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S-AFC/S-AFCII tuning guide.

19781 Views 0 Replies 1 Participant Last post by  turbolarry
The following is a basic guide to tuning an Evolution VIII with a S-AFC/S-AFC II. Special thanks to Kyle (kpt4321) and our sister site, for providing the basis for this article.

Step One: Setting up the car and S-AFC

Before we begin the guide on how to tune with a S-AFC, you must make sure the car is set up correctly to do so. Make sure all the fuel components are in good condition, and make sure you have no boost or vacuum leaks.

Make sure the S-AFC/S-AFC II is wired in properly;
DO NOT do the "blue wire mod", it has been proven to degenerate the O2 sensor's signal.

In the Th-Point section of the S-AFC, set the low trigger around 25%, and the high trigger around 65%.

In the NePoint section, set them to;
1k, 2k, 3k, 4k, 4.5k, 5k, 6k, and 7k,.

S-AFC II (adjust your points where needed):
1k, 2k, 2.6k, 3k, 3.6k, 4k, 4.6k, 5k, 5.6k, 6k, 6.6k, 7k

Now, you want to use baseline corrections for fuel injectors. These are just to start at, not to run at. If you have the stock 560's, leave both tables at zero. Other starting points:

660’s = -15% across, highs and lows
680’s = -20% across, highs and lows
720’s = -28% across, highs and lows
750’s = -33% across, highs and lows

If your on stock injectors, or your ECU has been reflashed, scroll down to the bottom of the page for a "high throttle only" tuning theory.

Step Two: Fuel trims and low throttle

Before proceeding past this point, you MUST have a logger of some sort!!

Once you have the S-AFC all set up, start by setting the low throttle points, using the fuel trims. Doing this will require a basic knowledge of fuel trims;

The ECU is, in essence, just a big set of spreadsheets (also known as "fuel maps"). It takes input from the MAS (in the form of Hz, temperature, and barometeric pressure) and comes up with a final value that represents the amount of air entering the engine. It also looks at the engine's RPM. With the RPM and an airflow value in mind, the ECU will look to the fuel tables, and find the amount of fuel it should inject into the motor.

Then the O2 sensor comes into play. The O2 sensor tells the ECU what the a/f mixture looks like, if it is rich, lean, or right in the middle (stoich.). If the O2 sensor says that the mixture is lean, then the ECU will add a bit more fuel on top of what the tables tell it, until the O2 values get close to stoich. If it has to do this for a certain period of time, it will take note of that in the fuel trims.

Evolution VIII’s only have 2 fuel trims, a long term fuel trim (LTFT) and a short term fuel trim (STFT). The STFT varies with the O2 sensor, an the LTFT goes for every rpm range. Since the STFT directly effects the LTFT, then you can just add the two together, and tune from there. For example, if the LTFT is +20%, and the STFT is -5%, you are at approximately +15%.

Now, on to the tuning. Set up your logger to display RPM, STFT, and LTFT. Start the car and let it fully warm up. Leave it at idle, and we will begin to tune the low throttle table in the S-AFC.

Now, look at the long term fuel trim. If it is positive, add a few percent on the S-AFC at the 1000 rpm point. This is not an exact science, but usually for about every 3 to 5% on the logger, you need 1% on the S-AFC. After adding or subtracting a few percent, let the car idle for a few minutes, and watch the fuel trims change. This may take a while.

Continue to do this revving and holding the motor at 2k, and 3k rpm, or your designated RPMS 3.6k and under. After you are done and are fairly confident they are close, take the car for a drive and see if they change. Try to get the fuel trims close to 0% (+/-5). Remember, if the fuel trim is negative, you have to lean it out a bit, and if it's positive, you have to richen it up.

Once they are at 0% (+/-5), and they have stayed that way for a drive, you can carry the numbers across up to 7k rpm. So, if you have -10% at 3k and 4k rpm, use -10% at 4.5k, 5k, 6k, and 7k. Then, you will also want to use -10% on your high throttle table, all the way across, until we begin to tune it in the next issue.

STEP 3: Hi Throttle

At this point you should have your fuel trims leveled out near 0% (+/-5), and that they have stayed like that for several days of driving. Also, this assumes that you have used the same correction factor that you used for the higher rpm's of the low table, all the way across the high table.

Now, it's time to do some real tuning.

Make sure you have your boost set where you want it.

Now set up the logger. You want to make sure to log RPM, timing advance, and O2 (front), and throttle.

Now, find a nice empty long road, turn on your logger, and make a pull in third gear. Make sure to go WIDE OPEN, really floor it, from 2.8k rpm to at least 6.8k.

