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Air Fuel Ratio Tuning Tips

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First, a quick overview of the DSM air metering system:

The goal of the DSM air metering system, and any air metering system really, is to be able to calculate the mass flow of the air entering the motor. Now, in order to keep this short Iím going to keep the chemistry to a minimum, if you want more description in that area ask or start another thread. The basic premise is the combustion of gas and air is a chemical reaction, for which you need a certain amount of air molecules and a certain amount of gas molecules. The mass of a fluid is directly proportional to the number of molecules of said fluid, and thus by knowing the mass of something you can figure out the number of molecules.

Chemically, it turns out that in order to have a reaction that uses up all of the gas and all of the air available, you need to have about 14.7 times the mass of air, as you have the mass of gas. For example, if you have 14.7 pounds of air, you need 1 pound of gasoline to burn both of them completely.

The ECU needs to calculate the amount of fuel to shoot into the motor. In order to know how much gasoline it needs to inject, it needs to know how much air is coming in, and what the target air to fuel ratio (A/F ratio) is at that point in time. This is the job of the air metering devices, and some of the code in the ECU, which interprets the information from these devices.

The DSM air metering system consists of four parts: the karman vortex sensor in the MAF, and intake air temperature sensor, the intake air pressure sensor (barometric pressure sensor), and the ECU.

The karman vortex sensor works by detecting the frequency of small vorticies that happen when a smooth airflow is presented with an obstruction. The number of vorticies is proportional to the velocity of the airflow (I believe it follows the form frequency = kv^2). When the MAF outputs a ďHzĒ signal to the ECU, what you are really seeing is the frequency of these vorticies inside the upper chamber of the MAF.

Now, if the ECU knows the velocity of the airflow, and if it is programmed to know the cross-sectional area of the MAF, then it can calculate the volumetic flow of air through the MAF. Think about it, if you have air moving at 1 foot per minute, through a tube that has a cross-section of one square foot, you will have a flow of one cubic foot per minute. The flow of the motor is obviously much higher than that, but itís the same theory.

If you remember high school chemisty, then you remember the ideal gas law. The ideal gas law states that PV=nRT, or when rearranged, that PV/RT=n. ďnĒ is the number of molecules of the gas (which, as we said above, it proportional to mass). P is the pressure, V is the volume, T is the temperature, and R is a constant depending on the units used to describe the aforementioned items. In order to find the mass of the incoming air (n), me need to find all of the ďother stuffĒ on the left side of the equation. We already solved for volume (in terms of volume per unit time), R is a constant stored in the ECU, and we can find temperature and pressure from the other two sensors in the MAF!

So, we do some math, and we get an air mass. Then, the ECU looks at a air/fuel ratio table (if in open-loop) or shoots for 14.7:1 (if in closed loop), and decides how much fuel it needs. Since the ECU knows how much the injectors flow, it can figure out how long it needs to open them up. For example, if it wants half the fuel that the injectors can deliver, then it will just open them up half the time.

Now, what happens when you install larger injectors? Larger injectors basically have bigger holes in the end, so if you open them for the same amount of time as the stockers, you squirt out more fuel. Of course, the ECU doesnít know that the injectors are flowing more, so it keeps opening them for the same amount of time. Therefore, in order to get back to the A/F ratios that you want, you need to reduce the pulsewidth of the injector for each set of conditions.

The way a SAFC, or any other piggyback, does this is by reducing the amount of air mass the ECU thinks it is seeing. If the ECU thinks it is seeing less air, then it will shorten the pulsewidth. Since itís not really seeing less air (the SAFC intercepts the airflow signal and reduces it), you get the same amount of air, plus a shorter pulsewidth, plus a larger injector, and the end result puts the A/F ratio back where it should be.

Basic SAFC operation: The SAFC intercepts the Karman frequency signal coming from the MAF. It then has a set of maps, indexed by rpm and load (TPS), which it looks to to find a correction factor. It multiples the airflow signal by that correction factor, and then sends the changed signal to the ECU. For example, if the airflow signal is 1000Hz, and the correction is -50% (maximum reduction) it will end up sending a 500Hz signal to the ECU.

