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Fooling the S-AFC

So heres the S-AFC, alot of people use these to tune thier cars after putting in bigger injectors with a turbo. Its got a nice graphic display and is relatively powerful.


Please read up on how the MFI system works before continuing on.

You might also want to read up on choosing injectors. That also covers how injectors work.

The Apex S-AFC and Pressure sensors

So now you know all the basics of MFI. The Apex S-AFC reads RPM and throttle position. Based on the settings you input, it will increase or decrease the MAP sensor voltage at each RPM and throttle position point. The MAP sensor reads the absolute pressure in the intake manifold. The PCM uses the the output of this sensor [and others] to determine how much air is entering the system. If you know the temperature and pressure of the air in the intake manifold, you can make a pretty good guess of how much air is there. Once you know how much air you have, you can compute how much fuel will be needed to combust with that air. If you're interested, I've included a diagram below that shows how absolute pressure relates to atmospheric and gage pressure. The MAP sensor reads absolute pressure. Gages, like fuel pressure gages and boost gages read in gage pressure [makes sense huh?]. Your fuel pressure gage might just say "psi" for units, you can pretty much assume that it means "psig". In general, when speaking of boost or fuel pressure, I'll just say "psi" when I mean "psig".

Atmospheric pressure, P(atm), is 14.696 psia. The "a" on the end of "psi" stands for absolute. A "g" would mean gage. The absolute designation means that the scale starts at zero and increases. There are no negative absolute values. Gage pressures start at atmospheric pressure and increase or decreas from there. A pressure lower than atmospheric is called vacuum, thats a negative gage pressure. In short, P(abs) = P(atm) + P(gage). Consumer gages usually read gage pressure. The specifications of pressure sensors are usually written in absolute terms. Pressure sensor specifications are also often written in metric units. So, you should know that 14.696 psia equals 101.325 kPa [thats kilo-pascals absolute]. That should take care of all the details.

I've already mentioned a couple of reasons why bigger injectors are favorable for use with turbo kits. But tuning the car to run like normal is a pain. You'll have to work out what fuel pressures [at vacuum and boost] will give you a little more fuel than you'll need. You can then use the S-AFC to lean things out. Don't rely on the S-AFC to do all of the work, you really do need to use a rising rate fuel pressure regulator [like the Vortech S-FMU]. Since the PCM calculates how much fuel to inject based on several sensors, not just the MAP sensor, you need to adjust fuel flow externally [with fuel pressure]. If you keep your fuel pressure static and use the S-AFC to make all fuel adjustments from idle to full boost, the car will bog when you quickly hit the gas. It injects too much fuel and you won't go anywhere. It will run fine under closed-loop mode if you press the throttle slowly. So it'll be fine under normal driving.

Now you might be wondering why this page is called "Fooling the S-AFC". Well, I didn't think that it made all that much sense to base the fuel trim on throttle angle. I think that boost pressure is probably more important as an indication of how much air is coming in. Wouldn't it be nice to have a S-AFC that adjusts fuel based on RPM and boost pressure? You can do it and it'll only cost you about $35 in parts. The S-AFC looks for a 0 to 5 volt signal from the throttle position sensor. That is 0 volts at closed throttle and 5 volts at full throttle. It varies linearly in between those points. So all you need is a pressure sensor that reads 0 volts at the lowest pressure you'll see and 5 volts at a pressure a little above your maximum boost pressure. It needs to vary linearly in between those two points. The sensor must be able to survive in the harsh environment of an engine bay [high temperatures and dirty air]. Finding a sensor with all of these qualities is not easy, especially for a novice like me. After much searching, I did find the proper sensor. Its a Motorola MPX 4250 AP. It reads from 2.9 to 36.3 psia and gives a 0.2 to 4.9 volt output. It is actually designed to be used in turbocharged automotive MFI applications. You can get it from Newark Electronics for about $22. You'll need an enclosure to protect it. A little black box, 1551GBK, works nicely. You need several feet of high temperature 24 AWG wire. Three different colors is the best way to do it. That way, you know what wire is what. Tap-in connectors will also be needed. Radio Shack sells some with built in bullet connectors [for quick disconnects]. The part number is 64-3089. A crimp tool, zip ties, bullet connectors, soldering iron, solder, flux, drill, quick-cure plastic epoxy, and double sided foam tape will also be needed.

Building and Installing

This is the easy part. Before anything else, get familiar with the sensor characteristics. Read through the specifications in this PDF file: MPX4250A.PDF. Be careful with the sensor. Don't touch the leads with your fingers. The sensor is labeled "static sensitive", so don't take chances. Now drill a 3/16" hole in the side of the box for the sensor port to stick out of. Drill the hole so that the sensor can sit against the bottom of the box. See picture below.

Solder a 1 foot length of wire to pins 2 and 3. Pin one is denoted by the notch in the lead. Solder a length of wire to pin 1 that is long enough to go from the sensor to your S-AFC wiring harness. Drill a 3/16" hole in the other side of the box for the wires to pass through. Drill a tiny hole in the top for an air vent. Thread the wires through the 3/16" wire hole in the box. Spread some plastic epoxy in the bottom of the box and lay the sensor in it. Make sure the vent hole in the sensor is up [you don't want to epoxy it shut]. Push the port of the sensor though the port hole as far as it will go. Use some more epoxy around the port hole. You need to hold the sensor in place while the epoxy cures. If you shopped wisely, it'll only take 5 minutes. Once the sensor is secured, you can screw the top on. You can use the foam tape to attach the sensor to something in the engine bay [preferably something cool]. Mine is on my cruise control box. Use the tap-in connectors to tap into the 5 volt power and ground for the stock MAP sensor. It is very important that you use these two connections from the stock wiring. The voltage is very stable here. Sensor output is related to its power source. The accuracy of the sensor depends on those connections. Now run the long wire to your S-AFC wiring harness. Use a normal bullet connector to make the connection to the grey wire of the S-AFC [throttle signal wire]. Keep the wire away from hot stuff and moving stuff. Use zip ties where necessary.

Here is a complete wiring diagram

This is the sensor built and installed


This is the toughest part. 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. Of course, don't do it in traffic. 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. You can interpolate any value you want. If your car is not running, you'll need a way to pressurize the sensor. You can tee the sensor together with boost gage connection. Use the boost gage to read what pressure you are giving the sensor. An electric tire pump with a bleed valve hooked in the vacuum line can be used to simulate boost [thats what I did]. I really can't tell you the best way to set up the S-AFC. You can set your low throttle point at full idle vacuum and the high throttle point a little above your maximum boost pressure. Remember the whole absolute versus gage pressure discussion when you make any calculations. You could also set your low throttle point at 0 psig boost. Reference the link above about fuel injectors to help you figure out what fuel pressures are appropriate. Try to run the car rich and then lean things out slowly. An empty parking lot makes a good place to tune in. You can simulate real driving without traffic to get in your way. There are other sites with good tuning methods for getting the right settings on your S-AFC. For example, there are some here.

Contributed by Corbin

Cars Modifications Power Tuning S-AFC Fooling the S-AFC

Document statistics: Last modified on 2008-12-14 13:09:27 by Corbin

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