I held an executive meeting with myself and the consensus was that it would be more fun to spend some of my limited time talking about the PCM, how it works (in regard to Ignition timing and Fuel Only. It does a lot more then what this will cover.), and how to make it do different things for you.
Here is a rather simple drawing of what goes on, under what conditions inside the PCM
The PCM receives Inputs from the sensors listed on the left.
Depending on WHICH Major Mode, Open Loop or Closed Loop, the engine is in, will determine what the PCM does with the sensor inputs.
Within the Major Modes (Open and Closed) there are sub-modes. I?m not going to retype them you can see them in the drawing.
IF the Engine is in any of the Open Loop Modes, the sensor input is collected by the PCM and run against a set of predetermined Memory Tables. Based on the values in these tables, the PCM will control the Ignition Timing AND Fuel Injector Bandwidth outputs.
IF the Engine is in one of the two CLOSED Loop Modes, the PCM will collect information from all of the sensors listed on the left
AS WELL AS the Oxygen Sensor. The PCM will then calculate (computer will actually do some work) the correct Ignition Timing AND Fuel Injector Bandwidth.
It is that simple folks. Where it gets complex is when you want to override either the predetermined tables (Open Loop) or the Calculations (Closed Loop.)
Tuning A/F Ratios
One way is to take advantage of the Adaptive Learning of the PCM, as described before. Kinda like teaching an old dog new tricks after the super charger has been installed. However, this is only going to get you so far and appear to have limits to how far the tables will adapt.
Another way is to find some really smart kid who knows how to re-burn the memory tables and let him do it for you. Provided you know what data should be in the tables to meet your needs.
Note: I believe but do not know for fact that is this what some blower company?s call ?reprogramming the PCM.? They re-burn the memory tables with new values. I sincerely doubt they change the Calculation Algorithm being it only effects Closed Loop Modes, Idle and Cruise.
A third way is to artificially vary the Input signals till you get the Outputs (timing and bandwidth) to your liking!
There are two input sensors that have a significant effect on the outputs, these would be the MAP Sensor and the Intake Air Temperature Sensor.
Both of these sensors are nothing more the variable resistors. As there resistance varies, the voltage signal being sent to the PCM will vary.
So, if you want to change that voltage input signal to the PCM all you have to do is raise or lower the resistance at one or both of these sensors.
There are a few device out there that do this. A friend recently sent me a unit made by Speedtweaks. It uses the IAT sensor signal.
By increasing the resistance on this sensor you are telling the computer that the incoming air is colder then it is (more dense) , therefore the PCM will dump more gas through the injectors. (by increasing the Injectors Bandwidth, within the injector physical flow ability to inject fuel)
HKS sells a similar unit that will tweak the MAP Sensor and trick the PCM into thinking the pressure is greater or less then what it really is, and adjusts the injector bandwidth accordingly.
These devices do work, but within there design scope.
If you need to just tweak things a little bit this is the way to go. You are limited to a small range of change as contained in the predefined memory tables.
Also, when you do tweak things this way, you are tweaking things across the entire RPM range, EXACTLY the same way.
If it is a little tweak, no problem, things should be ok.
But if it is a big tweak I think you will find a lot of negatives associated with this. Too rich A/F at some RPM?s and too lean A/F at others. It?s an all or nothing type of thing.
Also, none of these devices will help in managing A/F ratios for Closed Loop Modes.
The more expensive units, like the A?Pexi Super AFC or HKS AFR-D enable you to adjust (add or remove) resistance at the MAP sensor in 500-RPM increments thereby changing the A/F ration in 500-RPM increments.
In other words, at every 500-RPM you an opportunity to richen or lean out you?re a/F ratio.
Both units can and do manage the Air Fuel Ratios during Closed Loop Modes as well. This is particularly important during Idle when you have rather large injectors installed but no boost being blown into the cylinders.
Bottom line, the very first thing I will do if I find I have an A/F Ratio problem is to make sure all my sensors are working in peak order. (Read that replace them)
If I find my A/F ratio is across the board, a little lean or a little rich I will tune that out using one of the simple resistive units on the AIT or MAP sensor.
If I find I am rich or lean at different times, I will go with a piggyback A/F Controller. Probably the A?Pexi or maybe the HKS if I can learn more about it.
I hope this has been helpful to some. Please realize I am not an expert at this but I am more then a little knowledgeable on the topic.
The drawing was a simplification of the PCM in this regard. While the inputs, outputs and modes of operation are factually correct, there are many others that I have not shown to keep this simple.
Now it?s back to the salt mines to finish up the installation instructions.
Have a great day folks.
Frank