One of the main things I think people donít understand about the SAFC, is the way the rpm and load based interpolation works. While I wonít go off on a tangent on load here, the basic theory is that TPS is a representation of the amount of load on the motor (although not a very good one on a turbo car). The two different maps allow you to have different corrections under different load conditions, so that, for example, you can have the car richen up more as load increases in order to attempt to avoid knock. The use of the RPM points is obvious, as they just allow you to change the correction based on engine speed.

The question that comes up is, what happens if youíre between points? In reality, itís pretty simple. The SAFC just builds a curve (although it is built out of straight lines between the points, as with most engine management). If the correction is +10% at 2000 rpm and +20% at 3000 rpm (not likely, but it makes for an easy example) then the ECU will basically ďconnect the dotsĒ and when you are between those two points it just looks where that line points to on the correction map.

Examples: If you are at 2500 rpm, since you are halfway between the points, you will get +15% correction (15 is halfway between 10 and 20). If you are at 2250 rpm, you will get 12.5% correction, because that is one-quarter of the way from 10 to 20.

The SAFC does the exact same thing with the throttle points, between the high and low maps. If you are halfway between the two throttle points, then the SAFC will find the correction that is halfway between the corrections on the high and low maps. When you are below the lo throttle point, it just uses the low throttle settings, and when you are above the hi throttle point, the SAFC just uses the high throttle settings.

One of the main side effects of changing the airflow with a SAFC, is the profound effect it has on timing. The timing map in the ECU is set up as such: the ECU looks at the engine speed and the airflow (actually the airflow per rev, but we wont go there right now), and then finds the point on the timing map. The timing map is just set up like a spreadsheet, with the columns representing either engine speed or airflow, and the rows representing the other. The tendency of the timing map is that lower airflow (less load on the motor) gets more timing advance. This is for a couple reasons, but generally lower load means less heat and less cylinder pressure, which means you can use more timing advance to get the mixture to combust at the correct point. The effect this has is that if you reduce the amount of airflow that the ECU sees, it will move down on the timing map, and you will get more advance.

You need to be very careful with this. Not only do you get more advance at WOT, but you also get more advance at cruise and part throttle. In most cases, the WOT knock that can be caused by too much advance can be tuned out, but the knock caused at part throttle and when the turbo is spooling canít always be.

Written by kpt4321 at DSMtuners.com

Introduction
I am writing this Help file, to try and help all you people out there with SAFCs (Super Air Flow Converter from AíPEXi). I know how hard and frustrating it is trying to get help on something, and I hope that this answers most of your questions. This is all based on my experience, and what I have learned from others. If you have some comments to make, or some information tips to add, please feel free to submit them.

Quick Background Information
The SAFC is a computer that modifies the MAP Sensor (Manifold Absolute Pressure) signal to the PCM (a.k.a. ECU) of the car. This is really useful because it can "lie" to the carís PCM about how much air it is getting. Accordingly the PCM will give more or less fuel to the engine, thus helping you tune. Added use of the SAFC comes from the fact that you can play with the settings throughout the RPM range with different throttle positions.

Note: The SAFC does not officially support our Engine. Our MAP sensor is NOT listed in the manual. However, this isnít terribly crucial to the way the AFC works. All that the SAFC does is VARY a signal (in our case, a voltage). So, donít worry, the SAFC works perfectly fine, and what follows are some setup suggestions that have worked for me.

Gauges & Sensors
Before I start to talk about how to tune your SAFC, you have to know to whether your car is in tune or not! To do that there are a few things to note...

The best way to know if a car is running lean or rich is to use a wideband 02 sensor (Oxygen Sensor). But these cost way too much (in the region of $800 and up!). The next best thing to do is read the signal off your existing 02 sensor.

Basically, the voltage read from the Stock 02 sensor can be translated into three categories: Lean (too little fuel), Correct (Just Nice), and Rich (excess fuel). Unfortunately, the differences between the three categories are sometimes slightly blurred and it is hard to tell. For that reason you should always tune a little on the rich side.

To make your life easier, I highly recommend a digital A/F Gauge (the Air/Fuel Gauge displays the 02 Sensor Voltage). By digital, I mean with a numerical readout. Any gauge that just has blinking lights is extremely confusing, and pretty much useless. The gauge I would recommend is the Jumptronix. By getting this gauge, you are also supporting a fellow DSMer.

Along with your A/F Gauge, you need an EGT gauge (Exhaust Gas Temperature). The EGT gauge is used as a backup/check tool for the A/F Gauge. You basically tune the car using the A/F Gauge and you keep an eye on the EGT Gauge making sure that the temperature is within safe limits.

