First, a little background on MAP Sensors and Speed/Density ECU's

The Chrysler "MAP" or Manifold Absolute Pressure sensor generates a variable signal voltage proportional to manifold (absolute) pressure, (not a variable frequency signal). The typical signal voltage of such a MAP sensor, generates ranges 0.5 volt DC at lowest pressure (vacuum) to just under 4.9 vDC at maximum (boost) pressures.  In the Mopar Speed/Density-based ECU system, the MAP sensor provides critical information used to determine air density, which in turn can determine the correct Air/Fuel ratio. Understanding its interaction with the ECU will be one focus of our performance modifications.

The MAP sensor is produced in various vacuum/pressure ranges, depending on the intended engine application. The 3-Bar MAP sensor can measure boost levels up to 29 PSI and replaces the standard Mopar Turbo I/II 2-bar MAP (14.5 PSI) sensor. The stock Normally Asphyxiated TBI uses a 1-bar MAP for its required range of 30 - 0 inches Hg vacuum. Tthe chart below may be helpful :

Map Sensor	Vacuum	Boost (Pressure)	Engine Application
1-Bar	30 - 0 in Hg	0 (NA)	2.2/2.5 TBI/NA
2-Bar	30 - 0 in Hg	0 - 14.5 psi	2.2/2.5 Turbo I/II/III/IV (boost levels below 14.5 psi)
3-Bar	30 - 0 in Hg	0 - 29.0 psi	Typically used with Custom ECM calibrations (boost levels up to 29 psi)

Hitting the Wall: Over-boost = Fuel Cut-Out

Regardless of whether 1, 2 or 3-Bar MAP sensors are used, each range of MAP sensor still generates a signal between 0.5 to 4.9 vDC, depending on engine load. On a stock Chrysler 1990-91 TURBO I SBEC the ECM monitors the 2-BAR MAP voltage for over-boost (fuel cut-out) shutdown.  Safe, rapid, engine shutdown automagically occurs (via the ASD/Fuel Pump relay) whenever manifold boost pressure exceeds the factory-programmed set point within the SBEC.

This occurs roughly somewhere around 4.74 vDC (MAP sensor signal output) or approximately 12 - 14 psi relative manifold pressure (Mopar factory calibrations vary by year and turbo/drivetrain configuration). 

If you have bypassed the ECM wastegate control and you actually "hit the wall" it can be a little surprising. It's not like bouncing the spark when you hit the rev limiter, for hitting the over-boost wall actually shuts down everything and leaves it down, until you restart the engine. For the Average Joe, this is a good thing, as it would prevent a  closed wastegate failure from destroying the engine. Remember, the stock fuel injection and ECM will lean out at anything above 14.7 psi, don't defeat the shutdown feature without having CAREFULLY read the fuel section.

The Solution: "Fuel-Cutout" Defense

But you aren't the Average Joe, and you've taken care to feed enough high octane fuel at higher boost levels, right? So how do solve the overboost, fuel-cut dilema? They go by different names, such as fuel-cut defender, cut-out raiser, cut-out bleed, boost-cruncher, but they all server the same purpose:

Name	Type	Comments	Relative Cost	Weblink
Zener Diode	MAP electrical connector	Fixed-value, Zener Diode clamps 2-Bar MAP; signal totally defeating cutout	Dirt Cheap DIY project	Dodgegarage.com: How to
MAP Bleed Valve	Mechanical valve in MAP vacuum line	Adjustable, typically under hood	Gus Mahon's brilliant low-buck "Cut-Out Raiser" solution	Dawes Devices Cut-Out Eliminator
Boost-Cruncher	In Car and underhood electrical connection at MAP	Adjustable, electronic clamp, with three indicator LED's. Typically mounted inside car.	mid-priced solution	Forward Motion by Carroll Supercharging Boost Cruncher
Fuel Cut Defender (FCD)	In Car and underhood electrical connection at MAP	Adjustable, electronic clamp, with IN (SAFETY)/OUT (DEFEAT) Switch and two indicator LED's. Typically mounted inside car.	mid-priced solution	TurboXS "FCD" Fuel Cut Defender
3-Bar MAP with Custom Calibrations	typically under hood	Typically installed as part of total upgrade package (see below)	Most expensive	FWD Performance