| As I start collecting parts and making lists, I figured I'd start explaining a few things as I go along, both to help with my own thought processes and edify readers. I'll start by jumping into the benefits and capabilities of a modern EFI controller. |
The most complex part of any EFI system is the Engine Control Unit, or ECU. The ECU is the brains of the entire system, evaluating inputs and precisely controlling the amount of fuel delivered to each cylinder and the timing of the spark that ignites the compressed mixture. ECU's operate by receiving inputs, evaluating them, and generating outputs based on calculations and programmed predetermined values.
- Sensors and Inputs -
These devices can measure any number of variables: coolant, air, and exhaust gas temperatures, exhaust gas oxygen content, manifold and barometric pressure, throttle position, combustion temperature and pressure, vehicle speed, wheel speeds, lateral and longitudinal acceleration and many more. There are so many sensors that it's almost impossible to list them all, but there are two main types, digital and analog. Digital sensors output a binary value (either on or off) such as a wheel speed sensor that counts pulses per second or some variable-dependent switch. An example of a digital input is a switch - when depressed it sends voltage to the ECU which enables or disables some function (like a secondary rev-limiter for launching or a temperature-triggered switch for a fan) based on programmed values.
There are also analog sensors, which either output a varying voltage or current. For example, a coolant temperature sensor outputs a 0-5V signal; as temperature increases the resistance of the sensor increases and the voltage drops. An analog input functions in much the same manner, for example, the driver could adjust a potentiometer (or variable resistor) and the ECU could be programmed to retard timing or reduce the amount of fuel injected depending on the voltage the potentiometer is outputting. Another type of analog sensor is a knock sensor, which detects the characteristic sound of engine detonation or engine knock and retards timing as per tuner-programmed tables
- Fuel, Spark, and Outputs -
As the sensors continuously monitor and transmit data to the ECU, it processes it and responds according to its programming. Using a series of tables or maps, the ECU compares the sensor values to a set of pre-programmed and modifiable tables, and based on the values it reads, operates fuel injectors, charges ignition coils, and activates outputs. These are known as base tables. For example, an engine that is tuned for Load-Based (also called throttle position or TPS-based) tuning will have a base fuel table that looks similar to this:
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| Snapshot of fuel table from Performance Electronics software (www.pe-ltd.com) |
This table has Throttle Position on the y-axis and RPM on the x-axis. When the throttle is at a certain point and the engine RPM at another point, the ECU opens the fuel injector for the time specified by value in the cell (in milliseconds.) Instead of being metered by a specific and fixed-size orifice as in a carburetor, the injector can open for any duration or not at all, metering out exactly how much fuel the ECU (and tuner) determines is needed.
Tables also exist that add or subtract fuel from that base value depending on the air and coolant temperatures, battery voltage, and many other variables. These are called compensation tables as they serve to compensate for environmental variables that affect combustion. Similar tables exist for spark timing.
Acceleration compensation, which in a carburetor is achieved by a lever squirting a set amount of fuel as the throttle is depressed, is achieved by evaluating the throttle position. The tuner can define how fast and how far the throttle must open and at what RPM the engine must be at to inject additional fuel, as well as the duration and volume of the addition. It allows for almost infinite customization of the enrichment without exchanging any cams, levers, or diaphrams.
The ECU also has outputs that can enable or disable auxiliary equipment. The ECU controls the fuel pump, priming the fuel system before ignition, it enables and disables fans based on programmed and adjustable values, can enable nitrous oxide solenoids at programmed RPM and throttle inputs, trigger wastegates, and perform many other functions.
The brilliance of the ECU is in its tuning capacity. You don't even have to pop your hood at the track to change any number of parameters. You can save maps for the street, maps for the track, and play with variables as you drive if you so choose. Some ECU's even allow for closed-loop control, which uses an oxygen sensor in the exhaust stream to automatically control the amount of fuel added to achieve a predetermined air-fuel ratio. Tuning changes that used to take minutes to hours and involved replacing mechanical components can now be achieved with keystrokes, and it is this adjustability that I am seeking. I want to maximize performance from my existing combo, increase control over engine functions, and overall have a faster, more efficient car.
There are many more functions of ECU's, they can perform traction control functions, log data for later review, control shift points on an automatic transmission, and hundreds of other duties. This will affect the price you pay for the system, as well as the complexity of the setup, installation, and tuning.
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