What is speed density? Can ne of you tell me what it is and how to do free mods on a 91 Formula LB9 305 TPI

 

Speed density refers to the type of ECM the car uses to control the engine. The computer in the 90-93 f-bodies (yes the '93 4th gen is speed density too) uses the speed of the car and the density of the incoming air (based on vacuum in the engine) to calculate the amount of fuel to squirt into the engine. The 85-89 f-bodies, as well as the 94-02 4th gens use a MAF (mass-air flow) sensor to calculate the amount of incoming air to determine the amount of fuel the engine needs. Speed density is also called a MAP system (manifold absolute pressure), because it uses a MAP sensor to measure the amount of vacuum in the engine, and how much of a load the engine has on it.
There are several tech articles on this website in the tech article section that deal with free mods and mods for beginners.

 

Just to remove a possible confusion, the SPEED refers the ENGINE speed, not the speed of the car, hehe.

Since it is basically "guessing" the airflow from lookup tables, it doesn't handle large changes to engine airflow at all, unless you re-calibrate the PROM.

Some free (or cheap) mods:
Advance the timing a little
grind down EGR walls inside plenum (must remove plenum, and while you're at it, remove the base manifold and port it, or replace it).
K&N at a time when it's due for an air filter change anyway
Get into DIY PROM tuning, and lower the fan temps, calibrate the WOT fuel and spark, etc... Can be done pretty cheaply if you bargain shop for old chip burning stuff. If you start spending money on real parts, you won't regret being able to optimize the air/fuel and spark.
Buy performance replacement parts at a time when a part needs replacement - i.e. exhaust is totally worth it; good oils; spark plugs/wires
Make a cheapo high flow air filter duct and K&N on the end.