What is the

The "MAP sensor" mod connects a manifold absolute pressure (MAP) sensor to the AFC throttle input. The goal is to make the AFC load-sensitive rather than RPM-sensitive. As the boost level increases, the AFC can deliver more fuel accordingly. For more information, see this VFAQ by Corbin Behnken.

Here is a summary of the mod:

" Lots of questions about this mod. So here is a summary.

Why? Turn the Hi/Lo throttle reference for the NE points on the SAFC into boost references and not throttle position dependent. This makes the SAFC engine load sensitive.

How? Buy 3 bar sensor GM Part # 16040749 at http://www.gmpartsdirect.com/

wire Pin A = Ground - on firewall 
wire Pin B = Sensor output, cut the gray wire that previously attached the ECU's throttle position terminal, and wire the sensor output to this wire of the SAFC 
wire Pin C = +5v, recommend throttle position sensor wiring harness - turn ignition on and use a multimeter to find it.

The map sensor has special 3 pin weatherpack connector, these are available from the junkyard or the wiring accessory section of any good automotive store. Get the one with pigtail wires, usually these connectors have internals alignment grooves - shave with knife to fit. Splice into your boost gauge (manifold pressure and vacuum are necessary) with a 1/4" tee and run hose to the sensor nipple.

Drive car and observe the voltage on the SAFC that the sensor makes at max boost. Let's say YOUR car makes 19psi boost and the corresponding sensor output voltage is 4.1v , take 4.1 divide by 5.0 (max sensor output) and the result is 0.82 or 82% This is your Hi Throttle NE point on your SAFC. Set the Lo Throttle similarly. I used a sensor voltage of 1.15v, which corresponds to slightly above -5 in Hg. and is usually not seen at 70mph cruise, thus 1.15/5.0 = 0.23 or 23%. The Lo Throttle NE point is now 23%. I am still working on tuning this in. I pick up some part throttle knock under light acceleration, but mash it to the floor and it works very very well. The upper rpm is better than before I did this mod. I will make a VFAQ page for this someday.