How and why to build a MAF table

Assumptions for this document are that you own a TwEECer or EEC-TUNER and that you are running larger than stock injectors and/or Mass Air Flow meter (MAF).  

First lets start with a little theory. The way the ford EEC series processors determine the amount of fuel and calculate load values is by measuring the amount of air that is flowing into the engine via the Mass Air Flow (MAF) sensor. The MAF sensor works by heating a wire inside the sensor. As air flows past the wire it cools the wire. Depending on how much the wire cools and the radius of the MAF sensor you can calculate the physical volume of air flowing through the sensor. This measurement is sent to the EEC processor as a voltage reading between 0 and 5 volts. 

The EEC has a 30 point table pre-programmed into it that references a voltage reading to an amount of air in kilograms per hour (KG/HR). For simplicity lets pretend it is only a 5 point table and use simple round numbers to make calculations easier. For example:

The way the EEC works is to reference the voltage signal to a row in the table. If the voltage falls between two readings it calculates a value using the surrounding numbers. So if the MAF was sending a signal of 3 volts you would have 500 kg/hr of air flowing into the engine. If the MAF was sending a signal of 2.5 volts you may expect 440 kg/hr (the scale is weighted with larger increments in the higher voltage range).

The EEC then uses this table to calculate the pulse width for each injector to supply the proper amount of fuel to feed the measured air. This calculation is based on the values in LOW_INJECTOR_SLOPE and HIGH_INJECTOR_SLOPE (more on these settings later). For now assume that both are set to 20 to represent a 20lb injector.

Now lets assume you ordered a new MAF meter from Pro-M and had it calibrated for 40lb injectors (remember, these are all made up numbers). You installed a fresh set of 40lb injectors and bolt on the meter. How does the meter make the 40# injectors work with the stock programming? It does this by reducing the voltage from the MAF meter to represent a physical volume of air equal to the factory injector size divided by the new injector size (in this example 20/40 or ½). NOTE:  It is NOT reducing the voltage by this percentage, this is very important to understand.  The reduction in voltage is precisely engineered to make the EEC believe it is receiving intake air - reduced by this percentage.  This means that if 1000 kg/hr of air is actually flowing through the meter it will signal the EEC with 3 volts (from our table above). This tells the EEC that 500 kg/hr of air is flowing into the engine and the EEC opens the injectors (that it still thinks are 20# injectors) for long enough to feed 500 kg/hr of air. Since the injectors are actually bigger (twice in this case) the amount of fuel injected is actually proper for the 1000 kg/hr that is flowing through the meter.

You can also do the math in reverse.  At the same voltage reading from the stock MAF and the new MAF your actual airflow will be new injectors size/old injector size (in our example 40/20 or 2).  So, if the voltage is 3 volts from the new MAF which corresponds to 500 kg/hr of airflow when you look at the stock transfer table at 3 volts you would see 250 kg/hr of airflow.  

This works fine for most tuning needs but does mess with other calculations in the EEC like load values. You also don’t have the ability to accurately scale fuel unless your MAF table accurately represents the meter and if you plan on data logging it is nice to have the log show you actual airflow.  For all these reasons and more it is recommended you tell your EEC what is actually going on by entering actual flow numbers and actual injector sizes using the EEC-Tuner.  

Building a MAF table:

One assumption made is that you have an accurate flow report for the meter you are using.  Pro-M ships a flow sheet with each of its meters and the stock calibrations for most stock meters are available for download on the EEC-Tuner Yahoo Group sponsored by Performance Specialties.  

First lets start by reading a calibration form provided by Pro-M.  Here is an example for an 87mm Bullet meter that has been calibrated for 83lb injectors.  The important rows on the chart are [Actual Flow KG/HR] and [AVG. 90 SAMPLE VOLTS].  By taking these rows of numbers and using either the MAF Excel spreadsheet or Paul Booth's EEC software you can generate a full 30 point MAF table.  For the purpose of this document I will use Paul's EEC software to generate the 30 point MAF table.

