Stupid Question

[RDRZ]

What do you guys mean when you say you have a "speed density" car?

[bigred90gt]

86 to 88 cars are speed density except california cars. California went to mass air in 88. In the air tube between the filter and the throttle body, there is no mass air sensor on the speed density motors. They are harder to tune as you mod them because there is no meter to tell the computer how much air you are pulling in so it can adjust your fuel and timing curves. On mass air cars (89 and up except cali cars which was 88 and up), the mass air flow sensor tells the computer this info and the computer adjust the fuel and timing accordingly. Someone correct me if I'm wrong on anything.

[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