fuel injection
 

I am about to do a 5l conversion and I was wondering which fuel injecting system is better. Mass air or speed density?

Mass air is more adaptable and tuneable. The 86-88 guys will tell you the Speed density system can work fine, and may pump a few ponies more than a stock Mass Air system, but the Speed Density is pre-programmed for a known setup and will eventually give you problems as you deviate from the known values.

If you are converting a Mustang, you can swap in the engine harness and computer from a FI mustang, just make sure you get the complete system from the same car as Ford changed the setup each year, and sometimes during the year. For a non Mustang swap, stick with a universal kit like Ron Francis or FMS etc..

If you're looking to modify the engine at all, Mass Air is the only way to go. True...you can modify a speed density motor, but the state of tune becomes much more important. Like Chevyguy said...a speed density system might make a few more horsepower stock, but the adjustability of Mass Air is it's selling point.