Far too many people assume that because a carb isn't a mass of microchips, sensors and wires, that it is simple. Lawn mower carbs are simple, but automotive carbs aren't.
Some basic thoughts about two very different approaches to making power in an internal combustion piston engine:
Carburetors are mechanical devices that depend on pressure differences to meter fuel. The venturi is common to all type of carbs, it is what creates the pressure difference that makes carbs function. The float bowl is vented to the atmosphere just above the venturi in most carbs. The fuel is sucked out of the bottom of the float bowl through a metering system that controls the gross amount of fuel delivered to the engine. That metered fuel is then delivered to the point in the carb where the greatest pressure drop occurs. In some simple carbs, it is the area of the smallest part of the venturi opening. More complex systems may use a booster venturi to aid in fuel delivery.
Notice the emulsifier tube in the diagram: it has two purposes: It helps break up the fuel into tiny droplets and it serves as a suction breaker when the engine isn’t running. That keeps the siphon effect from sucking fuel out of the float bowl when the engine is shut down.
Also present in carbs are separate circuits that meter fuel at idle, a power valve that opens up to provide extra fuel at high power settings, and an accelerator pump to provide extra fuel when the throttle is suddenly opened. All these circuits with the exception of the accelerator pump depend on a pressure drop to make them work.
The primary limitation of carbs is that they are a mechanical device, and have no way to compensate for changes in temperature and atmospheric pressure. These two items can limit the power of a carb’d car. A race to the top of Pike’s Peak on a hot summer day would be an example of how this will affect a race car. Or a trip down the drag strip at Denver, Colorado in the summer compared to one in Los Angles during the winter. Definite power differences would be seen & felt. Adjustments can be made, but it requires some small amount of wrenching to do.
EFI does not depend on a pressure drop to pull fuel into the intake or help atomize the fuel. The venturi and associated restriction of the intake airflow is eliminated. Instead EFI measures either air pressure or airflow along with air temperature, water temperature, RPM, and throttle position to calculate the amount of fuel needed. The calculation is much more precise than the simple mechanical process of a pressure drop across a venturi. Since a pressure drop across a venturi is not part of an EFI system, pressurized injection is needed. The injectors are capable of consistently breaking the fuel into tiny droplets that flash to vapor and distribute themselves evenly in the individual intake ports. Couple the precision of the air/fuel measurement along with timed, pressurized injection of the fuel directly into the intake port, and you have a superior system.
EFI expands itself into engine management: not only does it control fuel delivery, but ignition timing as well. The computer can add or remove timing advance while the engine is running, according to the need of the moment. Inputs from sensors can detect detonation and retard the timing, or add timing if the engine can handle it. With an aftermarket tuner and a laptop computer, you have an infinitely tunable machine that can make changes on the fly while you are racing. Consider the possibility of the computer detecting a sudden increase in RPM due to wheel spin. The computer could reduce the timing advance to the point that the wheel spin was minimal and then increase it as the car picked up speed.
