[Urbanmyth]

How to street tune: Apex SAFC
Part One: Basic Operation Concept

Okay, I understand that not all of us are fortunate enough to own DSM Link or AEM EMS, so I have put together a basic tuning guide for you guys out there still running Apex’s Air Flow Converter.

Note: Hey guys, I will start by saying this is the first installment of my street tuning guides. Alot of this information has been pulled from various mags and websites, and helped me to culminate all this information into a single guide .This guide is to help you understand how the Apex SAFC works and is in no way a complete explanation on how to run or tune your car. I am not held responsible for you causing damage to your car. All examples given are just that, examples! Don’t assume any setting I have plugged in here will work. Every car is different and every tune requires attention to detail, and concepts of sound tuning knowledge. Also, before we start, I will assume you have read the ENTIRE instruction manual, right? Also, it is wired correctly, you know what all the menus are, and the settings required for your particular vehicle are correct. If you bought your AFC used, Google it, the instruction manual is available online.

Apex SAFC: Explained

Before we get started, we must first understand the concept of the Air Flow Converter.
Basically, (and without getting into complicated detail) the AFC “intercepts” the MAS Airflow Signal coming into the ECU (Hz) and “fools” the computer into thinking less air is entering the engine. Since fuel delivery is primarily based upon Airflow, this will allow you to compensate for larger injectors, by reducing injector pulsewidth.
When you run larger injectors, the ECU doesn’t have a “sixth-sense”. It doesn’t automatically know you have larger injectors, so it won’t adjust injector pulsewidth. Basically, this means if your running 550cc injectors, the ECU will spray them as if they were 450’s causing you to run rich, (but we’ll get to that later). For the most part, when you upgrade fuel injectors, all it means is you have an injector with bigger holes in the end of them that allow more fuel to spray for a given amount of time. (Hint: that’s why for the most part injector prices are pretty close regardless of size) And no kiddies, THAT DOESN’T MEAN YOU CAN PORT YOUR INJECTORS!!!

Concept of Operation:
Okay, let’s talk about basic AFC operation. As I stated before the SAFC intercepts the frequency signal from the MAS and converts it to a set value based on your settings. How it does this is the AFC has a set of maps that are indexed by rpm and engine load (TPS) in which it calculates a percentage correction. The percentage correction is exactly what it says: it will compensate the percentage you enter at each RPM and Throttle Position, off the actual value measured at the MAS.
For example, if the actual signal coming into the ECU is 500 Hz, and you have a correction factor of (-10%) the new value the ECU will calculate is 450 Hz. Make sense?

So if you have a larger injector, and your car is running rich or sluggish, you must make the correction factor in order to compensate. Ideally, after the correction the injector pulsewidth will be reduced, and the car should operate as though it were stock. Now there is a benefit to this regarding Open Loop Conditions, but we’ll get to that a little later on.

The next correction factor you must take into account when tuning with the AFC is Throttle Position Compensation. There are two TP setting included with the AFC, Hi, and Low. Although it seems simple, you need to understand that it’s not as straight forward as it looks. Below the low setting, the car will calculate airflow from the tables from your low throttle point settings. Above the high setting, the ECU will calculate airflow based upon the high throttle point settings. Everywhere in between is what confuses people:

The important note that needs to be take into consideration when using the Air Flow Converter, is your mid point settings. Most people are misconstrued by this feature, and it tends to complicate some. The AFC operation is not as straight forward as it seems. For example, it’s easy to think that if your correction factor for say, 3000rpm is (-10%) then at 3250rpm, (or throughout the 3K rpm range) will be (-10%). Not the case.
As I stated before, AFC corrections are based on indexed tables that will calculate correction factor. At these mid-points it will take the average of the two, just like the graphs you use to do in math class, same thing. The easiest way to figure these values, and allow you to make optimal settings is to determine the average between the to base value settings. Here is an example kiddies:

If your 4000rpm value is (-10%) and your 5000rpm value is (-20%), then your total correction factor at 4500rpm is (-15%) Get the picture? All you did was take the average of the two values base on the principles of tables and graphs, and found the value at a given midpoint. 4750 would be (-17.5%) and 4240rpm would be (-12.5%), so on and so on. For more info on graph principles, search it or go back to math class.

