Page 141 of 2008 Performance Parts by Ford Racing Performance Parts

properly selecting electronic fuel injection components continued 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 a maf based system air leaks can cause a check-engine light rough idling spark knock drivability issues and in extreme cases complete engine failure depending on their magnitude 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 that is 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 then 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 1.1 power bsfc a/f ratio example what is the max airflow a naturally aspirated 300 hp engine will ingest first assume a bsfc of 0.50 lb hr and afr of 12:1 max airflow 1.1 300 0.50 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 engine components 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 eec-iv it is recommended that this method be complemented with a vehicle chip tune on vehicles 1996 and newer the second method requires the ability to alter the calibration inside the pcm generally through the use of one of the aftermarket tools available when using this method the actual flow data for the injector available on our website 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 tuner this is 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 etc 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 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 as a general rule of thumb the following stock ford maf sensors will support the corresponding horsepower maf sensor 55 mm stock 88-93 mustang 70 mm stock 94-95 mustang 80 mm stock ford 90 mm m-12579-54 approximate max hp 275 hp 350 hp 425 hp 540 hp not legal for sale or use on pollution-controlled motor vehicles direct replacement part see pages 2-8 for important safety emissions and warranty information www.fordracingparts.com 139

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