If you own a classic car, muscle car or pickup truck that's an early 1980s or older vintage, your engine likely has a carburetor on it. There are a few exceptions such as 1950s and 1960s Corvettes and Chevys with Rochester mechanical fuel injection, and European imports that were equipped with Bosch D-, K- or L-Jetronic fuel injection. Other than these, carburetors were the primary means of delivering fuel to the engine. In the early 1980s carbs where phased out and replaced with various types of Electronic Fuel Injection (EFI) to meet ever-tougher emission and fuel economy regulations. So if your old carburetor has reached the end of the road, maybe its time to upgrade to fuel injection.
The main difference between carburetion and fuel injection is that carburetors are mechanical devices that rely on airflow, intake vacuum and atmospheric pressure to siphon fuel from the fuel bowl into the engine. Fuel injection, on the other hand, uses high pressure spray nozzles (injectors) to spray fuel into the engine. EFI also has electronic controls and various sensors to monitor engine performance so it can constantly adjust the air/fuel mixture as operating conditions change. This includes such things as ambient temperature (cold air is denser than hot air, which changes the air/fuel ratio), engine temperature, throttle position, engine RPM, vehicle speed and engine load.
A key difference between EFI and a carburetor is that EFI constantly "retunes" itself on the go while a carburetor does not. When a carburetor is adjusted (tuned) for a given set of conditions such as air temperature and barometric pressure (altitude), it will be out of tune as soon as those conditions change. That's why people who race carbureted engines at drag strips, circle tracks or road courses are constantly fiddling with their carbs and changing metering jets and other components to optimize the air/fuel mixture for prevailing track conditions. It's the same story for a street driven carbureted engines. A carb that's tuned to run great on a mild spring day will probably run too rich on a hot summer day, and too lean on a cold winter day.
The main advantage of EFI versus a carburetor, therefore, is better fuel atomization, fuel mixing and air/fuel mixture control under a wide range of operating conditions. With EFI, there is no waiting for the engine to warm up after a cold start on a cold day before you can drive away without the engine stumbling or stalling. And when you punch it, you'll get the optimum air/fuel ratio and fuel delivery for maximum power.
Which makes the most horsepower, EFI or a carburetor? In theory, you would assume a carburetor or EFi system would be capable of delivering the SAME horsepower if they flow the SAME amount of air and fuel (same CFM ratings). That's true. But because multipoint EFi provides better cylinder to cylinder fuel distribution and can instantly adapt the Air/Fuel ratio to changing weather and altitude conditions, EFi has the advantage and can usually make more horsepower.
One thing that carburetors and EFI share in common is that both atomize liquid fuel so it will mix with air and form a combustible mixture. However, with EFI the fuel droplets are much finer, vaporize more easily and form a more uniform air/fuel mixture that improves combustion efficiency, emissions, fuel economy and power.
Fuel distribution cylinder-to-cylinder is also better with Multiport Fuel Injection (MFI, also called Port Fuel Injection or PFI) because there is a separate fuel injector for each of the engine's cylinders. Fuel is sprayed directly into the intake ports rather than the intake manifold. This greatly improves cylinder-to-cylinder fuel distribution because no air-fuel separation takes place inside the intake manifold because only air if flowing through the curved and bends in the manifold.
With carburetors or Throttle Body Injection (TBI) that have the injectors in the throttle body rather than the manifold, the end cylinders on a typical V8 engine tend to run a bit leaner than the center cylinders. The uneven fuel distribution hurts power, fuel economy and emissions. It also makes the engine harder to tune because an "average" air/fuel mixture that's just right for the end cylinders will likely be a little too rich for the middle cylinders.
In the early 1980s, U.S. auto makers replaced mechanical carburetors with "electronic feedback" carbs that used a computer operated Mixture Control (M/C) solenoid to adjust the air/fuel mixture. Changing the dwell or on time of the M/C solenoid changed the air/fuel ratio. This feature gave carbs a limited ability to self-tune, but it wasn't good enough to meet future emission and fuel economy regulations. The feedback carbs also proved to be very troublesome and unreliable, so the next step in the evolution of fuel delivery was to replace feedback carb with TBI systems (which simply bolted on in place of the carburetor), or to convert to a Multiport EFI system with separate injectors for each of the engine's cylinders.
In recent years, most late model cars and trucks have been equipped with Gasoline Direct Injection (GDI). With this setup, fuel is injected under extremely high pressure (several thousand PSI!) directly into the engine's cylinders similar to a diesel engine. GDI significantly improves fuel economy and power (20 to 30% in many instances).
As we said earlier, carburetors can be tricky to tune and require frequent adjustments if you want to get the most performance and/or fuel economy from your engine. Most carbs also have to be rebuilt after so many years because gaskets, seals and diaphragms deteriorate with age.
If you choose not to upgrade to an aftermarket EFI system when your old carburetor needs to be rebuilt or replaced, there are plenty of aftermarket carburetors from which to choose. For street use, a new carburetor should have a manual or automatic choke for easier starting and cold driveability. Most replacement carburetors are more-or-less ready to run right out of the box, but to optimize performance the main jets may need to be changed to adjust the air/fuel mixture, and the idle mixture, choke and accelerator pump will likely require some fine tuning to achieve the best performance.
NOTE: The biggest mistake most do-it-yourselfers make when choosing a replacement carb is to install a carb that is too large for their engine. A carburetor with a larger air flow rating (in Cubic Feet per Minute or CFM) can produce more horsepower at higher RPMs, but only if your engine is capable of flowing the extra air. Installing a carburetor that has too high a CFM rating fro your engine can actually hurt low RPM torque and throttle response. A 600 cfm carb is more than adequate for a 300 to 350 CID V8. For a larger displacement 400+ CID V8, or w high revving (over 6500 RPM) 350 V8, go with a 700 to 850 cfm carburetor.
If you're tired of constantly fiddling with and rebuilding carburetors, upgrading to an aftermarket fuel injection system will provide an entirely different tuning and driving experience. Aftermarket EFI systems come in two basic varieties: relatively simple and affordable bolt-on TBI systems which can be installed in a few hours, and more race-oriented and expensive multiport EFI systems. The kits usually include any necessary sensors (such as an oxygen sensor), an electronic control unit, wiring (which is minimal with most bolt-on TBI systems), and a high pressure electric fuel pump to replace your old mechanical fuel pump.
Aftermarket bolt-on throttle body EFI units such as this Holley are a great replacement for carburetors on street engines as well as performance engines. This unit has four 100 lb./hr fuel injectors that can supply fuel for up to 650 naturally aspirated horsepower! The computer control unit has a self-learning fuel map and can control ignition timing, for better idle stability, driveability (especially following a cold start), and wide-open throttle response.
Various aftermarket fuel system suppliers also offer Multi-Port EFI systems which feature a separate injector for each cylinder. However, these are more complex EFI systems that require replacing the intake manifold as well as custom tuning on a dyno.
Aftermarket multiport EFI systems are the ultimate way to go if you want to upgrade to EFI from a carburetor. These types of systems are also self-tuning, but also offer more capabilities in terms of custom tuning and calibration for optimum performance. Systems like these typically sell in the $1800 to $2000 plus range.