Just like the title says...

Why does my boost gauge read 1-2 PSI higher when it's below 50 outside?

I am looking for FACTS.

This is one theory...

As a gas decreases in temperature, the viscosity also decreases. Many believe this leads to the compressor (or blower) to be able to "grab" hold of the air better. I believe this follows the argument that adiabatic compressibility is higher for the colder air. This increases the supercharger efficiency and allows for more boost at the same given speed ratio.


your thoughts?

 

cold air is more dense because the gas particles become closer.

 

Ok, by the air being denser, your saying you can get more air in the same space? or something else?

 

actually - it's backwards - viscosity INCREASES as temperature decreases.

 

yeah, more air in the same space. like with an intercooler, the air becomes more dense because it cools down.
Steve

 

Wrong! In the case of a gas (which air is, last I checked), viscosity decreases with decreased temperature. [Source: Unit Operations of Chemical Engineering by McCabe, Smith, Harriott]. However, in the pressure regions we are talking about (1-3 bars), I am guessing viscosity changes are fairly insignificant.

 

You're right.

 

I also think that because colder air is generally drier, that dryness makes it easier for the compressor to work, as the water vapor in the air is probably much harder to push around than O2, N2 and other gassy molecules in the air. Just a theory though. Also, just by looking at ideal gas theory, since colder atmospheric air has more moles/volume, when the same amount of work is done on 2 streams of air, with one being denser, that stream will have a higher final pressure, all other things being equal. I think it is a combination of a lot of factors: thermodynamic, chemical and physical.

 

Thanks for the FACT


Mizeree_X- never really thought about the drier air, but it makes sense thanks for the input