Originally posted by thewinner
5800 ft....bandimere...colorado...yes 23 is possible, dont tell me its not lol its on the time slip

Absolute Barometric Pressure, expressed in inches of Mercury ("Hg), is not the same as what is reported on weather forecasts, which is Sea Level Corrected pressure. Absolute is the actual air pressure at elevation. Roughly every thousand feet of elevation reduces barometric pressure by one inch of mercury. For instance, if you were in Colorado at 6000', the absolute pressure would be around 24 "Hg, while the Sea Level Corrected reading would be around 30 inches. By using absolute, you need not recalibrate for every new location, and you need not know the elevation of the track.

When the barometric pressure is higher there is more oxygen available for combustion in a given volume. During a typical 12 hour period the barometric pressure will change only 1 to 2 tenths of an inch of Mercury (for example from 27.24 to 27.40). An approaching front may bring in air that is higher or lower by 1"Hg (28.95" to 27.95").

Barometric pressure is caused by the gravitational pull on the 'column of air' lying directly above the place you are measuring. It is usually measured with an altimeter or barometer. Creative racers (with way too much time on their hands) may wish to experiment with a hypsometer, which indicates pressure by monitoring the boiling point of water.

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.


ELEV - STD BAR

Feet - "Hg

0 - 29.92

1,000 - 28.86

2,000 - 27.82

3,000 - 26.81

4,000 - 25.84

5,000 - 24.89

6,000 - 23.98

Different scanners can report the readings in different units. Some scanners read in "kiloPascals" (kPa). 100 kPa is roughly "one atmosphere" or 30" Hg. There is a table below, under MAP, showing the equivalent readings in "Hg, kPa and sensor Volts.

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. The table above is correct.