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Just For Fun!!!

Tech Tip No. 1

Ned Ritchie

Copyright 1995, 1998


The boost gauge on my Turbo Charged Audi reads in BAR. What is BAR and how do I compare that to PSI?

What we're talking about here is pressure; air pressure inside the intake manifold.

Suppose we hooked two gauges one reading BAR and the other PSI to the manifold so that both are sensing the same pressure. With the engine off what do we read? One guage reads about 1.0 and the other reads nothing. What does that tell you? Some pressure must be there because the BAR gauge reads something, but if that is so why does the PSI read 0?

Remember your physical sciences. . . the weight of the earth's atmosphere pressing down on us constitutes atmospheric pressure and is 14.7 "pounds per square inch" (14.7 PSI). Generally we ignore this 15 PSI of atmospheric pressure. An example is when we put 30 pounds (per square inch) of air into our tire. We put 30 pounds more than the 15 pounds into the tire. The gauge ignores the 15 pounds and indicates 30 PSIG. The PSIG refers to "pounds per square inch gauge" and what we usually think. Pressures less than atmospheric are negative gauge pressures that correspond to partial vacuums. If our tire gauge took into account the atmospheric pressure it would be an absolute pressure gauge and would read 45 PSIA "pounds per square inch absolute."

Absolute pressure is the total pressure including the effect of atmospheric pressure. An absolute pressure of 0 corresponds to a complete vacuum. Atmospheric pressure is 1.01 BAR.

The BAR gauge in the Audi is an absolute pressure gauge and it takes into account the atmospheric pressure. It is an international unit of pressure that is used in most countries. It is equal to 14.5 PSIA or approximately 1 atmosphere. This is why our gauge read about 1 BAR when the engine is off at sea level. Those of you at a higher altitude like in Colorado will notice your gauge is lower than that.

Some of you may have seen the aircraft gauge in my car that is hooked to the manifold. It uses "Inches of Mercury (inHg) to measure pressure. This is also an absolute method and is used in aircraft because atmospheric pressure changes with altitude and therefore normal pressure gauges won't work. Atmospheric pressure is 29.921 inHg. You've heard the weather person say something like the BARometric pressure is 29.54 and rising... They are referring to the pressure of the air around us. The pressure that we usually ignore.

Atmospheric pressure can also be ignored when it comes to BAR. Sometimes a european tuner will say the engine is boosted to... or is running 1.1 BAR of boost. In this case he is referring to what we would call 15.9 PSIG boost. He could also say the engine runs at 2.11 BAR.

Here is a pressure conversion table to file away for future use. Just remember the following:

When working with PSIG you must add 14.7 PSI to get PSIA

When working with PSIA you must subtract 14.7 PSI from PSIA to get PSIG.


© Copyright 1997-2004 Ned Ritchie