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	James_Littler	2010-12-24 , 18:03
	Posts: 820 \| Thanked: 436 times \| Joined on May 2010 @ Portsmouth, UK.	#207

Originally Posted by Just4pLeisure

OK, I think I'm starting to understand: No air at all (e.g. in outer space) is 0 (kPa, Bar, InMg), normal air in the atmosphere is 100 kPa, 1 Bar or whatever it is in InMg.

Atmospheric pressure is 1.01325 bar, which would be 101.325 kPa, this is at sea level, this will obviously differ according to altitude and alto temperature which is why most boost gauges have the ability to tare the reading.

Originally Posted by Just4pLeisure

Boost is 'extra' air compared to the atmosphere and this is what the turbocharger 'makes'. So far so good, but this doesn't quite explain why normally I see 30-60 kPa (which is -0.4 to -0.7 bar of boost?) which suggests that my engine has less air getting to it than is in the atmosphere, is that right or even possible?

MAF sensors are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure, so what you're seeing is atmospheric pressure + or - your reading.

On idle when a supercharger or turbocharger is producing no boost/venting to atmosphere there will actually be a vacuum within the plenum caused by the engine sucking air in.

This is what gives you the - value as the ECU takes a boost feed after the throttle body.

Last edited by James_Littler; 2010-12-24 at 18:07.

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