WHY CARBURETOR IS REPLACED BY EFI

What would fuel injection be without a computer controlling all its functions? Less powerful? Sure. Less efficient? Certainly. A heck of a lot more popular? We think so. We'd venture to say that most gear heads, ourselves included, are naturally wary of trusting a mysterious "black box" to control fuel and spark inside a several thousand worth of engine. Carburetors and distributors at least look the part. You can't blip the throttle and check the pump shot in EFI. There are no jets or metering rods to play with. Heck, most ECU’s aren't even mounted in the engine compartment. However, the functionality says it all.

How Carburetor Works:

A carburetor takes advantage of the venturi effect to meter a certain fixed ratio of fuel and air into the engine. A sharp "step" in each throttle bore accelerates the incoming air stream, increasing its velocity and creating a region of lower pressure under the fuel booster. The size of the main jets controls the amount of fuel that's drawn out of the discharge nozzles. Then it's drawn into the cylinders, compressed, and fired.
Now we've got the most bare-bones of carburetors that mix a certain amount of fuel for a certain amount of air we allow to pass through the Venturi, giving us a constant air/fuel ratio. If we change the main jets, the mixture will become richer or leaner--but it'll stay richer or leaner across the entire rpm range. If you need to vary the air/fuel ratio for economical cruising (leaner) or richen it for maximum power at wide-open throttle, this system won't do anything for you. Hence, change of plans.

How EFI Works:

A modern multi port EFI system works differently than a carburetor. No venturi here--fuel isn’t introduced into the engine until the injectors spray it into the base of the intake runners, so the only thing that’s mechanically metered is the air that’s sucked through the throttle body. Since the mixture of air and fuel can’t tumble around inside the intake runners, the injectors spray a fine mist of fuel at high pressure, which "forces" it to mix almost instantly with the metered air. Changing the air/fuel ratio is simply accomplished by varying the amount of time that the injectors are spraying fuel (usually measured in pulse-width, the number of milliseconds each injector is turned on).

Main Metering Circuit:

A carburetor’s main metering circuit operates solely on the venturi principle, which by it would fit our needs perfectly if a car ran only at a constant load and rpm, such as up a slight hill in First gear. But in the real world, engine load changes, and selecting the proper main jet size is only the first step in creating a well-tuned street machine. Carburetors keep the engine properly fueled over a range of conditions by sensing vacuum (engine load) and kicking in additional fuel via its enrichment circuits. Modern EFI systems use "maps"--a table of numbers corresponding to certain parameters (i.e., rpm) saved in the onboard computer to tell the injectors how much fuel is needed at any conceivable operating point. The base fuel map tells the injectors how much fuel to spray at a certain rpm for a given engine load. This isn’t a direct analogy to a carburetor’s main metering circuit because it recognizes that an engine under heavy load needs more fuel (enrichment) than a lightly loaded engine, so it’s already a little smarter than our "bare-bones" carburetor.
Most EFI systems enable you to type in the air/fuel required across the usable rpm and load range of the engine. More sophisticated EFI systems use feedback from a wide-band oxygen sensor (WBO2), installed at the header collector, to monitor and display the actual air/fuel ratio. The actual versus mapped air/fuel ratio is displayed as a percent correction. If a standard narrow-band oxygen sensor is used, the computer can only display air/fuel ratios in a narrow range, so tuning is slightly more difficult. Either way, the benefit of any O2 feedback signal (closed loop) is a huge advantage in tuning and performance.

Choke:

As you might have guessed, there’s no choke butterfly attached to a multi-port EFI throttle body--it’s all done with electronics. Tweaking the "Cranking Fuel" graph changes how long the injectors stay open, and consequently, how much fuel the engine is fed.

Conclusion:

The bottom line is that EFI systems and carburetors all need to do the same things to make an engine run, and once you get past the fear of that little black box, it’s apparent that EFI provides a level of versatility and ease of tuning that even the best carburetors can’t touch.