http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html
a.. "In determining pressure ratio, the absolute pressure at the compressor inlet (P2c) is often LESS than the ambient pressure, especially at high load. Why is this? Any restriction (caused by the air filter or restrictive ducting) will result in a "depression," or pressure loss, upstream of the compressor that needs to be accounted for when determining pressure ratio. This depression can be 1 psig or more on some intake systems. In this case P1c on a standard day is:
14.7psia - 1 psig = 13.7 psia at compressor inlet
a.. Taking into account the 1 psig intake depression, the pressure ratio is now:(12 psig + 14.7 psia) / 13.7 psia = 1.95.
a.. That's great, but what if you're not at SEA LEVEL? In this case, simply substitute the actual atmospheric pressure in place of the 14.7 psi in the equations above to give a more accurate calculation. At higher elevations, this can have a significant effect on pressure ratio.
a.. For example, at Denver's 5000 feet elevation, the atmospheric pressure is typically around 12.4 psia. In this case, the pressure ratio calculation, taking into account the intake depression, is: (12 psig + 12.4 psia) / (12.4 psia - 1 psig) = 2.14
a.. Compared to the 1.82 pressure ratio calculated originally, this is a big difference."
****Notice where your statement regarding Denver pressure being 11 psia is NOT correct, it's stated here as 12.4.
http://www.worldpath.net/~thompson/misc/cfm%20mystery.htm
"Air at SEA LEVEL has an atmospheric pressure of zero pounds per square inch as measured by a pressure gage (psig) and 14.7 pounds per square inch on the absolute scale (psia)."
http://www.ptcmetrology.com/Metrology_pressure.html
"Pressure is usually measured, either as absolute pressure (psia), or relative to atmospheric pressure(psig) such measurements are called gauge pressure. An example of this is the air pressure in a car tire, which might be said to be "35 psi," but is actually 35 psi above atmospheric pressure. Since atmospheric pressure at SEA LEVEL is about 14.7 psia, the absolute pressure in the tire is therefore about 49.7psi.
Atmospheric pressure is the pressure exerted by the air around us. The pressure varies both with altitude, and weather patterns. Standard atmospheric pressure at SEA LEVEL is defined as 1 atmosphere equal to 760 millimeters of mercury (760 Torr) and 101,325 Pascals. 29 117/127 inches of mercury 29.92 inHg 14.6959 psia or 0 psig (pounds-force per square inch, absolute or gauge lbf/in 2"
I've cited a few references supporting my position; can you you cite some references against my position? I don't believe that you can.
Mike B.
> Sorry to waste the bandwidth, but...
>> Sea level pressure IS assumed as the "0" reading on every pressure
>> guage designed for automotive use.
> Only uncalibrated ones. Gauge pressure is gauge pressure, and is
> referenced only to *local station pressure*, not sea level. That's why
> gauges have a zero adjustment (unless they're cheap junk gauges). The
> station pressure in Denver is about 11 psia, so a gauge calibrated at
> sea level will be off by roughly 25% unless re-zeroed, and vice-versa.
> Thus, "sea-level" is irrelevant for gauge pressure measurements.
>> As an example of my meaning; a new tire, once mounted on its wheel
>> rim, before ever having any air pressure added to inflate it to spec,
>> contains ambient air pressure of the atmosphere around you. This
>> would be measurable as 14.5 to 14.7 PSI (close enough, as most
>> automotive guages won't even read this finely) at sea level. Any
>> pressure guage reading must assume to be a value above standard
>> ambient sea level atmospheric pressure, or 1 bar. Read the following
>> quote from the wiki link to help you understand what I'm referring to;
> Mike, I've spent 15 years running a metrology lab, and calibrated
> thousands of gauge and absolute pressure gauges/transducers/manometers.
> I hardly need a Wiki explanation. And a tire gauge hardly even
> qualifies as a gauge as commonly used.
>
> When someone makes a real request for info about sender/gauge
> compatibility, and you want to make some snarky response, you really
> ought to at least make sure you're right. To quote; "what's so hard
> about that?"
>
> Keith Hughes
> '86 Westy Tiico (Marvin)