PLease help me get up to date/speed

[Steve=Ravet%Prj=Eng%PCPD=Hou at bangate.compaq.com]

| 'cat' writes,
| > ...
| > 1)  Speed density vs mass flow:

robert dingli <r.dingli at ee.mu.OZ.AU> replied:
| 
| Mass flow system still require a mapping between engine speed and mass
| air flow and injector output (which can exist in many forms such as
| desired AFR maps and/or volumetric efficiency maps).  

What is the mapping for?

| In fully tuned form
| they perform as well as speed-density systems and are more suitable for
| many production systems.  Aftermarket EFI is a totally different matter. 
| 
| MAP and MAF sensors are both useless for much serious performance work.
| Throttle position is the only reliable method in such cases.

Why is this?