Now, save the log, and bring it up. Once you organize the data, it should look something like this (stock 04 Evo RS);

RPM O2'S Timing TPS
2585 0.83 20 100
2753 0.87 7 100
3175 0.89 1 100
3664 0.89 5 100
4234 0.89 7 100
4718 0.91 8 100
5156 0.93 8 100
5550 0.93 9 100
5875 0.93 14 100
6171 0.93 15 100
6460 0.93 18 100
6738 0.95 20 100

Look at timing and O2 values at all RPM values. Now you have to decide if, at a certain RPM, you are rich, lean, or just right. If you are too rich, your O2 values will probably be pretty high (over .98v) and you will have a decent timing advance. If you are too lean, then you will have less timing advance.

You want to tune for timing advance and stable O2's. You want to keep the timing advancing nice and smooth and to peak at 19* (+/-2*) at 7k RPM. O2 values should also be flat across the whole RPM range with .92 to .94 volts, the ideal target range.

So, with that information, decide if, for example the 3000 rpm point is rich, lean, or just right. Then, add or subtract just a couple % of correction, depending on your findings. You want to only do a few percent at a time.

Then move on to the 4k rpm point, and do the same thing, looking at your data. Proceed with this up to 7k, and then make another pull with the logger to see the effects of your changes. This will get easier as you get more experienced, but it's not really that difficult.

If you consistently sees 100% throttle during pulls, it no longer needs to be logged, just monitored from time to time. This will give you a better sampling rate. If you are not seeing 100% when you should, your throttle sensor needs adjusting.

Tuning: Advanced

So, you have mastered the art of getting your fuel trims right at 0%, and you can make nice WOT pulls with a good timing advance. You've basically learned all that you need to know to have a car that runs well, but there is a little more to learn if you want run "really really well." This is where you will most benefit not just from this information, but from talking to others who have lots of tuning experience.

Timing vs. Airflow

Now, while the ECU has tables for the amount of fuel it needs to inject, it also has table for how much timing advance it should give you, and tables for how much it should advance timing depending on knock.

For 0-3 counds of knock, the ECU will advance timing. For 4-7 or so counts, it will leave timing where it is, and anything over 7 will result in the ECU bringing the timing down in an attempt to control the detonation. While we cannot view this knock sum on a logger, it is there, you just have to interpret what it is by the behavior of the timing curve.

Now, the timing tables in the ECU, just like the fuel maps, are indexed by airflow and rpm. With a S-AFC, this has an added effect. Since a SAFC intercepts that signal from the MAS to the ECU and modifies it, it can change the amount of airflow that the ECU "sees." If you have to correct your S-AFC into the positive range, than the ECU will see more airflow Hz than the MAF is outputting, and could change the timing map you are following. The problem with this is, higher airflow levels get less timing advance for safety, and lower airflow levels get more timing advance, because the ECU thinks you are pulling in less air.

By leaning out the S-AFC (big injectors, more fuel pressure, race gas) you decrease the amount of airflow that the ECU sees, and therefore you will get a bit more timing advance for power. This all assumes you have no knock, and also keep in mind that more timing advance will have an engine a higher propensity to knock.

Fuel Cut

Another issue involving the amount of airflow the ECU sees, and the correction factors of the S-AFC, is fuel cut.

For those of you who do not know, the ECU has a program that tells it to cut fuel when the airflow exceeds a certain amount. Now, this is with the final calculated airflow, not just the Hz signal, which means that temperature and barometeric pressure will effect fuel cut as well.

If you are to install, say, 660cc/min injectors, you will be able to pull the correction factors within the S-AFC down about -15%, perhaps more. This means that the ECU will see about -15% less airflow under a given amount of boost than it would have with the stock setup, which makes it much less likely for you to get fuel cut.

Tuning stock injectors or flashes for more high end power

ldstang50, one of our members, presented this idea regading high end tuning only. He says he's running this way on top of a flash and getting good results. I have yet to try this, but in theory it makes sense. It involves loading the Ne points into higher RPM's and only following the above tuning guide for high throttle only. This off course assumes your ECU/flash is handling whatever intake you have and your not having problems idling for any reason.
The theory is that since your still on stock injectors, or have been flashed, everything is fine down low. You disregard 1000-2000 RPM's, leave the low settings at 0% and let the ECU do it's thing. It's up high where you want to lean out the stock fuel map, or staighten out the flash up high because of its safty margin. You concentrate on high throttle and everything stays the same down low because there are no adjustments (0% across on your low settings). Ne point examples;

3K, 4K, 4.5K, 5K, 5.5K, 6K, 6.5K, 7K

2.6K, 3K, 3.4K, 3.8K, 4.2K, 4.8K, 5K, 5.4K, 5.8K, 6.2K, 6.8K, 7.2K

From there you follow the above tuning guide for high throttle only. You log like normal, make the necessary adjustments, and monitor properly. Again this is only if your on stock injectors, or flashed, and everything is running fine below 3000 RPMS.
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