Electronic Fuel Injection Primer - Get To Know Your EFI

A primer on the modern miracle of Electronic Fuel Injection

Chris Werner Sep 19, 2012 0 Comment(s)
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You love late-model muscle cars, but do you understand what makes them tick? Do you know why your '10 Camaro SS not only has more power than equivalent cars of the '60s, but also with vastly better drivability and fuel economy? Why the 1980 Turbo Trans Am was a lackluster machine, but the 1989 model was such a killer? There are a lot of reasons that engines experienced such improvement, and so many of them boil down to Electronic Fuel Injection. EFI so revolutionized hot-rodding that the original High-Tech Performance magazine was born (and became GM-only in 1998). Back then, GMHTP readers were a distinct minority compared to those who thought carburetors would forever reign supreme, but today even the most diehard old-school aficionados have been converted after seeing the incredible combination of power levels and drivability EFI makes possible. These days, you probably take EFI as a given, but it wasn't like this 15 years ago. Back then, many saw it as a hindrance to backyard mechanics fixing or modifying their own vehicles. To love EFI requires knowing the ins-and-outs of how it functions, so we figured a "get to know your EFI" article was in order. After all, there's a lot of unseen business that goes on between the time gas is pumped into your tank and it ends up as motion down the road; this article should be a good first step at figuring it all out.

What Came Before

Carburetors, that's what. Beautifully simple in their basic operation, carburetors took advantage of the basic premise of physics that a fluid in motion (in this case, air flowing through the venturis) is at a lower pressure than stagnant air, thereby sucking fuel into the flowing air stream and letting the mix find its way through the intake manifold to the intake ports, and past the intake valves into the cylinders. Carburetors fell by the wayside when emissions standards grew more stringent during the 1970s and 1980s; emissions-compliant carburetors proved too complex and expensive to be worthwhile. A better way to more precisely meter fuel was needed in order to achieve more consistent, predictable compositions of gases and particulates in the exhaust, thereby allowing catalytic converters to do their job reliably over the life of the vehicle. That better way was EFI.

(Note: Many muscle car aficionados are familiar with the mechanical fuel injection systems seen on Corvettes of circa-1960 vintage; while very progressive in design, their lineage did not lead directly to EFI. A study of these systems is left to the Google skills of the reader.)

Background

The first production cars with EFI systems date back to the late 1960s (with some notable aborted attempts a decade earlier), but it didn't start showing up under the hoods of GM muscle machines until the mid-1980s. Since then, EFI systems have evolved tremendously, advancing in step with increases in computing power, tightening of emissions limits, and desires to improve fuel economy. The reader's basic knowledge of the four-cycle spark-ignition engine is assumed for the purposes of this article, and we'll leave the components of the spark side largely out of the equation. Still, it's worth noting that while spark plugs still ignite the mixture in the cylinder, distributors and the snake's mess of spark plug wires have become a thing of the past as computer-controlled individual coils now send the energy to the plugs. Also, we're largely ignoring small details like Idle Air Control (IAC) valves and like tangential equipment that has come and gone as EFI has evolved, though some may be mentioned where they happen to come up.

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Types of Electronic Fuel Injection

What follow are the three main styles of EFI, their differences having mainly to do with the placement of their fuel injector(s). In increasing order of sophistication and cost, they are:

Throttle Body Injection (TBI): In operation, the earliest and most crude form of electronic fuel injection wasn't that different from a carburetor, employing one or more large, low-pressure fuel injectors attached to a throttle body assembly. Like a carb, this setup relied on the intake manifold to mix the fuel with the air and keep it mixed on the way to the intake ports. GM muscle car enthusiasts first saw TBI show up as the Cross-Fire Injection systems of late C3 and early C4 Corvettes, and they could be found on certain F-body engines through the end of the third-gen era. By the mid-1990s, GM finally removed these low-cost systems (and the not-much-more-advanced "Central Port Injection" setups) from its trucks and said bye bye to TBI.

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Port Fuel Injection (PFI): Also known as Multi-Port Fuel Injection (MPFI or MPI). Still the most common form of electronic fuel injection on production vehicles, the GM V-8s we know and love have had PFI ever since Tuned Port Injection (TPI) appeared for 1985. By using individual fuel injectors for each cylinder and placing them just upstream of the intake port, PFI systems do not rely on the intake manifold for air/fuel mixing purposes, instead precisely metering a measured amount of fuel to the intake port of each cylinder. PFI systems generally have the injectors mounted as close as possible to, and aimed directly at, the intake valves. (While the cathedral-port design of early LS engines certainly look to be designed largely for injector aiming, in reality the unusually tall port shape had to do more with maximization of port cross-sectional area between the pushrods.)

Direct Injection (DI): Also known as Spark Ignition Direct Injection (SIDI) or Gasoline Direct Injection (GDI) to differentiate it from the compression-ignition direct injection scheme of today's diesels, this type of fuel injection can be thought of as essentially the same as PFI, with one key difference: The fuel injectors spray directly into the combustion chambers. This type of EFI has seen increased adoption in production cars in the last few years, but thus far it's seen only limited use in the types of rides that concern GMHTP readers: the fifth-gen Camaro features DI, but only with the V-6, as did turbocharged models of the Solstice/Sky and Cobalt. Though no GM V-8 to date has used direct injection, the wait won't be long, as you can bet on it for the Gen V small-block debuting in the 2014 Corvette.

EFI System Inputs

The types of sensors used in EFI vary by system type, age, and its exact execution, but they're very close for both PFI and DI (let's just go ahead and ignore TBI from here on out, shall we?). There are two main types of information the EFI computer needs in order to do its job: characteristics of air entering the engine, and characteristics of exhaust exiting the engine. Both of these, along with other parameters such as engine RPM, coolant temperature, and throttle position, determine how much fuel needs to be injected at any given instant; required spark timing is also a function of many of these inputs, but again, the ignition system is a secondary issue for the purposes of this article (as is the self-diagnostic capability of modern engine computers). Making our way from the air filter to the tailpipe, the sensors and their most common abbreviations are:

Intake Air Temperature (IAT): Most often located between the air filter and throttle body, this sensor measures the temperature of the air being sucked into the engine. Its output is a voltage that varies with temperature. Crucial to the operation of speed density systems, it is also present on MAF-based systems though plays a smaller role.

Mass Air Flow (MAF): Always located between the air filter and throttle body. This sensor generally consists of a heated element that is held at a constant temperature; more air flowing past it has a cooling effect, hence more current is required to hold the element at a given temperature. Its output on most GM EFI systems is a wave signal that varies in frequency and is input to the PCM as Hertz. This frequency, which ranges from 0 to about 12,000 Hz or higher (depending on type of MAF and ECM), is converted by the ECM to g/s or similar measure of air mass per unit time.

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