The MAP sensor reads the pressure inside the intake. When you are idleing you arecreating vacuum inside the intake because the throttle blades are closed.. As you give it gas, the amount of vacuum in the intake decreases. At WOT (Wide Open Throttle), the pressure inside the intake manifold is close to 0, or Atmospheric pressure. This means the presure inside the manifold is equal to what it is outside because the throttle blades are wide open.. The MAP sensor reads these pressures and tells the computer. It helps the computer determine how much fuel to add.

Your engine from the factory was never meant to see any boost/any pressure over 0PSI. If you add anything (Turbo, or Supercharger), you are adding boost.If the Map sensor reads anything over 1PSI, the PCM signals fuel cut and shuts off the fuel injectors. It's trying to protect the engine because it doesn't know where this pressure is coming from. When you actually do add forced induction, you need to either divert any boost away from the map sensor with a check valve, or add a voltage clamp.

A voltage clamp works just like it says. The MAP sensor sends a voltage to the PCM based on the pressure it sees. Let's say AP (0PSI) is 4.8V, anything over that would signal fuel cut. So you set the voltage clamp to 4.8V, or even a little lower to be safe. The voltage from the sensor is allowed to be sent freely up until your clamp voltage, once it hits that, the voltage clamp will interupt that signal and set 4.8V to the PCM, regardless if it's higher coming from the sensor. So the computer will never "see" the boost.

 

You didn't have a "Complete waste of money" option on the poll.

 

That thing is just as useless as these: http://cgi.ebay.com/ebaymotors/Dodge...QQcmdZViewItem

Possibly even more so...