Now to complicate things even further, this principle holds true for your Engine Load Settings as well. (TPS) As I said before, when you’re below your low setting, it will compensate airflow flow from your low table, and when above your high table it will use your high settings. Everything in between uses the same average principles as described above. So an example is as follows.

Low throttle point is 30% TPS,
High throttle point is 80% TPS,
Engine load is 40% TPS.
Engine RPM is 3000rpm.
Low Pt Settings: 3000rpm: (-20%)
Low Pt Settings: 4000rpm: (-20%)
Hi Throttle Pt: 3000rpm (-10%)
Hi Throttle Pt: 4000rpm (-10%)

With these settings:
Correction factor with 40% TPS
@ 3000rpm is going to be (-15%)
@ 4000rpm is going to be (-15%)

This is because your taking (-20%) @ 4000rpm and (-10%) @ 3000rpm and finding the mid points in reference to throttle position. The average is used because 40% TPS is between you 80% HI and 30% LO TPS setting. Of course this is only an example to help you understand better, but you need to make your correction based on data logs and Air/Fuel Ratios. Also the AFC has a log feature that allows you to view your total correction factor based upon Airflow frequency signal. Now for all you cavemen out there here is your average equation to find the median. (Mid-point)

X + Y = Z / 2 = midpoint

(-20%) + (-10%) = (-30%) / 2 = (-15%) correction factor
Now for throttle points:

HI TPS (80%) 4000rpm (-20%)
LO TPS (30%) 4000rpm (-10%)
TPS of (31%-79%) @ 4000rpm will be (-15%)

So let’s complicate things a little to help you figure this out with mixed values, so you can see how to apply the formula.

Settings:
Hi TPS (80%)
Lo TPS (30%)
3000 Rpm Lo: (-14%)
4000 Rpm Lo: (-17%)
3000 Rpm Hi: (-8%)
4000 Rpm Hi: (-7%)

What is your correction factor @ 3500 rpm at 65% throttle?

Correction Factor @ 3500rpm: (-11.5%)
Correction Factor @ 3400rpm: (-11.4%)
Correction Factor @ 3300rpm: (-11.3%)
Correction Factor @ 3200rpm: (-11.2%)

Get the picture? First find the midpoint between throttle points, and then the midpoint between RPM’s given the midpoint calculated values from TPS percentage.

3000 rpm: (-14%) + (-8%) = (-22%) / 2 = (-11%)
4000 rpm: (-17%) + (-7%) = (-24%) / 2 = (-12%)

Midpoint that @ 3500rpm:
3000rpm: (-11%)
4000rpm: (-12%)

(-11%) + (-12%) = (-23%) / 2 = (-11.5%)

Correction Factor @ 3500rpm is (-11.5%) Now it’s easy to see where all the points on the table go for RPM by the tenth.

Okay, enough with the corrections, we get it, right?

Now there is one major change we have to take into account when making Airflow compensations. Ignition Timing:

For the ECU to calculate timing, it looks at engine speed (RPM) and Airflow/ per revolution (measured by Volumetric Efficiency). If you read my article on VE, you’d know what that means by now. 
Based upon those settings the ECU can calculate its own timing map in order to maintain the best efficiency. The timing map is set up just as your airflow map: A series of spreadsheet graph tables that represent all the above values.
The lower the airflow, the higher your timing advance. This is mainly because cylinder pressure is lower, and lower air generally means lesser engine load. You will get more advance to allow the Air/Fuel mixture to combust at the correct point. Now when you enable a correction factor device (such as an AFC) to “trick” the ECU into thinking there is less air, (when there really isn’t) your going to get the same advance with a higher airflow, which we all know what that can lead to: Pre- Detonation.

Now that you understand the basic use of the AFC and correction factors, I can take a little break.. my fingers hurt! Expect part two soon!

[BoostedCivic]

Wow, what a awesome guide man, I really liked that (Too long though!). Nice job.

[KyleAdkins]

Wow Urban.. Nicely written guide. Two thumbs up!