Ford Mustang EFI System Tuning - Inside The Black Box Part 2 - Tech

In the base OL fuel table, A/F ratio is typically determined by load and ECT. At higher loads, or colder ECTs, the desired A/F ratio is richer. The stabilized OL fuel table determines target A/F from load and rpm. Typically, at low loads, the OL A/F ratio can be close to stoichiometric, but as the load increases, you want a richer A/F ratio for maximum power (and to guard against detonation).

The final OL A/F ratio is the table cell value for the current combination of ECT, load, and rpm (interpolated if necessary), then several adders or multipliers may be applied. Lambda values are sometimes additionally modified for such things as IAT, vehicle speed, and some programmable "global" adders and multipliers (global meaning it is applied at all times). For many EECs, there is a fuel multiplier function for WOT versus rpm, where fuel-table Lambda values are multiplied by the WOT fuel versus rpm function values (when in WOT mode), to arrive at the final A/F ratio target. For simplicity, it's often best to zero out most fuel adders and set all the multipliers to one, then tune your A/F ratio from the appropriate fuel tables.

The ideal WOT A/F ratio for an engine varies, from as lean at 13.5 for some naturally aspirated engines, to as low as 10.5:1 for some non-intercooled supercharged applications. Keep in mind the ideal spark advance and A/F ratio are related, not only from the charge cooling effect you get from a richer mixture (which allows you to run more spark advance without detonation problems), but also because the A/F ratio will affect combus-tion flame speeds (more about this later).

For CL, target A/F ratio will be stoichiometric (14.64:1) at all times. Injector PW will be calcu-lated by the EEC in the normal fashion, then adjusted based on the O2 sensor feedback (the short- and long-term fuel trims discussed in Part 1). In most vehicles, CL fuel mode will occur only when loads are low to moderate, Throttle Position (TP) is below a set value, and coolant/ air temperatures are above a programmed minimum value.

The transition back from CL to OL is important for performance applications. For example, if your engine makes big, instant boost (like with a screw or Roots-type supercharger), you want the A/F ratio to go richer (back into OL) as soon as you stomp on the loud pedal. If the EEC stays in CL too long, you can be running a dangerously lean A/F ratio (14.64) for a few seconds under boost. The EEC is normally programmed to transition gradually from CL to OL as the load or TP increases, so for our performance applications, we'll need to reduce this lag time. For most vehicles, there is a scalar value for OL delay time, which should be set to zero for most performance applications.

Spark-Control Basics
Before we get into spark control program-ming, we need to understand the difference between the spark advance the engine needs for MBT, and the spark advance the engine will tolerate before detonation destroys it. Spark advance is definitely not an "if a little is good, more is better" thing.

Without getting into pages of combustion theory, the short story is the engine spark advance needs general change as follows: