Paul:

Please find below the response from "stealth316" re: Pressure Ratio.

Pressure ratio is the simply the ratio of two pressures. The 2 pressures being compared must be **absolute** pressures, abbreviated psia. Note that the pressure a boost guage reports is relative
pressure, often called gauge pressure, or psig. Gauge pressure is relative to ambient pressure at the gauge. A gauge will read "0.0" at any elevation if monitoring outside air pressure.

With regards to turbo pressure ratios, it is the ratio of the pressure leaving the turbo to the pressure entering the turbo, both on absolute pressure scales.

Example:

Pambient = 13.5 psi
Pin = 14 psia
Pout = 24 psia (it would read 10.5 psig on a boost gauge; 24-13.5)
PR = 24/14 = 1.714

Note in the above example that if ambient pressure was say 11.9 psi like it is where I live, then the gauge pressure would have been 12.1 psi. But the absolute pressure leaving the turbo is still
the same (12.1 + 11.9 = 24). When you see "29.92" as the current pressure in weather reports, that is the local value corrected to sea level; it has been adjusted and cannot be used with gauges
unless you really are at sea level.

See http://not2fast.wryday.com/turbo/boost_converter.shtml .

So if you are at 1000' above sea level (14.17 is "standard" ambient absolute pressure) and your boost gauge reads 11 psig, add 14.17 to that to get 25.17 psia. Now if the pressure going into the
turbo is 13 psi (due to a restrictive intake, and 1 psi loss is typical at high flow levels), the turbo PR is 25.17/13 or 1.936.
Later,

Jeff Lucius, http://www.stealth316.com/

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