SPEED DENSITY vs. MASS AIR FLOW
With a speed density system, actual intake manifold pressure is now measured using a manifold absolute pressure (MAP) sensor, as well as inlet air temperature (IAT), and in addition to the previously sensed TPS and engine rpm. Now the PCM fuel control programming includes a desired air/fuel ratio table, the injector flow rates, engine cubic inch displacement, a volumetric efficiency table, and the programs necessary to instantaneously calculate inlet airflow, required fuel flow (for the desired A/F ratio found in the A/F ratio table), and finally the correct injector pulse width. Any radical modifications made to the speed density engine causes the said system to go "full tilt" (in pinball or poker terms) because the load/fuel mixture tables are inflexibly burned into the processor. If you exceed these parameters by installing a cam that's too big, or an induction that flows too much, then the system can no longer effectively compute injector pulse width (fuel/air ratio) and timing. The net result is a loss of driveability, an overly rich condition, or in extreme cases, engine failure due to detonation caused by lean mixtures and/or over-advanced timing.
With mass air flow, the air entering the engine is actually measured using a mass airflow (MAF) sensor. Injector pulse width is still calculated in the same manner as shown previously, however, now the airflow is actually measured instead of calculated. In a sense, things happen before the fact, rather than after the fact. The big advantage of a MAF system is that you can change things on the engine that affect airflow and maintain driveability. In most cases, the MAF sensor will realize the change in airflow, and the fueling will still be correct. It makes the MAF system the most forgiving for engine modifications. Mass airflow offers improved drivability, fuel mileage, and smooth performance.
The following is a list of items to consider prior to converting. Please contact us directly if you have any additional questions or concerns.
1. INTERNAL ENGINE COMPONENTS - We routinely see customers that were recommended a camshaft that is not EFI compatible. You will need to ensure that the camshaft you select is compatible with computer controlled applications. This usually results in a lobe separation of 112 degrees or greater. Cylinder head upgrades must complement the camshaft being used, in addition to any additional valve train requirements. We recommend giving COMP Cams a call at (800) 999-0853.
2. DIAGNOSTIC FAULT CODES - like any EFI system, our kit is designed to operate free of any diagnostic fault codes. It is best to ensure there are no codes present prior to the conversion. We routinely see codes associated with the removal of emissions equipment (CANP, EGR, EVP, TAB, TAD) that can negatively impact idle quality and drivability. Although, the severity of these effects can vary between sensor/solenoid, it is critical that they are all dealt with, either through the OEM, eliminator kit, or performance tuning.
3. VACUUM READING – A healthy engine vacuum at idle should be between 18-20 inHg. It is a good idea to check vacuum if any issue arises. There are many things that can result from a variety of degraded conditions. Testing for vacuum should be accomplished with the use of a gauge attached directly to the upper intake manifold/vacuum tree. Once a vacuum leak is confirmed, an isolation test should be conducted.
4. MASS AIR METER AND FUEL INJECTOR - Mass air meter and fuel injector size should compliment the CFM and fuel requirements of your engine. Our products include item descriptions that provide recommendations. At a minimum, 24lb injectors are recommended for use in applications using both aftermarket camshaft and cylinder heads.
5. TFI MODULE(LIGHT GREY vs. BLACK) - The replacement PCM included with our kit is compatible with light grey modules. Some aftermarket replacements include a black TFI module, which is compatible with 1994 and later applications. If you have a 1993 and earlier model year you will need to ensure you have a light grey module.
6. AIR INTAKE/FILTER - Our kits are designed to work with either a factory airbox or aftermarket air intake. The F-Series MAF airbox is not recommended for use in applications with aftermarket camshaft and cylinder heads. If you are designing a custom setup you will need to ensure it models the engineering design of the OEM or aftermarket.
7. PERFORMANCE TUNING - All of our supplied PCMs support aftermarket tuning with the use of a performance chip (SCT, TwEECer, Moates Quarterhorse). A performance tune is recommended for applications smaller than 351ci using 42lb or larger injectors. Please keep in mind that aftermarket tuning is not required to maintain idle quality or drivability, as the factory programming will maintain a safe air/fuel ratio even in a modified application. Tuning should be used to maximize power by achieving a performance oriented air/fuel ratio. If you find yourself having to tune to maintain either idle quality or drivability there are other issues present that must be resolved.