Electronic fuel injection

Many car crafters roll their eyes and tune out when bench racing turns to electronic fuel injection (EFI). We're not here to hijack you into believing that EFI is cheap or even simple. Carburetors will always have their place. And yes, EFI is more expensive and more complex. That's old news. Even if you never take the opportunity to apply what EFI can do for an older muscle-car, it's still worth learning the basics of electronic engine control. What this all comes down to is that the more you know about EFI, the less intimidating it becomes. Don't worry; there's no test at the end, and we promise not to whack your knuckles with a yardstick if you lose focus for a second or two.

Basic Components
While electronic fuel-injection systems became the norm during the 1980s, these were advanced digital versions of simple designs that were first developed back in the mid-'50s. The first domestic-production EFI system appeared as the Electrojector on the '57 Rambler Rebel, using vacuum tubes instead of microchips. Chrysler also dabbled with EFI in the late '50s with disappointing results. Bosch is generally given credit for the first true full-production-run EFI system 10 years later on the '67 Volkswagen using D-Jetronic FI. So EFI has been around for more than 50 years.

Before we get into how EFI works, we should start by looking at the components. The computer is the brain of the system and is the control mechanism often called the electronic control unit or module (ECU or ECM). Lately, both factory and aftermarket systems are integrating automatic transmission controls into the ECM and referring to these as either powertrain control modules (PCMs) or vehicle control modules (VCMs).

In order for the ECM to make proper decisions about air/fuel ratio or spark timing, it requires a voluminous stream of sensor information. For a bare-minimum EFI speed-density system to operate, it needs to know engine rpm, throttle position (TPS), engine load from intake-manifold vacuum (manifold absolute pressure or MAP), and preferably coolant temperature (CLT). Along the way, factory and aftermarket EFI systems have added mass airflow (MAF), knock sensors, inlet air-temperature sensors, and oxygen sensors. Most of these are designed to inform the computer of engine conditions leading up to the combustion event and operate on a very simple system that converts a physical property like coolant temperature or manifold vacuum into voltage. Automotive sensors generally operate on a 0- to 5-volt scale.

The oxygen sensor, usually placed in the exhaust stream ahead of a catalytic converter, is the only feedback sensor that informs the computer of what happened after the combustion process. The oxygen sensor reads the amount of free oxygen in the exhaust, converting those levels into voltage. This information is extremely useful to the computer to help it maintain a given air/fuel ratio while the engine runs down the road. All gasoline-fueled production EFI engines are designed to run at part-throttle at the lowest common exhaust emissions level achieved at 14.7:1 air/fuel ratio, something engineers call stoichiometric, or ideal chemical balance. At part-throttle, the ECM helps the engine maintain that 14.7:1 air/fuel ratio. At WOT, when maximum power is demanded, the computer must adjust the air/fuel ratio to a richer 12.5:1 to 13.0:1.

We also have to have a way to accurately introduce fuel into the engine. Most performance fuel-injection packages employ a multipoint system that specifies one injector per cylinder. These multiple injectors are then fed fuel at high pressure through a common fuel rail. Generally, most multipoint systems are designed to run at 3 bar (three times atmospheric pressure: 3 x 14.7 = 44 psi). One bar is actually 0.9869 of an atmosphere, which is why 43.5 is the 3-bar pressure reference number. With a set pressure, the volume of fuel the injector can deliver is determined by the amount of time the injector is open. This is called the injector pulse width. Properly sizing the injectors improves part-throttle performance because oversized injectors are difficult to control at very short pulse widths.