A9L Stock MAF Transfer ( 15.9998, 835.509 ) ( 4.76807, 835.509 ) ( 4.44312, 695.465 ) ( 4.177, 595.977 ) ( 3.88599, 499.658 ) ( 3.54395, 399.219 ) ( 3.35498, 347.574 ) ( 3.14893, 295.612 ) ( 2.90991, 243.334 ) ( 2.69409, 201.828 ) ( 2.58203, 182.5 ) ( 2.44995, 161.272 ) ( 2.31592, 142.261 ) ( 2.15405, 120.083 ) ( 1.98901, 100.122 ) ( 1.88989, 90.2996 ) ( 1.80298, 82.3786 ) ( 1.69092, 72.8734 ) ( 1.57397, 62.4176 ) ( 1.46802, 54.1798 ) ( 1.32104, 44.6745 ) ( 1.18188, 36.7535 ) ( 1.09106, 32.6346 ) ( 0.884033, 23.7631 ) ( 0.75, 18.6936 ) ( 0.571045, 13.6242 ) ( 0, 13.6242 ) ( 0, 13.6242 ) ( 0, 13.6242 ) ( 0, 13.6242 )

Now lets look at the stock MAF table from an A9L EEC and compare it to the Pro-M calibration for 83lb injectors.  Using the math from above the new meter should be telling the computer that 83/19 of the airflow is occurring at a particular voltage.  Lets use the voltage 1.32 as it exists in both charts.  When the meter signals the EEC with 1.32 volts the EEC believes that 44.6745 kg/hr of air is flowing into the engine.  By reading the Pro-M sheet we can see that 198.3 kg/hr is actually flowing.  If you do the math 44.6745 * 83 / 19 = 195.157 which is very close to the true Pro-M airflow.  This is how the meter tricks the EEC into injecting the proper amount of fuel.

All makes sense so far right?  If not re-read the preceding section and try the math yourself.  It is important that you understand why this works before you start altering the EEC.  You can destroy your engine if you do it wrong!

If you load Paul Booths EEC software and choose [Binary->MAF] from the menu bar you will be presented with an empty graph and cells to enter data.  Enter the row [AVG 90 SAMPLE VOLTS] into the first column and [Actual Flow KG/HR] into the second column.  Make sure to enter all the data points including all decimals.  After entering the last number make sure to press the [ENTER] key.  This will cause the MAF table to generate a chart and register all the values in the software.  You will also notice that many of your numbers you entered will change slightly.  This is because of the way the EEC stores values.  At this point I recommend saving the data you have entered.  This is what you should see (using the numbers from my sample sheet).

Now press the icon on the top bar that has [30] on it.  This will generate your 30 point MAF transfer from the data you have entered.  This is what you should see:

The numbers on the right are the your numbers for you 30 point MAF transfer function.  Either you can continue using Pauls software and copy the numbers directly into your configuration file or move them into the Shiftmaster or GUI EEC software.   Some of the other cool things you can do now in Paul's software is to lean/richen the fuel mixture by using the arrow keys at the top and selecting the range in the drop down next to the arrows.  You can also save a curve as a baseline and compare your changes to it as a reference.  Before you get carried away remember the first step is to just get your car running again.  To do that you need to set your injector slopes.

Injector Slopes:

You will find in your .eec file (or in the Scalar Values tab in the GUI) two settings for Injector Slope.  They should look like this:

LOW_INJECTOR_SLOPE     26.0995     # Injector slope lbs/hr
HIGH_INJECTOR_SLOPE     19.6416     # Injector size lbs/hr

The HIGH_INJECTOR_SLOPE is the actual size of your injectors.  If you were installing the mass air meter I used in my example you would probably also be installing 83 lb injectors so you would enter: HIGH_INJECTOR_SLOPE   83.  The cool part though is you wouldn't necessarily need to run 83 lb injectors with this meter.  Since your EEC now knows the actual air flow you could run any size of injector as long as you entered the proper size here.  

The LOW_INJECTOR_SLOPE is a fictitious number the EEC uses to size the injector for low RPM/LOAD settings.  Since injectors are sloppy the EEC uses whatever number you put in this key as the actual injector size during conditions where the injector would normally be inaccurate (low RPM for example).  What is normally done is you multiply your actual injector size by 1.2 and put whatever number you get into this key.  I don't agree with this philosophy an here is why.  IN MY OPINION:  All injectors are sloppy but not sloppy by a percentage, more by a scaled amount.  With 42 lb injectors you may only need to multiply by 1.1.  With the 83lb injectors I run my car runs best with this set at 89 or only 1.08 times the actual injector size.  

Tuning Advice:

1 - Don't tune around a problem.  It is possible with the EEC-Tuner to actually tune around a major vacuum leak, a bad cylinder, or bad injectors.  In the long run you are only hurting yourself by doing this.  If you program everything right and your car runs way to rich or way to lean either you made a mistake in the programming or (more likely) something is out of whack on your car.  Check the mechanical before assuming it is a problem with the EEC-Tuner.

(more to come.....)
Pegging a meter:

A FAT meter:

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