inlet manifold.
To control the amount of fuel/air mix entering a carbureted engine, a simple butterfly valve (throttle plate) is generally fitted close to the start of the intake manifold and at the end of the carburetor.
If the engine is operating under light or no load and low or closed throttle, there is high manifold vacuum.
Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase—but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible.
On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed.
The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed.
A pressure gauge measuring the manifold pressure can be fitted to give the driver an indication of how hard the engine is working and it can be used to achieve maximum momentary fuel efficiency by adjusting driving habits: minimizing manifold vacuum increases momentary efficiency[citation needed].
During this time a huge variety of vacuum switches, delay valves and accessory devices were created.
The manifold is connected directly to the air intake and the only suction created is that caused by the descending piston with no venturi to increase it, and the engine power is controlled by varying the amount of fuel that is injected into the cylinder by a fuel injection system.
Since low-pressure is only created on the overrun (such as when descending hills with a closed throttle), not over a wide range of situations as in a petrol engine, a vacuum tank is fitted.
