I'm not the one to explain it. You might want to look it up more on megasquirt.info. I have some books with the definition too but I don't have them handy.
Basically there's 3 ways to calculate engine fuel required. We're worried about two:
Alpha-N : Based on RPM and throttle position. Throttle position doesn't fluxuate even if you have a crazy cam so this can deliver a good idle overwise not possible. I think the problem was resolution of the throttle position sensor. At some positions a very small change in throttle would change air flow a lot and the resolution wasn't good enough for good management.
Speed-Density : Based on RPM and intake pressure (density). This has good resolution and good performance so long as the intake manifold pressure is fairly stable. This is true almost all the time -- unless you're trying to idle an erratic cam and the vacuum is all over the place.
I'm not sure what it's current status was, but MS was working on hybrid configurations. You ran Alpha-N until a certain RPM or throttle position or something then the system switch to Speed-Density automatically and smoothly. Best of both worlds.
I hope I got the idea across. Greg
