Fuel Injection Basics

If there is such a thing as automotive high-performance witch doctors, then their voodoo has to be a steaming cauldron filled with all sorts of evil doings known as electronic fuel injection. The problem is too many hot rodders believe that those shrunken heads hanging from the witch doctor’s door belong to everyone who has tried EFI and failed.

Nothing could be further from the truth, but it’s a tough sell to nonbelievers. If you’re curious enough to stick with us, over the next few months we’ll dive right into that steaming pot of hoodoo-voodoo, which just might turn out to be more like chicken soup. So grab hold of your best lucky charm and let us enlighten you.

Carburetors have been around since the first internal combustion engine. They perform well and keep improving all the time. For racing and street use, they do an admirable job, and they’re tough to beat for the money. But ask a carburetor to deliver outstanding part-throttle efficiency and specific fuel delivery per cylinder, and the job is just too great. In the mid-’80s, the new-car manufacturers were forced to adopt electronic fuel injection (EFI) in order to meet increasing demands for better emissions, improved fuel economy, and seamless driveability.

The advances made in computer technology even back in the fledgling days of the Radio Shack TRS-80 computer allowed Chevrolet and the rest of the new-car companies to adopt the microprocessor’s incredible speed and decision-making capability to accurately control the delivery of fuel and spark to an engine. This isn’t just techno-hype. Not all that long ago, carbureted, leaded-fueled, point-triggered engines had to be tuned up every 10,000 miles. Today, the majority of new cars don’t require a tune-up for 100,000 miles. It’s computer control that creates reality out of what would have been fantasy back when musclecars were king.

EFI Alphabet Soup

There are basically two types of EFI designs—throttle body and multipoint. Throttle body injection (TBI) uses a unit that looks a little like a carburetor mounted on a conventional intake manifold. TBI usually mounts two (and sometimes four) large fuel injectors that are controlled by an electronic control module (ECM), sometimes referred to as an electronic control unit (ECU). Throttle body systems are very similar to carburetion in that fuel is injected into the airstream as the air enters the throttle body, then into the manifold. This is referred to as a wet-flow system, in which the fuel is introduced upstream of the intake port.

The second, more complex type of EFI is multipoint fuel injection (MFI). Multipoint systems inject fuel using individual fuel injectors located in each intake port. A typical MFI small-block Chevy, for example, employs eight separate fuel injectors, one for each cylinder. The injectors are usually located downstream of the throttle body, most often at the entry to the cylinder head intake port. Each injector is also pointed in the direction of the intake valve, with the injector electronically controlled by the ECM, to deliver a precise amount of fuel at a specific time in the engine’s intake cycle.

Most early EFI systems were batch-fire systems where the ECM fired all eight injectors simultaneously. Usually batch-fire systems fire the injectors once per engine revolution. This way, the injectors could be sized small enough to be more easily controlled at idle. Later, sequential EFI systems were refined to fire an injector a few degrees before the intake valve opened. Generally, sequential injection offers more precise fuel control at the price of increased complexity. But on production engines, the benefits are more in the area of emissions and driveability than in performance.

Sensors

While all this sounds complex, once EFI is broken down into systems, it becomes less intimidating. The key to EFI is sensors. A typical EFI system employs at least a half-dozen sensors, usually many more. These sensors are the ECM’s eyes and ears and are used to determine how the engine is performing. Based on that information, the ECM will then change the fuel-flow rate, spark timing, or idle speed to compensate. The accompanying sidebar (“Sensor-Tivity Training”) will outline what the sensors are and what they do.

Fuel control is EFI’s most important job. All EFI systems control the fuel delivery to an engine by referring to what is called a base fuel map. This map is usually an “x-y” grid using two critical inputs. For speed density, the inputs are rpm and load. The rpm input to the computer is just like hooking up a tach lead on the negative side of the coil. Load input is also not much different from the information you would receive from a vacuum gauge. As you probably know, high vacuum indicates low load with the throttle barely open. As the throttle opens, engine load increases and vacuum drops. Chevrolet uses what is called a manifold absolute pressure (MAP) sensor to convert this “vacuum” (also called manifold pressure) reading into an electrical signal that the ECM can understand.

As you can imagine, given enough data points, you could break down every possible rpm and throttle opening point (load) to create a fuel map that would cover each of these engine situations. That’s what the base fuel map accomplishes. The map takes the rpm and load and creates a number that represents the amount of time the injectors should be turned on. This is called an injector pulse width. The longer the injector is pulsed (or turned on) the more fuel is delivered to the engine. So idle and low-load portions of the base fuel map would need short pulse widths, and wide-open throttle at high rpm would require long pulse widths to deliver enough fuel to produce the proper air/fuel ratio for the engine. This is a simplistic description of how the ECM controls the fuel. We’ll get into more detail on electronic fuel control in the coming months.

Speed Density And Mass Airflow

There are several different ways to control the air/fuel ratio. The earliest Chevrolet factory EFI system was configured for what is called a speed-density design. Speed density requires just two main inputs to establish a base fuel map: engine rpm and load. Speed density assumes that a certain amount of air will enter the engine at any particular combination of rpm and load.

For production engines like the early Tuned Port Injection (TPI) engines, this works as long as the engine remains stock. Modifications to the engine to increase airflow (and therefore power) would tend to make the engine run lean, since the engine would inhale more air at that rpm and load point than it did when it was stock. If you take in more air, a proportional amount of additional fuel must also be delivered to maintain the same air/fuel ratio. Speed-density systems cannot perform this function without reprogramming.

Later versions of the Chevy TPI system were outfitted with a mass airflow (MAF) sensor. This sensor measures the amount of air entering the engine, giving more precise control over the air/fuel ratio. MAF systems are more accurate but also more expensive, since the MAF sensor tends to be pricey. Aftermarket EFI systems like ACCEL/DFI, Electromotive, Haltech, FP Performance, and others employ the speed-density design mainly as a way to reduce the cost and complexity of the system but also because all these systems offer easy access to all the ECM maps to make changes to fuel flow and spark timing. Factory systems have always been designed to prevent easy changes to these base fuel and spark maps since the factories consider this to be “tampering.”

Alpha-N

There is one other EFI control system that is generally used only in racing called an Alpha-N system. This control system’s major inputs are throttle position and rpm. This system was developed because race engines often operate at idle and part-throttle with very little manifold vacuum. This makes using a MAP sensor difficult. This system is less precise than speed density or MAF and is therefore generally only found in racing or on heavily modified street engines with big camshafts. A MAP sensor can still be used with Alpha-N, but it is generally employed as a barometric pressure sensor to detect altitude changes.

This first installment of our EFI basics series has painted a broad picture of the entire system so you can get a handle on how these systems operate and understand what all these terms mean. Next month, we’ll take a look at the fuel-delivery side of EFI, because that’s an area most hot rodders can easily understand. In the coming months, we’ll peel back the layers of mystery around how the ECM actually deciphers all this information and makes decisions based on the inputs from its sensors. Stay tuned, it’s gonna get interesting.