What is speed density ?
 

I've got an 88GT and I've heard something about my car being a speed density instead of mass air. Is this true - and what exactly is the difference between the two.

'Speed Density' refers to the fact that the computer is looking at the speed and density of the air entering the intake, whereas 'Mass Air' measures the actual mass per time of air coming into the engine.
Speed density cars tend to run a little stronger stock than mass air, but will encounter idle and driveability problems if more aggressive mods are added (heads, cam, etc). Mass air systems are better at compensating for mods.

Actually a Speed Density car has no idea how fast the intake charge is coming it. It has no ability to measure the quantity of incoming air, just the air density. A speed density car relies on a database of information stored inside the computer as tables for the injection cycle.

A Mass Airflow car uses a MAF Meter to physically measure the volume of air coming into an engine, then refers to a table like SD.

As far as a speed density system not being able to handle modifications, that's completely false. That's like saying a MAF car cannot handle larger injectors. They both take tuning. You can tune SD by adding fuel pressure, or larger injectors, or having a custom chip burned, and you can tune the MAF to accept larger injectors by having the MAF recalibrated.

There are a couple people on this site in the low low 12's/high 11's with SD.