BAR (Barometric Pressure):

This is the barometric pressure in the area where you are driving. At sea level, the barometric pressure is about 30" Hg (inches of Mercury). As your elevation increases, the barometric pressure decreases, since this is effectively the weight of the air pressing down on you from the very top of the air layer that surrounds the earth. In Denver, for example, at 5,800 ft. elevation, the barometric pressure might only read 24.0 "Hg. As atmospheric conditions change at any location, the barometric pressure will change slightly. When a storm approaches, the barometric pressure will drop.

Different scanners can report the readings in different units. Some scanners read in "kiloPascals" (kPa). 100 kPa is roughly "one atmosphere" or 30" Hg.

In effect, in a naturally aspirated engine, this is the maximum pressure you can see in your cylinders as they pull the air/fuel mixture into the cylinder. The higher the barometric pressure, the more air can potentially be fed into your engine. Just remember, there is nothing you can do to change BAR…. It is a reading of the atmosphere, not your engine, and should be very close to the barometric pressure reported by the local weather service, or printed on your dragstrip time slip.

If you want to check your stock "1 Bar" sensor, the output voltage should be about 4.8 Volts with the key on, but the engine not running. If you have a supercharged engine, and are running a speed-density or N-Alpha system, you might have a "2 Bar" or "3 Bar" sensor.

The MAP sensor measures the barometric pressure when the key is turned on, but before the engine starts. The sensor is about the size of a domino, black plastic, and mounts at the front/passenger side of the intake manifold.

The barometric pressure readings shown above for various altitudes were challenged by an aircraft pilot, who claimed he had been to cities like Denver and never seen anywhere near 24"Hg. What he failed to understand was that pilots are provided with a barometric pressure that is "corrected" to sea level. They need this data to calibrate their altimeters, entering the barometric data in the Kolisman window on the altimeter. But this data is not applicable to the power output of an internal combustion engine.

MAP (Manifold Absolute Pressure):

Here is where you can do something about the pressure that forces the air/fuel mixture into your engine. The air cleaner is exposed to atmospheric pressure (BAR). As the air passes through the filter, it loses some pressure. It loses more pressure in the ductwork that feeds it into the engine, the MAF sensor, the throttle body, the intake runners and the restriction of the open valve. Each of these items reduces the air pressure slightly. And gives you a pressure LOWER than atmospheric pressure in your intake manifold. This pressure is called manifold absolute pressure, to avoid using the negative numbers required to measure vacuum.

When a stock LT1 is idling, the "MAP" reading should be about one-third of the "BAR" reading. Check the "BAR", and remember it - it won't change for a while, unless you're in the middle of a hurricane… Then display "MAP". If the BAR = 30.0, MAP should be at 10.0, or below. On my relatively stock LT1, with a BAR = 30, I would see MAP readings at idle of 9.3 to 9.5. The readings would fluctuate slightly with the idle. Maybe as low as 8.9, and maybe as high as 9.8.

If you have a cam that is more aggressive than the stocker, you will see a higher MAP reading at idle. With a fairly radical cam, you might see reading of 18 or so.

The difference between MAP and BAR is your manifold vacuum. If BAR = 30.0 and MAP = 9.5, then your vacuum is 30.0 - 9.5 = 20.5 inches of Hg. If you connect a vacuum gauge to the correct port on the manifold, this is what it should read.

When you start driving, the MAP will fluctuate with the throttle position and the engine load. Close the throttle completely at 2,000 RPM, and the MAP will drop well below 9.0. Floor it at low RPM, and the MAP should shoot up to a number equal to, or very close to "BAR". The output of the MAP sensor is very important, because it tells the PCM where to look in the fuel and ignition timing tables. High MAP = high engine load. Low MAP = low engine load.