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· Registered User
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Just wanted to expand further on the comments regarding the pressure ratio (y-axis) on the compressor maps. Although a PR of 2.0 logically equates to around 1 bar of boost, there are some factors that you need to take into account for an accurate calculation.

The formula I use is as follows:

pOut/pIn = (Pm + Pic + Patm) / (Pit + Patm)

Where Pm = Relative boost pressure @manifold.
Pic = Pressure drop through IC.
Patm = Absolute ambient barometric pressure.
Pit = Pressure [email protected]

Units of measurement are PSI.

So let's say there is a 3psi drop through the stock intercooler (not outlandish for a small tube-fin IC), plus a 0.5psi drop at the turbocharger inlet (guesstimate). Sea-level barometric pressure is 14.7psi

Given these conditions, a PR of 2.0 would figure as follows:

2.0 = (Pm + 3 + 14.7) / (-0.5 + 14.7)

Solving for Pm:

Pm = 2 . 14.2 - 17.7

Pm = 10.7 psi

This is important in the selection of compressors based upon flow map data because you generally need to run a higher PR than it might first appear. i.e Your turbo needs to work to overcome the pressure drop across the IC (and at the inlet). 1bar of boost at the compressor outlet does not necessarily equal 1bar of manifold boost.

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