Here are some guidelines for reading your gauges:
For reading the 02 Sensor voltage (remember all readings are taken at Full Throttle). From OV - 0.88V (lean), 0.92V (just nice on pump gas) and 0.96V - Up (rich). Note: When you are at partial throttle (or when the car is idling), the 02 sensor voltages will cycle from and display anything from 0.2V and 1.0V. So donít panic, itís normal!

If the temperature probe for the EGT Gauge is placed on the exhaust manifold, the temperature should NEVER exceed 1650įF (I would recommend using 1600įF as a max just to be safe). If your car is Turbocharged, and your probe is placed on the Downpipe (after the turbo), use 1400įF as your max safe temp (1450įF is cutting it dangerous).

OK, now that all that is clear, letís get on to using the SAFC.

Setting Up the SAFC Correctly
Once you have installed the SAFC, there are a just few things that you should set up before you can start tuning.

First turn the key in the ignition all the way to the end, without actually cranking the engine. Basically you wan the SAFC to turn on with the engine off, so that you can set the initial SAFC settings without accidentally screwing up the engine.

Tuning
I will not give you any preset settings. This is for many reasons. All cars set up differently, and EVERY car has unique settings that you have set (no two cars are 100% identical, even if purely stock). Also, I am sure that someone lazy would just skip to this step, stick in some numbers, and do something stupid to their engine. Even though I am not liable, I still care....

Lo-Throttle Tuning
This is probably the hardest tuning to do, because the 02 Sensor Voltage is cycling while you are trying to set your settings.

First start off by going to the Settings Menu and by choosing Lo-Thrtl. There you will see a graph. The left & right arrows, let you select the RPM setting at which you want to tune at, and the up & down arrows let you change the percentage, in 1% increments.

Start off at 1000rpm. This is the most important setting for people with raised fuel pressures, or bigger injectors, because it basically helps you set your idle. What you want to do is make your car idle smoothly. 99.9% of the time most cars idle RICH (i.e. too much fuel). First you take note of the cycling voltage. You will notice that the voltage keeps flashing a maximum value. This is the value that you should keep paying attention to. If the value is ABOVE 0.92V, you have to start leaning down the fuel. On the SAFC press the down arrow and do it slowly so that you do it in 1% increments. You want to keep on leaning the fuel until the maximum voltage that shows up is around 0.88V-0.92V. Once you have that set up, let it idle for 30 seconds and make sure that it is still cycling with a correct max value. Sometimes the PCM tries to compensate for the SAFC changes. If the value has changed a little, then just simply lean it down or richen it up as necessary.

Now you can keep doing this tuning routine for other RPMs by slowly stepping on the gas pedal to rev and hold the engine at 2000rpm. This is actually quite hard and requires some practice. After you tune 2000rpm, tune 3000rpm. At this point you have a pretty good idea of the graph you are setting up, and in my experience, for every next rpm increment just go down 1% from the previous rpm setting.

Example: 1000rpm -16%, 2000rpm -15%, 3000rpm -14% etc...

HOWEVER, when you reach 6000 & 7000 rpm drop 2% from the previous values

Example: 4000rpm -13%, 4500rpm -12%, 5000rpm -11%, 6000rpm -9%, 7000rpm -7%

This is a good technique to use as a starting point for tuning.

What you have to do next is take a nice little drive on a long empty road, with a passenger. You need the passenger, because you will run off the road otherwise! You can really get carried away staring at the cool blinky-lights!

OK so, now you get to about 2000rpm in third gear and make sure that the SAFC is displaying rpm and throttle position (you have to make sure that your TPS value is less than what you set the Lo_Thrtl ó about 3 5%).

Now, just drive keeping the TPS below you Lo-Thrtl setting and checking your rpm and 02 sensor voltages. Remember if the value is below 0.92V at a certain rpm, just richen it a little (up arrow, to increase fuel) and if it is above 0.92V lean it up a little.

Remember, sometimes, it takes more than a 1% change on the SAFC to make the O2 sensor voltage change.

I am assuming that you have read (or at least skimmed) through the SAFC manual. You should know things like where to find the Settings or the Display menus.

In the Settings Menu, go to Th-Point. This is where you tell the SAFC which settings it should use. I recommend Lo 35%, and Hi 50% throttle. This is based on my driving habits; you should drive around a little and see at what throttle percentage you consider "low throttle".

