A map sensor is necessary when tuning an engine via speed-density. Most standalone ECU's run off of speed density instead of air flow. Whether your car had an air flow meter from the factory, or your factory MAP sensor does not go high enough for the amount of boost you may be running, getting an aftermarket map sensor will tell the ecu how much pressure is in the system, a value that is needed for tuning.

AEM Map Sensors are high quality, durable, accurate, and stand up to the torture tests of racing. When picking a map sensor, you should pick the smallest map sensor that will provide you with enough "gauge pressure" to read the amount of the maximum boost you plan on running. This list below should help you

3.5 bar Map Sensor will read a maximum of 36.0 psi of Boost 

Car Model: Universal
Material: Stainless Steel
 
3.5 BAR / 50 PSIa MAP Stainless Steel Sensor Kit. Stainless Steel Sensor Body. 1/8" NPT Male Thread. Includes: 3.5 BAR / 50 PSIa Stainless Steel Sensor, Connector, Pins, 1/8" NPT to -4 Adapter & 1/8" NPT to 3/16" Barb Adapter.
Sensor Performance Specifications: 
Accuracy: +/- 1% Full Scale over -40C to 105C includes Repeatability, Hysteresis and Linearity 
Operating Temp: -40C to 105C / -40F to 221F
Burst Pressure: 500PSI 
Response Time: < 1mS 
Wetted Materials: 304L & 316L Stainless Steel 
Vibration: 100 to 2000Hz, 20g Sinusoidal, 3 Axes
Weight: <85g 
Supply Current: <6mA at 5 Vdc
Output: .5 to 4.5Vdc Linear
Transfer Function: PSI = (12.5*(Voltage))-20.95
Elec. Termination: Integral weatherproof connector, includes mating connector & pins(Supply Voltage 5Vdc +/- .5, Ground, Output)