Now we've got the most bare-bones of carburetors that mixes a certain amount of fuel for a certain amount of air we allow to pass through the Venturis, 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. In fact, the main metering system won't even allow the motor to idle because the pressure drop (or "signal") isn't strong enough to pull fuel from the boosters at low rpm. This is why a modern carb has a separate idle circuit, one or more enrichment circuits, and a choke to keep the mixture artificially rich on cold days before the motor's hot enough to atomize the mixture nicely.
How EFI WorksA modern multiport EFI system works differently than a carburetor, but it's still bolted to the same motor and installed in the same car, so it must provide at least the same level of functionality. No venturis 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 pulsewidth, the number of milliseconds each injector is turned on). You can probably imagine how a choke, idle, or enrichment circuit can be designed into this system. It's easily done by increasing (or decreasing) the pulsewidth to richen (or lean out) the mixture whenever it's needed. It sounds simple because it is! So let's break down some of the basic functions of a carburetor and see how an EFI system accomplishes the same tasks with the aid of a computer.
ConclusionThe bottom line is that EFI systems and carbs 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 carbs can't touch.
The initial cost is greater, and the learning curve is a bit steeper, but in the end, knowing how EFI works will actually make you a better carb tuner.
Common MisconceptionsWe're amazed at the amount of EFI-related misinformation and BS we overhear at car shows, races, and straight from the mouths of "experts." So we told some real experts from FAST, ACCEL, and Holley what we heard and had them set the record straight.
What we hear: "EFI gives way better fuel economy than a carb."Reality: It depends on how the car is driven. EFI allows more efficient part-throttle cruising, but if you have a street/strip car that's driven hard, don't expect 20 mpg.
What we hear: "Your injectors and throttle body can never be too big."Reality: If the injectors are too small or too large, the computer will not be able to control them accurately, resulting in poor performance. A big-bore throttle body isn't necessarily a bad thing, but as the butterflies get bigger, that same throttle angle will cause more air to flow, so it may become more difficult to modulate the engine power (especially off-idle).
What we hear: "You need huge fuel lines to support a high-horsepower EFI motor."Reality: It's usually OK to have an oversized fuel system, but since most modern multiport EFI systems operate at 40-plus psi of fuel pressure, an identically sized line flows a heck of a lot more fuel than in a 6-psi carbureted system. We've seen 1,000-plus-horsepower EFI'd engines happily fed by AN-8 (11/42-inch) lines.