Back again in the Settings Menu, select NePoint. This is where you set the RPM increments for which you would like to later tune the SAFC. The points that I recommend are 1000rpm, 2000rpm, 3000rpm, 4000rpm, 45000rpm, 5000rpm, 6000rpm, 7000rpm. I like the 4500rpm setting because you need that extra point for tuning at 4500rpm, as the 4000-5000rpm range is quite crucial especially at high throttle.

In the etc. menu, go to Sensor Type and select Pressure. Start off with In 06 & Out 06. For some unknown reason (maybe because the SAFC was NOT made for our car??) the setting In 07 & Out 07 works better in some cars.. It doesnít depend on the year of the car...

Now, Go back one screen and enter the Car Select menu and choose 4 Cyl and the Thr arrow pointing UP.

This basically covers the crucial settings. The can play with the rest of the settings like VFD Bright later (Vacuum Fluorescent Display Brightness, basically the brightness of the screen!).

Now turn on the car and make sure it idles! It will probably take two tries to turn it on the first time. This is because when you installed the SAFC, the car battery was unplugged and the ECU had lost all power. You did disconnect the battery..right?

Of course if the car doesnít start, then make sure that the battery is hooked up :-) Lost of people forget this because they are so excited after the install...


Hi-Throttle Tuning
Hopefully by now, you have a good idea of how the tuning works, and you are comfortable playing around with the SAFC.

First, start off by filling in the values for 1000rpm and 2000rpm. These should be set at the same values you used for Lo-Thrtl. When you are flooring the gas pedal, these rpms fly by so fast that it really doesnít matter. As a baseline for the 3000rpm setting, use the value for Lo-Thrtl at about 3% richer.

Example:
LO-THRTL: 1000rpm -16%, 2000rpm -15%, 3000rpm -14%
HI-THRTL: 1000rpm -16%, 2000rpm -15%, 3000rpm -11%

For the 4000 - 7000rpm start off with the SAFC at values at 0%.

Well, you know the routine by now: floor the car in 3rd or 4th gear, watch the 02 sensor voltages and make tuning adjustments. Remember donít make huge drastic changes in once step. Do it reasonably gradually. Also donít ever forget to keep your eye on the EGT gauge. Lookout for excessively hot EGTs (see above for the values, depending on your setup).

Some things to note:
The settings at 6000 & 7000rpm are usually pretty similar to each other. At these rpm settings, I would usually recommend running slightly rich like at about 0.94V. This is to be on the safe side in order to try and prevent knock which usually can occur at these high rpms.

Some other things to note for both Lo-Thrtl & Hi-Thrtl. After you have tuned the SAFC to run nicely in the first four gears, remember to check the settings in fifth gear. Due to the gear ratio of the fifth gear, sometimes the settings at higher rpms are too lean/too hot (EGT). The only way that we have found to get around this is to re-tune the car, on the fly, to run richer when you are going to be traveling at high speeds. In any case, you should always keep an eye out for your EGTs. This is your warning that something is running lean.


Blue Wire Mod
What is it? Well, after you have installed your SAFC, you will notice that some wires from the harness are unused. That is because they are there for applications of the SAFC in other cars.

One of the unused wires is blue, and the idea here, is to hook it up to the 02 Sensor. If you do that, and you go to the etc. menu and select the Sensor Check option, under #2 you will see your 02 Sensor voltage.

Now this may sound very nice and convenient, because it means that you donít need a Digital AIF Gauge. However, personally, I highly recommend against it. Here are a few reasons why:

- Itís a pain navigating between the Hi-Throttle and Lo-Throttle settings and the Sensor Check menu, especially while driving, when you need to be able to make quick adjustments and see the results very fast, so that you can continue adjusting the tuning graphs.

- People have reported getting wrong 02 Sensor readings when using the blue wire mod. It is suspected that the SAFC puts too much extra load on the 02 Sensor reading and so the reading is somewhat affected. This is especially true if you have hooked up more than one gauge to the 02 Sensor.

- The blue wire is actually meant for a second Air Flow Meter on some Nissans. Some people suspect that the SAFC sometimes tries to send a signal back through the wire. Now if itís connected to the PCM (ECU).. . .you can guess the rest.

Written at 2gnt.com