Stupid Question

[PKRWUD]

When it comes to EFI, most vehicles are either Speed Density (SD), or Mass Air Flow (MAF). Those two terms describe the method with which the ECM figures out how much air the engine is getting, which is one of several factors it uses when deciding how much fuel to inject, and how much ignition advance it gives.

SD relies on a MAP sensor, which measures the pressure in the intake manifold, an Air Charge Temperature (ACT) sensor, which measures the air temp, and the engine's rpms (speed) to determine how much air the engine is getting. The ECM has pre-programmed fuel curves that it uses based on these readings. Unfortunately, when you make drastic modifications to the engine, say with a big cam for example, the manifold pressure is affected, and so it doesn't provide the best amount of fuel, and the result is often a poorly running engine.

MAF relies on the MAF sensor, which uses a very hot wire to determine the amount of air going into the engine. This wire is placed in the path the air must travel to get in the engine, and as the air rushes by it, the cooling effect it has on this wire is measured, and the amount of air is then known. MAF is much more accurate, and adaptable to any mods. For this reason, MAF is the system of choice if you plan to heavily modify your engine, but because of the traditionally rich fuel curves used in SD engines, a stock, or near stock application will be faster with SD.

SD was used well into the 90's in some vehicles, but the Mustang has been MAF since 1989, like Caymon said.



Take care,
~Chris