A very popular upgrade classic car owners are considering these days is ditching the carburetor for EFI, or Electronic Fuel Injection.  There are many good reasons to do so, however carburetors do still have their place.  Let's start by discussing the problem with carbs and why to consider EFI in the first place. 

Carburetors are an old technology mechanical fuel system - although tried and true - they are not without their issues in the modern world.  For decades, carburetors worked just fine.  With most carbureted vehicles off the road these for long periods of time between drives AND the heat they can experience during modern traffic patterns, they aren't always optimal.  EFI by contrast is a modern technology typically electronically controlled by a computer and is generally very reliable and consistent.

Basic carb operation works by a mechanical fuel pump filling the float bowls on a carburetor at a rate of about 5-6 PSI. The float bowls of the carb are what actually feed the engine when it demands fuel by opening up the needle from the seat to fill up the bowl, then through a power valve, accelerator pump and squirt nozzle into the venturi or the carburetor and into the engine's intake manifold. This worked fine for years - and still works fine for many race engines - but there are problems with carbs for use in today's world.  First off, the gasoline is not the same as it used to be. Most fuel has 10% or more Ethanol mixed into it which is not good for rubber, plastic or soft aluminum parts that most carburetors are made of.  Second, depending where you live, there is generally much more traffic on the road then there was in the "old day" which means engines are going to run hotter due to sitting in traffic more with not much air flow.  Gasoline can boil at as low as 100 degrees and has a max high boiling point of 400 degrees.  Underhood temps can easily get up to 250 or more degrees when sitting in traffic which can cause a condition called Vapor Lock. When fuel begins to boil, there are air pockets in the fuel supply in the line to the float bowl or at the extreme end, the fuel in the float bowl boils away before it can be refilled. Sometimes an electric fuel pump helps with this issue as it provides constant pressure to the bowl even when fuel starts boiling.

EFI by contrast uses an electric fuel pump with a much higher pressure (typically around 60 PSI) and fuel injectors that only fire when the computer tells them too. The trick is how the computer knows when to fire the injectors and how to adjust the air fuel mixture based on feedback it receives from the engine sensors.  EFI uses a coolant temp sensor, an oxygen sensor in the exhaust, a throttle position sensor, and sometimes a knock sensor, intake air temp sensor and mass air flow sensor. This all depends on the particular EFI system in use. Whiel semi complicated to wire up and install, the results can be increased reliability and consistency in starting, idles, cold starts, hot starts and no vapor lock. Unlike a carburetor, the EFI computer is constantly testing the air/fuel mixture and making enhancements and corrections (known as self-tuning) - carbs must be manually tuned, driven and tuned again if needed for changes in weather and altitude.