Once the engine is fully up to temperature and you're in a steady cruise mode, it's best to bring the AFR to what is called "stoichiometric" which, for gasoline, is 14.7 parts of air to 1 part of gasoline. This is the most efficient AFR for completely burning the gasoline and having nothing harmful dumping out the exhaust. It's also the hottest-burning AFR, so when you jump on the throttle, things have to change again or you could melt some pistons after a while.
At wide-open throttle (WOT), the AFR must go rich again in order to generate maximum power while lowering combustion temperatures. Finally, on deceleration, you'll want to shut off fuel flow to the engine to help with engine braking, unless rpm is getting below about 1,500. Then you'll want a slightly lean AFR so the engine is ready to go when you hit the pedal again.
So it seems that we expect a lot from the engine, and keeping it happy requires a number of adjustments. Carburetors typically offer a limited number of adjustments that can't really compensate for all possibilities. You may be able to set up the baseline AFR by changing needles or jets and set the idle speed easily. What is far more difficult to handle, for example, is the change from summer to winter gasoline blends, or compensating for altitude changes while on a trip across the mountains. A carburetor is completely unable to adapt to these changing conditions as you drive, and is known as an "open-loop system." At best, your setup is a happy medium, but once you stray too far from that medium, you're not going to stay very happy.
Close The Loop
In order to make the necessary adjustments to keep the engine running as we like it, there has to be some feedback to a control system that can make the adjustments. Now, in the modern context, we're really talking about full electronic control, which includes spark timing as well. The fuel injectors themselves are a key component in the whole system because they can be controlled, and that adjusts the amount of fuel delivered to the engine. Controlling the fuel injectors is an electronic control unit, roughly equivalent to a desktop XT computer from the '80s. Finally, there is a broad array of sensors that are read by the controller. From your perspective, the most important sensor is the throttle position sensor because that is how your go-pedal instructions are read by the ECM.
Other sensors are also connected to the ECM so that it can read information about the engine, such as coolant temperature, rpm, and crankshaft position. A few other sensors are used to determine information about the incoming air and the outgoing exhaust gas. In creating a feedback, or closed-loop, control system for the engine, the oxygen sensors in the exhaust plumbing are the most important.
We previously mentioned air/fuel ratio and the particular ideal or stoichiometric ratio, where gasoline is fully consumed along with the supplied oxygen. In general terms, it's the primary objective of the ECM to adjust the fuel supply so that the oxygen sensor reads zero oxygen content in the exhaust gas. This indicates that the highest efficiency with lowest emissions has been achieved.There is now a closed-loop control system looking after engine operations. When the signal from the oxygen sensor deviates from the target level for a given set of conditions, the ECM can change the amount of fuel delivered, which will affect the amount of oxygen in the exhaust gas. If that change is in the right direction, smaller adjustments are made for fine-tuning. If the change is in the wrong direction, other appropriate changes are made. This kind of feedback was never used with a carburetor.
EEC Evolution
Over the years, there have been several versions of Ford's Electronic Engine Control (EEC) systems. The original EEC I debuted in 1978 to control ignition timing, exhaust gas recirculation, and secondary air injection by way of the Thermactor Air pump. EEC II arrived the following year, adding oxygen sensor feedback and digital fuel volume control by way of a stepper motor.Central fuel injection (CFI) debuted in 1980, along with EEC III, which was updated to control one or two central fuel injectors located in a housing that sat where the carburetor used to be.
 One thing we found when building...  One thing we found when building our fuel system for our Factory Five Roadster is that Ford increased the fuel-pump outlet-line size in 1994. So for all you Fox Mustang owners looking to run -6 line with a larger in-tank pump, do yourself a favor and score the pickup bracket from a '94-'95 Mustang GT. |  Speaking of fuel, if you're...  Speaking of fuel, if you're increasing pump capacity and line size due to large horsepower numbers, adding big-bore billet fuel rails should be on your list as well. A supercharged engine can suck stock fuel rails dry under boost; the same goes for huge injectors. |  If you're running stock fuel...  If you're running stock fuel rails and a fairly basic EFI system (more for driveability than all-out power), you can probably get away with the stock fuel pressure regulator. But if you're running big power or an aftermarket fuel system (lines, rails, pump), then a regulator should be on your shopping list to properly control the fuel pressure of your system. |
 For those planning an EFI...  For those planning an EFI conversion for their carbed Ford, there is a multitude of fuel system choices, including complete fuel system kits that have a pump, regulator, lines, rails, and more under one part number from the likes of FAST, Aeromotive, Mass-Flo EFI, and others. |  Aftermarket EFI systems are...  Aftermarket EFI systems are another way to get the performance, mileage, and driveability you're looking for in your Ford project. This is the Mass-Flo EFI system that we used in our Factory Five Roadster project. It's based off the Ford EEC IV system, using an A9L ECM, custom intake, throttle body, and a GM-based mass air meter that resides inside your air cleaner housing. This system easily handles our 427ci Windsor stroker, making 500 hp. |  Once installed on our Smeding...  Once installed on our Smeding 427 Cobra Special and hidden by a Ford Racing Performance Parts Cobra oval air cleaner, the system is very low-key. With the color of the fuel rails blending into the intake, it can often cause people to do a double, even triple, take before realizing this is an EFI-equipped engine. |