What is the difference between a MAP style TPI and a MAF style horsepower-wise or why did GM switch to it?

 

A rough & quick explanation of the two.

MAF system was commonly used throughout the
early to late 80's engine, and some late model cars
used MAF throughout the european and import cars.

the MAF sensor is the single most vital part of
a MAF TPI, calculating the air that flows through the
engine, giving the ECM information so it can adjust it's
running parameters. These systems respond well to engine
modifications without alot of the tuning hassle, but you
have to deal with the bottle neck of the sensor.. limiting
the air taken in.

MAP is the exact opposite. Requires no sensor before the
Throttlebody. MAP's make more of a educated guess of how
much air is consumed. Any internal modifcations will change
the way the car runs itself, and most require some sort of
tune. Alot of people run MAP's due to it's no hassle, tune &
go functionality, & and the fact it looks cleaner.

and, it is somewhat more economical, but eh..

Im sure there is more info on the MAP systems, use
the 'Search' function. Or check out the TPI OBDII sticky.
Im sure you can pick up a few things in that thread.