Note the difference between...
Note the difference between a 42-lb/hr injector calibrated MAF curve versus a 19-lb/hr MAF curve. Since the ECU maxes out at 5 volts, the 19-lb/hr MAF would "peg" at airflows around 800 kg/hr (good for around 300 hp, depending on the A/F ratio and engine efficiency). Higher horsepower means greater airflow, so you need a MAF that will measure it.
In Part 1 of this series, we mentioned the aftermarket MAF sensors that were calibrated for larger injectors. The technique was to reduce the voltage output for the same airflow, thus fooling the ECU into thinking there was less air flowing into the engine, so the ECU would calculate a shorter injector PW, and everything should work out without having to reprogram the ECU. The problem with this plan is the ECU load calculations become inaccurate, which could cause other problems.
But a good (and necessary) thing also happens with the larger injector calibrated MAF sensor approach. While the MAF sensor output voltage is reduced for a given airflow, you can also view it from the other perspective, where for a given voltage, the actual airflow is greater. The result is now a MAF that will measure airflow much greater than the stock 835 kg/hr before outputting 5 volts. For example, 42-lb/hr calibrated MAF sensors will measure anywhere from 1,700 to 2,900 kg/hr at 5 volts, depending on the size and type.
In many cases, the dynamic range of a MAF sensor calibrated for larger injectors works out closely with the dynamic range of the injectors themselves. In other words, just when you're at a horsepower level where you're running out of fuel-flow ability from a given size injector, you're about at the limit of measuring the corresponding airflow for a MAF sensor calibrated to that injector size. Installing a MAF sensor calibrated for larger injectors therefore becomes a necessity when forced to upgrade to larger injectors. But how do we get around the problem of the incorrect load calculations? Simple, by reprogramming the ECU so it knows the actual airflow for any given MAF sensor voltage, and the actual injector flow rates. If the ECU isn't being fooled, it will correctly calculate all necessary parameters.
Two-dimensional parameters...
Two-dimensional parameters are known as "functions," like the WOT fuel multiplier versus engine rpm parameter shown. The multiplier values here act on the desired A/F ratio, so lower numbers mean richer.
Now that we've explained the up front stuff, we can finally go into the ECU to tune. Keep in mind that you can not tune around a mechanical/ physical problem with the engine or fuel system. Doing so will only be a Band-Aid solution, and usually temporary at best. To get the best idle, part throttle, and WOT performance, you need to have a healthy engine and the proper parts to support the air and fuel requirements.
EEC Basic Tuning Concepts
In the Ford EEC universe, all tuning parameters are grouped into either "scalars," "functions," or "tables." Scalars refer to one-dimensional variables, such as idle rpm, engine cid, and so on, or "switches," where a feature can be enabled or disabled. Functions refer to two-dimensional parameters, such as WOT spark advance versus rpm. Tables are used for three-dimensional parameters, such as spark advance versus load and rpm.
Again, our focus for this series will be on fuel and spark tuning, with honorable mention on idle control tuning. Of course, there's lots more in the ECU beyond the scope of this article, dealing with things like emissions control systems programming, automatic transmission controls, and charge motion control/variable camshaft timing/electronic throttle control in the later vehicles. We'll save that stuff for the professional tuners, while we cover the basics of tuning here.
Three-dimensional parameters...
Three-dimensional parameters are set in tables, such as this startup fuel table. Numbers in this table subtract from the base OL fuel table A/F values, so larger numbers in the startup table means richer.
Fuel Injectors
For anything but a stock or mostly stock engine, you'll probably need to upgrade to larger fuel injectors, so you'll need to reprogram a few things in the ECU. Unfortunately, it's not just a single scalar that needs to change when reprogramming the ECU for larger injectors. Injector tuning parameters include the injector slopes, injector breakpoint, injector voltage compensation, and often, cranking fuel pulse width (PW).
If you plotted the mass of fuel flow per injection event, versus injector PW, you'd find the slope of the plot changes with PW. At low PW the slope becomes steeper due to the sloppiness of the injectors at really short PWs. At greater PWs, the fuel flow becomes more linear and stabilizes at a lesser slope. The fuel injector low and high slope scalars refer to the injector flows at short and longer PWs, respectively, which is somewhat confusing since the low-slope value ends up larger than the high slope. The fuel-injector high-slope scalar corresponds to the typical injector flow rating, so for a 42-lb/hr injector, the high slope scalar would be close to 42. The low slope would be a value somewhat greater than 42, depending on the injector.
Back on our fuel flow versus PW graph, the point at which the slope changes from high slope to low slope is called the fuel-injector breakpoint. This value corresponds to the fuel flow when the slope changes, not PW, and is another scalar that needs changing when larger injectors are installed. The ECU uses the breakpoint value to know at what fuel flow it needs to change its PW calculations from using the low slope to using the high slope, and vice versa. If your injector breakpoint value is incorrect for the injectors you're using, it can lead to idle quality issues.
Fuel injectors do not deliver...
Fuel injectors do not deliver a perfectly linear amount of fuel with increasing pulse width (PW). There's a point in the fuel delivery curve (the breakpoint) where the slope changes. Proper tuning requires the ECU to know both the high and low injector slopes, and the breakpoint.
Fuel injector flow rates will also change with battery voltage. Since injectors are electrical solenoid devices, as battery voltage drops, the injection opening events will become more sluggish. The EEC can compensate for changes in voltage, such that it adds a bit more to the calculated PW as the voltage goes lower. All Ford EECs include a function for injector compensation versus battery voltage. Function values are dependent on the size and type of injector, so when upgrading your injectors, this function should also be revised as necessary.
For Ford EEC, when starting the engine, the injector PW is not calculated during engine cranking, it is merely looked up in a function of PW versus ECT. If nothing else was changed on your engine but the injectors (i.e., the starter, cam, and throttle body are all the same, so the same air will flow into the engine during cranking), the cranking fuel PW function in the EEC should be revised, since larger injectors will flow more fuel at the same PW. Again, if nothing else changed, the values in the cranking PW versus ECT function can simply be multiplied by the ratio of old injector size divided by new injector size. If your upgrades included anything that would affect cranking airflow, then all bets are off to find the ideal cranking fuel PW for your engine. Test and tune accordingly.