SAFC Tuning Mini-faq

This is a mini-FAQ for how to tune an AFC. The preferred method is to use a datalogger, but a method using some kind of O2 sensor voltage display is also provided.

Basic tuning theory

The factory Engine Control Unit (ECU) employs a narrowband Oxygen sensor to check the results of a closed-loop fuel ratio control process. The ECU comes with fuel maps that are based on the input from several engine sensors: The Intake Air Temperature sensor, Barometric Pressure sensor, Engine Coolant Temperature sensor, and Knock/Detonation sensor. The details could fill a large book, but the easiest way to think of it is that once the engine is at a normal operating temperature, the Fuel Injectors are opened for a certain amount of time, which is determined mainly from the Airflow reading. DSMs use a Karman Vortex style of Mass Air Flow sensor (MAF) that the air filter assembly attaches to directly. It is important to note that the airflow value is determined BEFORE the air is pressurized by the turbo, and that any air leaks in the piping between the MAF and the throttle-body will skew the airflow reading. A leak before the turbo will result in the ECU getting a smaller airflow reading than it should, and a leak after the turbo will result in the ECU getting a larger airflow reading.

The ECU operates in two modes. The first mode referred to as "Closed-Loop" is what the ECU uses for part throttle and cruising operation. In closed-loop operation the ECU cycles the air fuel ratio between a little rich and a little lean using the Oxygen Sensor Feedback Trim as shown on a datalogger. In this mode the actual Oxygen Sensor Voltage is unimportant, but the fact that it is cycling up and down is useful since it means the ECU is operating in closed-loop properly, allowing the ECU to fine-tune the Air/Fuel Ratio (AFR).

The second mode of operation is called "Open-Loop" and is used mainly at full throttle or whenever the engine RPM is over 4500. The ECU also uses Open-Loop if the fuel trims are out of range (I.E. you don't have it tuned right). In this mode the ECU adds fuel based on a direct lookup of the airflow on a fuel map. No fuel trims are used to adjust the base maps in this mode. The oxygen sensor voltage output will be fairly constant in this mode and can be used to determine whether the AFR is rich or lean. In short, if the O2 voltage is cycling up and down several times a second then it is in Closed-Loop, and if the O2 voltage is steady then it is in Open-Loop.

The fuel trims are adjusted by the ECU automatically to provide the "proper" AFR during all cruising/part throttle situations. This process works very well and the stock ECU is able to compensate for most sensor calibration problems. The ECU cannot compensate for larger injectors, or a crazy big fuel pump, or a modified MAF, this is where the AFC becomes useful.

How the AFC Works

The AFC is a simple device that alters the airflow signal that the ECU reads. The AFC sits between the airflow sensor (MAF) and the ECU, and works similar to an EQ for a stereo. The AFC adjusts the airflow value that the ECU reads up, or down based on RPM, and switches between a LO and HI adjustment map based on throttle position.

The MAF changes the frequency of the output signal based on airflow, so the more airflow the higher the frequency. The AFC takes the frequency input from the MAF, looks at the engine RPM, then looks at what the setting is on the AFC for that RPM. Since the AFC has a limited number of RPM points, the AFC interpolates the values when between two RPM points. So if the 4000 RPM setting is +15% and the 5000 RPM setting is +12%, and the engine is running at 4500 RPM then the AFC would adjust the frequency that the ECU sees by averaging the two values. In this case it would be (15 + 12) / 2 = +13.5%.

There are two maps on the AFC. One is called the LO map and the other the HI map. Which map it uses is determined by the throttle position. When the AFC is initially set up, the throttle position for the LO and HI map is configured. The default is something like 30/60. This means the LO map values will be used exclusively from 0-30% throttle, then it will interpolate values between the LO and HI maps from 31-59%, then use the HI map exclusively from 60-100% throttle position. For a DSM application, it is easier to tune if the maps are either on or off, instead of mixing the values between them. Set the Th-Point to 69/70 so it uses the LO map up to 69% throttle and then goes to full HI map beyond that. This separates the maps into closed loop and open loop maps, making it easier to tune.

