I get questions from readers about the AFC NEO, the legendary piggyback computer from Apexi that led the way for tuners since the mid 80s.
These questions vary from the advanced to the idiotic, but after getting a patch of comments asking questions about the unit and how it works, I’ve decided to go in depth and explain just what the AFC does.
To fully understand what the Super Air Flow Controller does, you must first understand what exactly you are trying to accomplish.
When adding a power adder such as a turbocharger, supercharger, upgrading your fuel system or upgrading your power adder can result in undesirable results without tuning your ECU.
When you add more fuel, or add more air, you need the ability to adjust fuel trims to reach the desired performance from your vehicle. To do this however, you will need a tuning computer, piggyback or standalone as well as understanding the 3 operating modes of the ECU itself.
A piggyback like the AFC is an easy, cheap alternative that allows you to get the job done with minimal hassle and cost. By splicing into your MAF or MAP sensor, the AFC gives you the ability to adjust the amount of air the sensor reads.
What does this change? When you lower or raise the mass of air seen by the ECU, you can lower and raise the IPW and subsequently the injector cycle.
By lowering the airflow value before it reaches the ECU, the ECU drops the IPW and basically compensates for the larger injectors or fuel pump you have installed.
While there are those members of the tuning community that likes to turn their nose up at such a crude and simplistic method of tuning. And while a piggyback isn’t IDEAL, it’s more than enough to get the job done, and I’ve tuned countless cars to the 400-500 hp range with nothing more than just a AFC for tuning.
But they do have a point… how you ask? We’ll get to that point later in the read.
Onward to your ECU’s modes, and explaining what each one does.
Limp Mode : This mode is when the ECU sees conditions that the manufacturer deemed “unsafe” and thus puts the car into this mode to prevent any serious damage. When one of these parameters is violated or breached, the ECU switches the car into limp mode, making the car overly rich and circumventing what could be certain disaster.
Limp Mode isn’t fun, but it will allow you to get the car home and / or to a mechanic for help.
Closed Loop : Closed Loop mode is where you as a tuner should spend the majority of your time. This loop describes the connection from the 02 sensor and your ECU / PCM. This mode helps the ECU determine if the previous combustion cycle was rich or lean, and what to do about it.
The ECU is aiming for stoichometric air fuel ratio, which would be 14.7:1, which is the ideal mixture for maximum fuel efficiency. Reaching this mixture will allow you to extract every last single bit of mileage from your vehicle.
As the ECU receives the narrowband voltage reading from your 02 sensor to determine the engine’s mixture, it sets values that we will speak about later.
These values are STFT ( Short Term Fuel Trim ) and LTFT ( Long Term Fuel Trim ) to adjust the fuel deliverability for the next combustion cycle. Whether or not your car is running lean or rich is an easy determination, once you have these fuel trim values.
Open Loop : Thirdly, but not lastly the final mode is what is commonly called “Open Loop” or Wide Open Throttle for those scoring at home. In this mode, the ECU reads values such as Intake Air Temperature ( IAT ), Barometric Pressure, and Manifold Absolute Pressure ( MAP ) to calculate the mixture required to operate the motor at WOT.
Based on the values the ECU reads from the motor, it calculates the amount of fuel needed to reach the desired AFR. While at WOT, the knock sensor informs the ECU when there’s more knock ( or pre-detonation ) than what it’s been programmed to accept. Using the feedback from the knock sensor, the ECU monitors the engine’s operation to ensure that things stay in one piece and don’t start melting.
In the event that the ECU sees an overly lean condition in the previous combustion cycle, it will then increase the value of the STFT, which in turn increases Injector Pulse Width ( IPW ) in an attempt to restore a stoich mixture.
In the event of an overly rich condition ( too much fuel ) the ECU will then lower the value of STFT, which in turn decreases the IPW to bring down the mixture back to the desired 14.7:1 / or .50 voltage via 02.
Now when the vehicle has run for an extended period of time, consistently rich or lean the PCM will turn the STFT value into the LTFT value, and return the STFT to zero. When tuning with a piggyback for maximum gas mileage, you want these 2 values as close to 0 as possible to prevent any modification to the Injector Pulse Width.
When tuning for maximum power however, ignition timing is the name of the game. If all other values such as IAT, boost and engine load are the same, your tuning changes to your AFC will dictate the ignition timing of your engine.
As the knock sensor is the ECU’s “eyes” at open loop operation, it’s important to monitor this value, because of the inherent nature of the SAFC.
Earlier, we spoke about possible downfalls of using a piggyback like the SAFC. Because you are “fooling” the ECU into thinking there is less air, there is more timing available right out of the box from the ECU.
Problem is, you haven’t truly changed the amount of air the engine is ingesting, you are merely fooling the ECU into giving you the IPW you need to get the car running. So when tuning your vehicle on a dyno at wide open throttle under load, you can see how this can become a problem.
So when you’ve upgraded your injectors to larger units, you need to remove more and more fuel ( or air flow reading to ECU in this case ). But since the injectors aren’t stock, despite the ECU treating them as such, the “factory IPW” creates additional fuel, that shouldn’t be there.
Additional fuel = rich condition = decreasing STFT until your car throws the good old P0170 CEL, Fuel Trim Malfunction (Bank 1).
So when tuning for maximum power, you must monitor knock to maintain maximum ignition advance in relation to your AFC.
When tuning for maximum gas mileage, you must monitor STFT, LTFT and the ACTUAL airflow value from your MAF or MAP. Please remember when monitoring your MAF or MAP, the reading given to you by a datalogger is the value AFTER the AFC has adjusted it.
To achieve true airflow value, you must take the value from your datalogger and divide it by the AFC Correction factor in decimal form.
For example, we have a Evolution XIII showing 19 lbs/min of airflow at 4500rpm, at 80% throttle you have adjusted the airflow reading by -21 on your AFC.
19lbs/min / 1-.021 = Actual Air flow.
Next up : WOT TUNING