SPEED DENSITY vs. MASS AIR FLOW
With a speed density system, actual intake manifold pressure is measured using a manifold absolute pressure (MAP) sensor, inlet air temperature (IAT), and 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 to the speed density engine cause the system to go "full tilt" (in pinball or poker terms) because the load/fuel mixture tables are inflexibly burned into the processor. Suppose you exceed these parameters by installing a cam that's too big or an induction that flows too much. In that case, 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 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 measured instead of calculated. In a sense, things happen before 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 and any additional valve train requirements. We recommend calling COMP Cams 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 no codes are present before the conversion. We routinely see codes associated with removing 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, they must be dealt with through the OEM, eliminator kit, or performance tuning.
3. VACUUM READING – A healthy engine vacuum at idle should be between 18-20 in Hg. It is a good idea to check the vacuum if any issue arises. There are many things that can result from a variety of degraded conditions. Testing for vacuum should be accomplished using 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 - The mass air meter and fuel injector size should complement your engine's CFM and fuel requirements. Our products include item descriptions that provide recommendations. At a minimum, 24lb injectors are recommended for use in applications using aftermarket camshafts and cylinder heads.
5. TFI MODULE (LIGHT GRAY vs. BLACK) - The replacement PCM included with our kit is compatible with light gray modules. Some aftermarket replacements include a black TFI module compatible with 1994 and later applications. If you have a 1993 and earlier model year, you must ensure a light gray module.
6. AIR INTAKE/FILTER - Our kits are designed to work with either a factory air box or aftermarket air intake. The F-Series MAF air box is not recommended for aftermarket camshaft and cylinder head applications. If you are designing a custom setup, you must ensure it models the engineering design of the OEM or aftermarket.
7. PERFORMANCE TUNING - Our supplied PCMs support aftermarket tuning using a performance chip (SCT, TwEECer, Moates Quarterhorse). A performance tune is recommended for applications smaller than 351ci using 42lb or larger injectors. Please remember 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, other issues must be resolved.