How Carburetors and EFI Work - Main Metering

What we hear: "You will never be able to tune your EFI without a professional tuner and a dyno."Reality: Hey, we pulled it off with our Pontiac 455. We're not tuners, and the car's never seen a chassis dyno.

What we hear: "EFI makes more power."Reality: Remember, EFI is almost infinitely tunable, so you might see more total power across the RPM range of the engine. But don't expect peak power to change much if you decide to shelve that Holley HP carb for a Commander 950 EFI setup.

Main Metering CircuitA carburetor's main metering circuit operates solely on the venturi principle, which by itself 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. Carbs keep the engine properly fueled over a range of conditions by sensing vacuum (engine load) and kicking in additional fuel via its enrichment circuits, but that's a topic we will address a little later.

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" carb. The idea is to match the perfect injector pulsewidth across a full range of steady-state rpm and load (manifold pressure or vacuum) to keep the motor at a happy air/fuel ratio across the board. Steady-state simply means that the engine is allowed to settle out and remain at that operating point. Other maps take care of adding or subtracting fuel depending on throttle position and rate of opening, so it's important that the base map be tuned only after the engine has reached steady-state. After the main fuel map is dialed in, it serves as the "fingerprint" of the motor and shouldn't be altered unless major changes are made to the combination.

Some EFI systems use volumetric efficiency (VE) as the main variable in the base fuel map. The VE is defined as the maximum amount of air that the engine can theoretically suck into the cylinders, divided by the actual amount of air that the engine ingests. One hundred percent VE means the engine is "perfectly" efficient; forced-induction powerplants often have greater than 100 percent VE as the intake air is pressurized, and would fill a greater volume if allowed to expand at atmospheric pressure. Knowing the VE at various load and rpm points, as well as the injector flow rate, engine displacement, and number of cylinders, the computer calculates the injector pulsewidth needed to supply the right amount of fuel. Using VE instead of pulsewidth can save time if you do an injector swap-you simply type in the new injector flow rate instead of having to change every pulsewidth value across the base fuel map.