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Definitive Piggyback Tuning Guide

The following is a guide to tuning your car with a SAFC, AFC, VPC, AFR, etc. It starts with basic techniques and proceeds to moderately advanced ideas.

Enjoy!


STEP 1: Setting up the car and the SAFC.

Before we begin the guide on how to tune with a SAFC, 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. Also, if you have a 255 lph or larger fuel pump with no adjustable regulator, then either get a new reg or don't try to use the fuel trim techniques outlined below.

Second of all, setting up the SAFC. At this point, I will assume that you have it wired in properly, if you do not, there are plenty of directions in the VFAQ. Also, may I suggest that you 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 SAFC, set the low trigger at 30%, and the high trigger around 80-85%.

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

Now, you want to use baseline corrections for fuel injectors. If you have 450's, leave both tables at zero. If you have 550's, put them around -10%. If you have 660's, usually around -18% would be a fine starting point. If you have a hacked MAS, then you will want to use about 5% MORE than these values.

The next section will cover fuel trims, and how to set the low throttle table.

STEP 2: Fuel trims and low throttle

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


Once you have the SAFC all set up, you should first start by setting the low throttle points, using the fuel trims. Doing this will require a basic knowledge of fuel trims, so I have outlined them below:

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.

Example: You are pulling in 30Hz of air at 800 rpm (idle). The ECU looks this up, and decides to inject 2.1 ms of fuel. However, the O2 sensor decides this is not enough. The ECU bumps this up to 2.2 ms, 2.3 ms, and finally 2.4 ms, when the O2 finally says that is perfect. If this keeps happening over a period of time, the ECU will increase the Long Term Fuel Trim to 114%, since 2.4 is 14% more than 2.1. It will, from then on, add 14% more fuel whenever it is in the range of that Fuel trim.

1g: 1g's have 4 fuel trims. The low trim is for idle and low rpm cruise conditions. The middle trim is for medium cruse rpm's (1500-2500ish) and the high fuel trim is for 2500+ rpm. The O2 trim is constantly changing with the O2 sensor, and it is what will cause the Long term fuel trims to change.

The approximate airflow ranges for the three trims are:

Low: 0-125 Hz
Mid: 100-175Hz
Hi: 175+ Hz

2g: 2g'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%.

You can also do this addition trick on a 1g with a TMO/Pocketlogger type setup.

Whew, that was exciting, but I think I covered it all. Now, on to the tuning. Set up your logger to display RPM, Airflow, and all the fuel trims your car has. 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 SAFC.

Now, look at the low fuel trim (2g's only have the LTFT). If it is positive, add a few percent on the SAFC at the 1000 rpm point. This is not an exact science, but usually for about every 3-5% on the logger, you need 1% on the SAFC. 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, especially in a 1g, so just wait.

When this is done, free rev to 1500 rpm and hold it there. Do the same thing, it will probably still be on the low fuel trim.

Continue to do this at 2k, and 3k rpm. 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 100%, plus or minus 10% Keep in mind that in a 1g, a perfect fuel trim is 100%, but in a 2g it's 0%.. That means that in a 2g, 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 within 5 or 10%, and they have stayed that way for a drive, you can carry the numbers across up to 7k rpm. So, if you have +5% at 3k and 4k rpm, use +5% at 4.5k, 5k, 6k, and 7k. Then, you will also want to use +5% 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, I will assume that you have your fuel trims leveled out near 100%, 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.

Also, make sure that you have no bad phantom knock, and that your base timing is set correctly to 5 degrees (on a 1g).

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

First, set up the logger. You want to make sure to log RPM, knock (if you can), timing advance, and airflow, and not many more.

Now, go make a pull. It is best if you can make one in third (or fourth) gear, but if you really have to do second, that might be ok to start. Make sure to go WIDE OPEN from 3k rpm to well above 6k. Also, make sure you have your boost set where you want it, it is actually easier to tune if you set it a few psi BELOW where you want it.

Now, save the log, and bring it up. Look at the 3k rpm portion of the graph, at knock and timing. Now you have to decide if, at 3000 rpm, you are rich, lean, or just right. If you are too rich, your O2 values will probably be pretty high (over 1.00v in a 2g, and over .95v in a 1g, approximately) and you will have no knock (although you can have rich knock, but we'll come back to that), and decent timing advance. If you are too lean, then you will have less timing advance, and knock.

On a 1g, you want to tune for no knock, end of story.

On a 2g, you want to tune for timing advance. You want to keep the timing advance graph on the logger above, say, 15-16 degrees, and you want it to be nice and smooth.

So, with that information, decide if 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 the logger. 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.

Tuning: Advanced

So, you have mastered the art of getting your fuel trims right at 100%, and you can make nice WOT pulls with no knock and/or 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 my information, but from talking to other members of this board as well. I also ask that guys who have lots of tuning experience (you know who you are) add their input here as well.

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 2g guys cannot view this knock sum on a logger, it is there, you just have to guess 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 SAFC, 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 SAFC 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 SAFC (big injectors, more fuel pressure, race gas) you decrease the amount of airflow that the ECU sees, and therefore you usually 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.

I have heard of 1g guys with 660 cc/min injectors getting timing advanced as much as 28+ degrees at WOT, because you hve to pull the SAFC correction factors down a lot due to the fact that 660's are 47% bigger than the stock 450's.

Fuel Cut

Another issue involving the amount of airflow the ECU sees, and the correction factors of the SAFC, 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, 550 cc/min injectors, you will be able to pull the correction factors within the SAFC down about 10%, perhaps more. This means that the ECU will see about 10% 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.
 
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