Initial AFC Setup

A few installation tips first. Be sure to use the Pink and Orange wires for the airflow signal wires, and NOT the Yellow and White wires. The AFC is a universal device and is setup to handle both Speed-Density and Mass-Airflow type systems. The Pink and Orange wires are for Mass-Airflow systems like Mitsubishis.

In the Setting Menu, set the Th-Point to 69 for Lo and 70 for Hi. Set the Ne-Point so the RPM points are 1000, 2000, 3000, 4000, 4500, 5000, 6000, 7000 for stock rev limiter, or 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000 for raised rev limiter.

Setup tips provided by Mike Welch at Road Race Engineering

In the Etc Menu, set the Sensor Type to "Karman" for a DSM. The Car Select should be set to Cyl 4 with the Thr arrow pointing up meaning a rising rate throttle position sensor. Set the GRPH Scale to 30, Initialize will reset everything to factory defaults so it should be set to No.

1. Monitor

For monitoring gauge functions, we like the following settings:

1 Gauge RPM
2 Gauges RPM and Karman
3 Gauges RPM, Karman and Correction
4 Gauges RPM, Karman, Correction and Throttle Position

You will not be able to see any values for "Air Flow" or "Pressure". Why? Because you don't have an air flow meter or a pressure sensor to monitor. These are for other cars. If you push briefly both "Prev." and "Next" at the same time, you jump from which ever screen that you are on in Monitor to the last screen you were in on Setting. It allows you to jump directly from monitoring throttle position and RPM directly to the Lo-thrtl setting menu for adjustment. When on the Lo-thrtl menu, push "Next" to jump to Hi-thrtl. By pushing "Next" you can jump back and forth between Hi and Lo.

2. "Blue Wire Mod"

This allows you to read Oxygen Sensor Voltage on the AFC display. On the wire harness for the AFC there are three wires that normally not used on an Eclipse. These are White, Yellow and Blue. The Blue wire is meant for the second Air Flow Meter on a Nissan 300ZX. You will connect this wire to the O2 sensor wire right at the ECU. See the Jumptronix Install instructions for info on where to get the O2 sensor signal. To read the O2 sensor voltage, scroll down to and select the "etc." menu. Select the "Sensor Check". Of the three readings, O2 voltage will be the middle one (#2)

3. Common Problems:

RPM Signal off by half (1G)

* Because of the way that Mitsubishi triggers the ignition coils, this is normal. To fix it, just set the Cylinders to "2"

Car runs like crap, wont rev past 2-3K rpm, No change in the Hz reading on the Karman display

* You read the wrong section in the manual and chose the yellow and white wires to modify the airflow signal. Wrong, you should have used the pink and orange wires.
* You have a bad connection on either the pink or orange wire.
* You have the pink and orange wires reversed.
* You didn't select "Karman" for the air flow meter type.

No display or other power problems

* You have been swinging the AFC around by the wire harness. This will pull the harness out of its internal socket. To check it or fix it, carefully unscrew the back cover with the proper small size Phillips screwdriver. Make sure the little connectors are plugged in all the way.

Actually tuning the thing

So hopefully at this point the AFC is installed and the car starts up and at least sort of runs. The first thing to do is to figure out where to start. The main factors are the type of Airflow sensor and the injector size. If the injectors aren't the factory size, first determine the difference in the size of the injectors between the upgraded injectors and the original injectors. The original injectors will be 450cc for 1G and 2G turbo models, and 380cc for 1G automatic turbos. If you upgraded from 450cc to 660c injectors for example, take 450 / 660 and it works out to roughly 30% bigger than the stock injectors. If the MAF is stock then you just need to worry about the injector sizing. If the car is a 1G with a 2G MAF, the MAF alone will read about 20% leaner than the stock MAF, add the Injector sizing and MAF differences together and those are the starting points on the AFC. For example, a 2G with stock MAF and 550cc injectors would set the AFC to -18% from 1000-4000 RPM to start. A 1G with a 2G MAF and 660cc injectors would use -30% for the injectors and +20% for the 2G MAF, so that works out to -10 on the AFC from 1000-4000 to start.

