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Tuning Your Ford EFI System - Inside The Black Box Part 1
Understanding and Tuning your Ford EFI System
Some of us have been there. Others never have. For those who have seen the inside, we realize it's a scary and confusing place. We slowly found our way around the "safer areas," then ventured further as we felt more comfortable. Still, years later, there are unknown caverns where we fear to tread. The place we speak of does not exist in the physical world-it's in a virtual one. We're talking about the world of the Ford Electronic Engine Control system.
Not too many years ago, the means to get inside and reprogram the Ford Electronic Engine Control (or EEC, pronounced eek) system was reserved to few who possessed the specialized systems and electronic engineering knowledge to read and "reverse engineer" the Ford EEC system. But in this day and age, in addition to several stand-alone electronic engine control systems (that totally replace the stock system), there are many aftermarket systems available with the ability to reprogram your existing Ford EFI system, either through an add-on chip or by reflashing the stock processor.
In this three-part series, we'll give you the background info on how the EEC systems work. In Part 2 we will discuss simple tuning specifics (as far as the Ford EEC system parameters go), while in Part 3 we'll go through some actual tuning techniques (again, very much simplified). In the end, you'll hopefully understand what your hired tuning expert is doing.
The Simple ExplanationThe Electronic Control Unit (ECU, aka Powertrain Control Unit or PCM) is the brain of the entire EEC outfit. It uses various sensors to "see" current running conditions and under-stand driver demands, then make decisions and perform calculations based on its internal programming (on board memory). Finally, it sends electronic outputs to a host of actuators which control the fuel and spark delivery for the engine, emissions control systems, coolant fans, automatic transmission functions, and so on. As EEC systems have evolved and continue to do so, the ECU gains control over more and more variables. In this series, we will focus primarily on the fuel and spark control, since they have the greatest influence on engine performance.
Fuel Control BasicsBefore we go into detail on the three different fuel control strategies, you need to understand the fundamental similarities between all EFI systems. That leads us to the electronic fuel injectors and the EFI fuel system. It's a simple enough concept. For a constant air/fuel (A/F) ratio, as the airflow into the engine increases (from increased rpm, increased throttle opening, or increased boost, for example), the fuel flow must also increase proportionately. But with electronic fuel injectors, the added fuel flow does not come from increasing the flow through the injectors in the same way, as say, the throttle valve increased the airflow by opening the passage in the inlet path. Electronic fuel injectors are actually digital (on/off) devices.
An electronic fuel injector is an electrically controlled on/off valve for fuel flow. When it's on, it flows fuel in proportion to its nozzle size, and fuel pressure. When it's off, it flows no fuel at all. To vary the amount of fuel flowing, the injectors are pulsed on and off, meaning they are opened and closed quite rapidly. If more fuel is needed, the "on" pulse gets longer. The amount of time the injectors are open is termed Pulse Width.
Or, the time between successive pulses gets shorter. This is important, since many people don't understand how you can flow more fuel with a shorter PW. The ratio between injector "on time" and on time plus "off time" (total cycle time) is known as the Duty Cycle, DC. For example, if the PW (on time) is 2.5 milliseconds (ms), and there are 5 ms between injections, the DC will be 0.5, or 50 percent.
Injection events can either be done in batch fire mode, where groups (banks) of injectors are fired at the same time (usually once per engine revolution), or in sequential mode, where individual port injectors fire for their cylinder only, normally following the engine firing order (once every second engine revolution for four stroke engines). Sequential EFI (SEFI) systems have some advantages for fuel control over batch systems (like improved emissions and rev limiting ability), and are therefore used on all modern OEM EFI systems.
For a given injector size, at a fixed fuel pressure, the fuel flow rate will increase directly proportionally to the duty cycle. That is, until you reach 100 percent DC. At that point, the injectors are theoretically open all the time, and fuel flow can't increase unless the fuel pressure is increased-we'll get to that in a minute. If you get to the point of saturating the injectors (100 percent DC), your A/F ratio will go lean if additional air flows into the engine. Hence the reason to install larger flow injectors. In practice, you typically don't want to have the DC regularly exceed 85 percent, since the injectors can overheat from all the applied current. Also at high DC the injector pulse width can become unstable, and with some injectors, less fuel will actually flow at high DCs.
So maximum fuel flow will then be limited by the size of the injector, right? Not totally. The other variable is the fuel pressure acting across the injector (from the supply side to the intake manifold side). For a given nozzle size, you can push more flow through with a higher fuel pressure. This is why injectors are flow rated at a specific fuel pressure. It is also why you can get away with smaller injectors when using a Fuel Management Unit (FMU) that's included in many supercharger kits. The FMU basically cranks the fuel pressure way up under boost to force more fuel through the smaller injectors. It's really a Band-Aid solution to having properly sized injectors, because the high pressures can shorten the life of your injectors and your fuel pump.
So for a given injector size, we now have two variables to control our fuel flow: DC and Fuel Pressure (FP). To make life easier for the ECU to control the fuel flow precisely, we'd like to have only one variable, so the FP is fixed at a constant value, typically 39 psi for most stock EFI Fords. But if we want constant fuel pressure, why do we regulate it with manifold vacuum in order to maintain a constant pressure across the injector? As intake manifold vacuum increases, the intake pressure decreases, therefore we reduce the fuel rail pressure the same amount to maintain the constant pressure drop across the injector. For boosted applications, we need to do the same thing in the other direction, i.e., as boost increases manifold pressure, we need to similarly increase the fuel pressure (although most stock fuel pressure regulators will not do this). The confusing part is we normally talk about manifold vacuum in units of inches of Mercury (in. Hg.), while boost and fuel pressure are usually measured in pounds per square inch (psi). If you do the units conversion, at 15 in. Hg manifold vacuum (idle with a mild cam), we should reduce fuel pressure by about 7 psi.