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Electronic Fuel Injection - EFI For The Masses

Is Electronic Fuel Injection For Everyone?

Controlling EFI
The computer used to control EFI is called the ECU, or Electronic Control Unit. It's a self-sufficient device that, once properly programmed, can tune the motor on its own and will keep it running the most efficiently under all conditions. The ECU uses a pre-programmed fuel map (see Fig. 1), which the user/tuner inputs into the ECU to decide how long to hold the injectors open (known as injector pulse width). This directly controls how much fuel gets into the engine and is displayed in milliseconds in the boxes on the fuel map. By increasing or decreasing pulse width on any part of the map, the engine will run either richer or leaner. The ECU is constantly altering the injector pulse width in its search for the perfect air/fuel ratio, which it measures in the exhaust using an O2 sensor.

An easier way to understand what the ECU does would be to think of it as your own personal little carb tuner hidden under your hood and the fuel map as his playing field. While you're driving around town, your personal tuner runs up and down his playing field looking for how much fuel to give your engine, based on the demands you've placed upon it. All around his playing field are buttons he can hold open for any length of time to inject fuel into the engine. Let's say you're just cruising around. He goes to the "cruise" area of you fuel map (which, in reality, will be somewhere in the a high-vacuum, low-rpm, and low-throttle position vicinity on the map, middle left in this case) and sees that it tells him to inject fuel for around 30 milliseconds at each injector.

He holds the button down for 30 milliseconds (the little guy works fast), but then you want to pass someone. You mash the throttle about 3/4 of the way to the floor. Inside your engine, the vacuum drops, the throttle opens farther, and the engine's speed starts to climb. Your tuner runs to the spot on the fuel map that closely approximates the area you are in, and it tells him to now inject fuel for 60 milliseconds. This cycle constantly repeats itself the whole time you're driving (the little guy never gets tired). The ECU is always recalibrating the air/fuel curve to make the engine run its best. But how does it know when the engine is running its best? That's controlled by the parameters pre-programmed into the ECU, and in fact, most ECUs have the ability to go beyond those parameters, because they know that no computer program can make the engine run perfectly all the time.

The ECU can "self-adjust" to better match conditions. What that means is that the ECU may only be sending 85 percent of the fuel map's recommended injector pulse width, because the conditions that day are not meeting the specified parameters of the ECU's map. The engine could be hot, or the air going into it could be cold, or any numbers of factors could be calculated, and injector pulse is constantly adjusted based on what the ECU determines to be the correct setting.

As you can tell, programming the ECU properly is pretty critical to performance. And it's not something that can be done with just a few clicks on the keyboard. Tuning the ECU to its maximum potential is done best on the road and at the track. With a laptop plugged in and a computer-knowledgeable passenger punching the keys, drive all over the place keeping throttle positions constant while tuning takes place.

Identifying EFI Components
This Edelbrock
Multi-Port-Injection system is a classic example of an MPI EFI kit. Shown here are all the mechanical components of the EFI as well as the electrical sensors, except the O2 sensor.

1.Fuel Pressure Regulator2.Fuel Rail
3.Throttle Position Sensor (TPS)
4.Throttle Body
5.Manifold Air Pressure Sensor (MAP)
6.Idle Air Control Valve (IAC)
7.Manifold Air Temperature Sensor
8.Fuel Pressure Checking Port
9.Fuel Injectors
10.Vacuum Reference for Fuel Pressure
11.Coolant Temperature Sensor
12.Fuel Return to Tank

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