Start the engine and let it warm up to full operating temperature. Turn off the motor and reset the ECU by pulling the fuse for 10 seconds. This is accomplished by pulling up on the yellow fuse holder in the fuse block by the BOV on 2Gs, or by removing the lower-right corner fuse labeled ROOM on 1Gs. This will clear the fuel trims that the ECU is currently set with.

Tuning without a Datalogger

Life is much easier with a datalogger, but not everyone has one. To tune right the car will need some way to view O2 sensor voltage (see Blue Wire info above), and an Exhaust Gas Temperature Gauge with the probe preferably mounted somewhere in the #1 or #2 exhaust runner. For the LO map, start at idle with the engine fully warmed up, and set 1000 RPM until the O2 voltage is cycling up and down and not holding a fixed value. Usually the O2 voltage will be switching back and forth between .3 and .7 volt. This as discussed above is referred to as Closed Loop operation. Once the O2 voltage is cycling, reduce the AFC setting until it stops cycling, then increase the setting until it starts cycling again, then add another 2 clicks on the AFC setting. With the Idle now set, cruise at a constant low throttle for each of the remaining RPM points and do the same. Cruise along at 2000 RPM until the O2s are cycling, then lean it out until the O2s stop cycling, then add fuel again until it starts cycling and then go another 2 clicks up. Do this for 2000-4000 RPM. With the cruising area set, take the 4000 RPM value and use that for 5000+, all on the LO map.

For the HI map, copy all of the settings from the LO map over so the HI map is set the same from 1000+. With the boost set at a conservative level, around 10-13PSI, do 3rd gear pulls from 3000-7000 RPM until the O2 voltage is pretty even at a constant value. The value should be .85-.88 for 1G pistons, and .92-.94 for 2G pistons. Watch the EGT gauge and let off the throttle if it exceeds 900C. Adjust the AFC as needed until the O2 voltage is in the ballpark and the EGTs are staying under 900C. This is the baseline setting and you can adjust up or down from there based on best judgment.

Tuning with a Datalogger

Set the logger to monitor RPM, Timing, Knock Sum, O2 Voltage, Lo Med and Hi Fuel Trims, and O2 Feedback trim, Throttle Position, plus whatever else you like. With the engine fully warmed up and the ECU recently reset, set the Idle 1000 RPM on the LO map until the O2 Feedback trim is averaging in the middle of the scale. While cruising around at part throttle, hold the RPM steady at each set point on the AFC from 2000-4000 and do the same until each of the RPM points have the O2 Feedback trim averaging in the middle of the scale. Pay attention to the Fuel Trims and if they are adjusting then compensate with the AFC. If the Fuel Trims are going leaner, then reduce the setting on the AFC and vice versa. The goal is to have the O2 Feedback trim averaging in the middle of the scale while in closed loop operation, and have the Fuel Trims right in the middle of the scale or slightly positive. Try to avoid having the Fuel Trims in the negative part of the range since that means the AFC is set too rich. Once the baseline settings are done, monitor the Fuel Trims over the next few days without making changes to the AFC and see where the Fuel Trims wind up. If they are within +30% of the middle of the scale then that is close enough. Set the 4000+ RPM range the same as 4000 for the rest of the LO map.

For the HI map, copy over the settings from the LO map to start with. Ideally the car will have an EGT gauge for reference purposes. This is not to exceed 900C while tuning. Let off the throttle should the EGTs get too high or the Knock Sum gets really high on the logger. With the boost set at a conservative value in the 10-13PSI range, make 3rd gear pulls from 2000-7000 RPM until the O2 voltage is in the .85-.88 range for 1G pistons or .92-.94 range for 2G pistons. Adjust the AFC until the O2 voltage is even across the whole RPM range at full throttle, and within the proper voltage range. Pay attention to the knock sum value and if it is high, try adjusting the AFC settings higher and see if that reduces the knock sum.

After the baseline values are set for both the LO and HI maps, turn up the boost slowly and monitor the EGT and Knock Sum. Ideally the EGTs will peak at 880C and the Timing on the logger will be above 15 degrees. Adjust the AFC up or down until this occurs. Keep in mind that if the boost is too high for the fuel octane, no matter how high you crank up the AFC the EGTs will still be really high. It is better to have less fuel and less boost than to have to add lots of fuel to control the EGTs.