Hello all, I am posting this based on the interest and feedback I got from my post in the Greddy chat forum. S-AFCs are inexpensive and easy to find and may be a good choice if you're not ready for a standalone ECU, but still want to run larger injectors. I am running 430cc injectors with my AFC and am very pleased with the results thus far.
This guide is to help you install an Apexi S-AFC into your 1990-1994 Miata. This guide can be used for both naturally aspirated and turbo cars.
STEP 0: What does the Apexi S-AFC do?
The Air Flow Converter takes the pulse (frequency) from the Mass Air Flow meter and either speeds it up or slows it down at different RPM points to make the ECU run the injectors either richer or leaner. All by itself, (as in N/A cars) it can only give you a little more bottom end power by leaning out the mix some. If you try to richen the top end you will just hit 100% fuel injector pulse sooner.
The best use for the AFC is to install larger injectors (runs richer) and then adjust the AFC leaner to compensate.
Installation is simple, and only requires installing a simple MAP sensor underhood and tapping into 4 wires and cutting one to intercept the air flow signal at the ECU.
There are 8 RPM adjusting points to adjust the air flow + /- 50%. These points are user defined within a 1,000 RPM band so that you can get finer control in certain areas if you need it. The rpm points come pre set at every thousand rpm from 1,000 to 8,000. The graph is highlighted for the rpm range you are setting.
The Super AFC also allows for two different maps according to load. It makes the load calculation based on the Throttle Position Sensor. You choose what is high and what is low.
STEP 1: Wiring the AFC
The chart below is from the Apexi S-AFC wiring guide. It tells us that for the 90-94 Miata, we'll use wiring diagram Z5-c. Ignore "sensor type" for now, we'll cover that later
As mentioned above, the AFC is wired into your ECU and MAP sensor. The MAP sensor is required since the Miata does not have a linear throttle position sensor as with many other vehicles. Due to this, we must use a MAP sensor for the throttle input signal on the AFC. This is actually a benefit, since it allows for more accurate load based fuel control. We'll cover the MAP sensor install later.
After disconnecting the battery, start with your ECU wiring. The diagrams below tell you which wires to tap in the stock ECU harness and the proper way to tap into them.
ECU Wiring Diagram
AFC Red wire: connect to "1G Power"
AFC Green wire: connect to "RPM Signal"
AFC Brown Wire: connect to ground (closer to the ecu: see diagram below)
AFC Black wire: connect to ground (1cm further away from brown wire: see diagram below)
AFC Yellow wire: connect to airflow signal output (to ecu)
AFC White wire: connect to airflow signal input
AFC Gray wire: connect to throttle signal (To MAP sensor signal wire)
STEP 2: Installing and Wiring the MAP sensor
A manifold absolute pressure sensor (MAP) is connected to the intake manifold to monitor intake vacuum (and boost). It changes voltage or frequency as manifold pressure changes. Since the Miata does note have a true linear throttle position sensor for the S-AFC to use, We'll instead use a MAP sensor. There are many different MAP sensors out there, and most all can be used. For turbo cars, what you need is one that registers high enough for the boost levels you plan to run.
GM 3 Bar MAP Sensor
The GM sensor is a popular choice because it is widely available new and used. This is the sensor that I chose. The instructions below are for the GM 3 bar, but most any MAP sensor will be wired in a similar fashion.
When installing your MAP sensor, find a good location and install "above" the throttle body (high on the firewall is a good spot) and use a clean vacuum/boost source from the intake manifold to connect to the vacuum input on the MAP sensor. Run the wires carefully away from heat and moving parts.
If using the GM 3 bar, you'll need a plug for it. You can get these used off of GM vehicles, or they are available from many online performance shops. The GM MAP is wired according to the illustration above and the corresponding list below.
B: Sensor Signal To AFC Throttle signal wire (Gray)
This wire will need to be passed through the firewall into the ECU area where the AFC harness is located. There is a little pass-through on the passenger side firewall on Miatas that is perfect for this.
C: 5V power in
(Finding a good 5V signal is key. On my Miata, I tapped into a AFM wire. There are others in the engine compartment, just be sure to use a simple multimeter to test)
Picture of the sensor installed in my 1992 Miata
STEP 3: S-AFC Setup!
Now that you're all wired up, it's time to reconnect your battery and adjust your S-AFC preferences and settings. If you bought your S-AFC used, or borrowed it from another car, it will have a memory of that car's settings. First off, follow the instructions in the owner's manual linked below and "Initialize it" before getting started. This is especially important when using a MAP sensor, because the AFC actually "learns" the voltage of the sensor you choose.
One of the early menu settings that you'll adjust is "Sensor Type" You'll use the setting "FLAP" and "08 08" ("flap" refers to the Miata AFM which is a flap type sensor, the "8" is simply a setting in that same menu in the AFC.) See pages 24 and 27 in the manual below for instructions on how to set this.
Apexi S-AFC Intstruction Manual: Right click and save (PDF)
Next is setting up your "low" and "high" throttle points
When using a MAP sensor, you set your low throttle point at full idle vacuum and the high throttle point a little above your maximum boost pressure.
Since the output of the pressure sensor will read in "throttle percent", you need to do some testing first. First, you can turn the key to "ON" without starting the car. Now you can record what throttle percent is 0 psig.
Start the car and you can record what throttle percent is full idle vacuum. How you get the throttle percent at full boost depends on your car. If your car is already tuned right [without the S-AFC], you can boost and record throttle percent. Be careful whenever you are driving and trying to read the display. Its best to have a passenger do the reading for you. Keep a pad of paper to write it all down on. The throttle percents should vary linearly between the data points.
Another method is to pressurize the sensor. You can tee the sensor together with a boost gauge. Use the boost gauge to read what pressure you are giving the sensor. A bike tire pump hooked in the vacuum line can be used to simulate boost.
Try to run the car rich and then lean things out slowly. A wideband o2 sensor is highly recommended. There are other sites with good tuning methods for getting the right settings on your S-AFC. Here is an example: http://www.roadraceengineering.com/newafc.htm
Good luck and enjoy!
This guide is to be used for information purposes only. Thanks to http://www.RoadRaceEngineering.com for some of the AFC explanations and details used above.