I would like to know the differance between the two and the details of each as well.

Also, if anyone knows, I have a 1991 F-250 with the FI 351W, which one (mass air or speed density) do I have?

Thanks.

 

Your truck is SD.

The difference lies in how the computer determines engine load and thus how much fuel and spark to apply. With SD the computer gets input in realtime from the MAP sensor which measures engine vacuum and it uses that and the pre-programmed data it has on the engine(displacement, injector size, etc) to calculate the required parameters. As you can imagine that works fine as long as you don't drastically change the engine displacement or do something to mess with the vacuum signal... like install a lumpy cam.
MAF also has the same pre-programmed data but instead of a MAP it has a meter in the intake airstream(next to the airbox) to measure in realtime how much air the engine is consuming, so it doesn't care about engine vacuum or the cam and will automatically and accurately compensate for engine airflow improvements.

 

Thank you.