Fuel Injection Basics
Chevy High Performance’s Guide To Understanding Electronic Fuel Injection


Photography: CHP Staff


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.



Sponsored Links

Remanufactured Throttle Body Injection units at Wholesale Prices
Supercharge Any Car
Up to 35 HP 31% MPG for any car. Lifetime Warranty. Only $69.95
Rebuilt pumps and pump assemblies with 12 month warranty
New & used Chevy Crate Engine. Check out the deals now!
TWM Induction Systems
TWM Induction and Fuel Injection TEC3, Haltech, HKS, Spearco

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.”



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.




Perhaps the most readily identifiable small-block EFI system is the GM Tuned Port Injection (TPI) system. Chevrolet used the TPI system from 1985 until the introduction of the LT1 in 1992. The TPI engines used both speed density and mass airflow computer control over the individual fuel injectors. The TPI system has become a very popular EFI addition for older Chevys.


This simplified diagram will give you some idea of how the electronics of an EFI system work. The ECM receives critical engine-operating information from all the different sensors. The ECM can then make decisions based on what the sensors report the engine is doing.


Aftermarket EFI systems abound and can be used on either factory intakes or on aftermarket manifolds. This is Edelbrock’s multipoint fuel injection system installed on a small-block Chevy. The simplicity of this system makes it easy to spot all the specific components. The throttle body (A) controls air inlet to the engine. Located on the throttle shaft is the throttle position sensor (B). The coolant sensor (C) monitors engine coolant temperature, while the manifold absolute pressure (MAP) sensor (D) keeps track of engine load.


One important sensor for feedback systems is the oxygen (O2) sensor. This sensor monitors exhaust-gas oxygen content and reports this information to the ECM. The O2 sensor is located in the exhaust collector but ahead of any catalytic converter. Later onboard diagnostics II cars are equipped with a second O2 sensor that monitors how well the catalytic converter does its job.


Besides fuel and spark control, most EFI engines also control idle speed with what is called the idle airspeed control (IAC) motor. This small, electric stepper motor is controlled by the ECM and allows a certain amount of air to bypass the throttle blades through a controlled orifice. The IAC motor does a great job of maintaining engine idle speed when additional load is placed on the engine, such as engaging the air-conditioning compressor or when heavy electrical loads are placed on the alternator. The IAC motor meters additional air into the engine to raise the idle speed.


This illustration shows how simple the fuel delivery system really is on an EFI engine. The main difference is in the return line. The return reduces the load on the fuel pump and circulates fuel back to the tank rather than allowing it to sit in the fuel line where it can heat-soak and possibly cause a vaporlock problem. Most EFI systems operate at 43 psi of line pressure, which is controlled by the pressure regulator.


This is a multipoint Mini-Ram bolted on a small-block Chevy. One advantage to multipoint EFI is that it allows tremendous freedom with intake manifold design. This intake is designed for high-rpm horsepower and very short runners.


For a time, mechanical fuel injection was considered the high-water mark of fuel control. Early fuelie Vettes and ’57 Chevys made it work for a while, but eventually the carburetor was still considered king.


The mass airflow (MAF) sensor uses a tiny wire strung across the inside of the sensor.
While EFI engines look intimidating from a wiring standpoint, they are actually fairly simple. This illustration reveals that only 15 separate connections are required to completely wire an ACCEL/DFI stand-alone EFI system. All the sensor connectors have their own dedicated connection so that the system can’t be improperly connected. Besides the sensors, you need connections for the fuel pump relay, ground, battery positive, starter solenoid, and tach pickup, along with a switched 12-volt connection. That’s it!.
This is an ACCEL/DFI-controlled engine in an early Chevelle. The intake is a Lingenfelter-designed ACCEL SuperRam sitting on a 525hp, 420ci small-block. The ACCEL system is a speed-density, batch-fire system operating in closed loop at part throttle. Anything above a 70 percent throttle opening kicks the system into open loop. The advantage of aftermarket EFI is the total control of the fuel and spark curves. Factory systems are far less flexible when it comes to changing the fuel or spark curves.