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IGNITION, FUEL SYSTEMS AND ELECTRICAL PROPERLY SELECTING ELECTRONIC FUEL INJECTION COMPONENTS (continued...) MASS AIRFLOW SENSORS On EFI systems that use a MAF sensor, this is the single most important sensor on the engine for determining a proper Air/Fuel (A/F) ratio. Unfortunately, it is also one of the most misunderstood sensors on the engine, as well. The engine’s air/fuel ratio and spark advance are determined by the PCM primarily from the input received from the MAF sensor. This is also why it is of critical importance that there are no air leaks (defined as air entering the intake stream between the MAF and the combustion chamber) in an MAF-based system. Air leaks can cause a “Check Engine” light, rough idling, stalling, spark knock, electronic throttle control failure mitigation modes, drivability issues, and in extreme cases, complete engine failure, depending on their magnitude. As with fuel injectors, changing the MAF alone will not result in more horsepower on an otherwise stock engine. A different MAF sensor should only be considered after engine modification which either causes the stock sensor to become a flow restriction or when the stock MAF sensor electronics are insufficient to measure the airflow that the modified engine is capable of ingesting. This latter point is critical in understanding when a MAF needs to be replaced. It is possible to have 2 MAF sensors that are equal in size, but capable of different maximum power levels. This is because the electronics in each MAF are different and are capable of measuring different maximum airflow, despite the fact that the size of the MAF housing is the same. For example, you can have two different 90 mm MAF sensors but one will be capable of measuring 60 lb/min of air, while the other can measure, say, 100 lb/min of air. They both present the same airflow restriction (which is dictated primarily by their physical size) but they are definitely NOT interchangeable. So how do you know how much air your MAF needs to be capable of measuring? If you have an approximation of the engine’s BSFC at WOT, as well as a target air/fuel ratio in mind, the amount of air that your MAF sensor needs to be capable of measuring (in lb/hr) can be calculated as follows. Note that this formula includes a safety factor of 10%. Max airflow = 1.10 x (Power x BSFC x A/F Ratio) Example: What is the max airflow a naturally aspirated 300 hp gasoline engine will ingest? First, assume a BSFC of 0.50 lb/hp-hr and A/F ratio of 12:1. Max airflow = 1.1 x (300 x 0.50 x 12) = 1980 lb/hr Now that we know the minimum size fuel injector and MAF that we need, we have to consider what the PCM will do with this new hardware. The two main methods of dealing with the installation of a new MAF and injectors are to either “trick” the PCM by careful selection of injectors and a “matched” MAF, or by changing the calibration in the PCM to match the MAF and injectors that you selected. The first method requires a MAF sensor that has been “curved” to a certain flow rate of injector. For instance, let’s say your engine originally came with 19 lb/hr injectors and you replaced them with 39 lb/hr injectors. To use this method, you will need a MAF with electronics that have been modified such that it will output a signal proportional to an airflow that is 19/39 times as great as the stock MAF would measure. This will result in the PCM delivering the correct amount of fuel despite the fact that the injector size has been increased from 19 lb/hr to 39 lb/hr. The downside of this method is that many other variables such as spark advance are determined from the MAF sensor through a parameter called “load.” For a given engine RPM, as load increases, required spark advance decreases. Since, by using this method, the MAF outputs a signal that is lower than the stock MAF, the calculated load will also be lower. This means that commanded spark advance will be higher than it should be, which can potentially result in spark knock and other concerns. While this method works quite well on less-sophisticated electronics, such as the EEC-IV found in a Fox-body Mustang, it is not recommended for newer vehicles which have a much higher dependency on the calculated value of load. The second, and much preferred method requires the ability to alter the calibration inside the PCM. When using this method, the actual flow data for the injector (available on our website for all FRPP injectors), as well as the “transfer function” for the MAF are entered into the calibration in the PCM. Generally, it is recommended to test the new calibration on a dynamometer to ensure that the engine receives the correct A/F ratio at all speeds and loads. Provided this is performed by a competent and experienced operator using proper equipment, this is by far the best method and will result in the best part-throttle drivability and idle, and the least amount of trouble with “Check Engine” lights, returnless fuel, electronic throttle monitors, transmission shifting, etc. Ford Racing performance upgrade kits and their associated calibrations are designed to work together seamlessly, taking much of the hard work out of upgrading the performance of your vehicle. Prior to tuning on a dyno, you should be absolutely certain that the ground circuits for the EFI system are in pristine condition. Doing so will help to ensure that the calibration you and your tuner develop on the dyno will also work when you leave the shop. It can’t be overstated that prior to the vehicle being tuned in any way, all vacuum leaks, electrical issues, etc., need to be resolved. Fixing them before you go to the dyno will always be cheaper than paying for dyno time while you’re wrenching on your car. If you have any questions not addressed in this tutorial, please contact the Ford Racing Techline at 1-800-FORD788 and we will be happy to assist you. For important information about the proper usage of performance parts, please see page 14. 216 Techline (800) FORD788 See pages 286-292 for important safety, emissions and warranty information.


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