Specific Uses for SAFC

[f2tzoomzoom]

hi all, i'm new to the board as you can see, and i've been reading up the last couple days, goin through faq's, etc.

I know that the SAFC is an air fuel controller, but what exactly does this enable you to do?
Control fuel? control how much air the ecu sees? is it just a monitor?

i didn't see any thorough explanation of what it does, maybe i missed it, but hopefully someone can lend a helping hand

 

[DGajre777]

"Basic tuning theory

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.

On a DSM 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 DSMs.

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.

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.

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."

Source: http://www.dsmtuners.com/forums/showthread.php?t=206021 and ftp://68.37.199.223/DSMtunersFTP/DSMtuners/Documents/Basic tuning theory.doc

 

[DGajre777]

"Kyle Tarry, 2003

This is the new revised version.

This was originally posted on www.racingknowledge.com by myself, in order to serve as a tuning guide for some "newbies" there. However, I have found it to be invaluable, and hence have converted it to HTML and have put it up here for reference.


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.


I guess this guide has been getting around the internet a bit, so let me specify a thing or two: This was originally intended for DSM's, so the setup and fuel trims sections in particular may not apply to other cars.


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.

UPDATE FOR SAFC-2: If you are using one of the newer SAFC-2's, which came out in early 2003, then you have to use very similar settings, but they are going to differ a bit. For one thing, the SAFC-2 has 12 adjustable NEPoints, rather than 8 like the original unit. This just means that you have 4 more rpm points to play with, I would suggest adding them around 3.5k, 4.5k, 5.5k, and 6.5k rpm, but that is your choice.



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 barometric 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. One thing you can do to speed up the process is reset the ECU before you start to tune. This will reset the trims back to 100, so you can tune by the STFT (O2 trim) alone.

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. The timing will almost always dip lower than that when the boost comes on at lower engine speeds, around 3k-4k rpm. Then, the timing should rise SMOOTHLY to a solid 16+ degrees. Any dips in the curve, or flat spots, tend to indicate knock.

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.

If you richen the SAFC and the timing drops, that means you're not knocking and the increased airflow is causing the timing to go down. This is covered in the next section.

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.

UPDATE: The old DSM myth that 0-3 knock sum means timing gets advanced, and so on, is just that. While it looks like it's true in most cases, the real method used by the ECU is different.

In reality, the amount of timing the ECU takes out is directly proportional to the knock sum. The ECU takes the knock sum value, and multiplies it by 90/255, and thus comes up with "degrees of knock retard." 90/255 is very close to one-third, so by dividing the knock sum by 3 you get very close to the amount of timing retard.

If the timing maps say that you should have 18 degrees of advance, but you also have a knock sum of 3, then you will end up with very close to 17 degrees of advance.

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 give 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 have 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.

Timing and Air/Fuel Ratio Curves

With the rpm based adjustability of the SAFC, you have the option to use different correction factors at each rpm point, even if the SAFC is reading the same load. This allows you to do a couple of things, which some people don't realize are possible, or beneficial.

The timing and air/fuel curves do not need to be flat at wide open throttle! As a matter of fact, they shouldn't be. Timing generally needs to be at the lowest advance as boost comes on to keep the motor from knocking, which it does automatically. Then, once you are at full boost and WOT, the point where you need to be the most careful with timing is around 5k rpm, which is the torque peak. The torque peak is the point of highest cylinder pressure, and thus the point where knock is more likely to occur. As engine speed increases past this point, you can usually squeeze out a little more advance, for two reasons. First, as VE drops off (you can se this reflected in the torque curve) cylinder pressure goes back down. Second of all, for a set amount of advance in degrees, the amount of time BTDC decreases as engine speed increases. You may want more timing to compensate for this.

The A/F ratio curve can do a similar thing, transition richer around the torque and boost onset areas, and lean out at higher rpm's.

Just be careful, as you're more likely to run out of fuel at higher engine speeds (less time for the injector to open), and you're also more likely to do damage if you do knock.



Using the Monitor Function:

One thing it seems to me that a lot of people aren't familiar with, is the monitor function of the SAFC. There are a whole bunch of choices, but there are only a couple that you will really find useful:

RPM: This is obvious, it lets you see the speed of the motor.

Airflow: This will allow you to read the Hz output from the MAF, before you apply SAFC correction factors to it. This will let you see the airflow if it goes over 1606 Hz (the maximum a logger can read), and will let you compare numbers to compare performance gains.

TPS: This shows you the throttle position as a percentage. This may not match the number on the logger (mat say 0% when the logger says around 10%), this is normal.

Corr: This lets you see the correction factor that is currently being applied. This isn't good for a whole lot, but it can give you an idea for how the SAFC applies corrections when you are between RPM or throttle points.

It also won't hurt for you to become familiar with the save and replay functions of the graph mode, but I'm not the manual, so you figure that out."

Source: http://www.dsmtuners.com/forums/showthread.php?t=206021
and ftp://68.37.199.223/DSMtunersFTP/DSMtuners/Documents/DSM_Tuning_Sheet_v2.1.xls