On 8/2/2017 at 10:05 PM, downscc said:

Robert:  I found a generic gas centrifugal performance curve in a Wiki article that is representative of actual curves I've seen for other applications:

The N lines are constant speed lines and flow rate is related to pressure ratio as a function of the rotor speed. Quantitative values for this family of curves can be easily found for auto racing compressors and turbocharger, I have not found one yet for the two speed R-2800 supercharger.  I can play with the above typical by presuming P&W is maintaining something along the peak efficiency line, then our decrease RPM scenario is accomplished by moving from one N line to another while also moving to the left as a result of less flow.  I can imagine a small decrease in speed and a realitively significant change in flow (as in the change from 2400/40 to 2200/30) maintaining a relatively uniform pressure ratio (supercharger boost directly related to MP).

Not so fast Mike, I will read your references but how much air might better be measured by SCUF (standard cubic feet, a standard unit of air mass) than pressure.  I can pack a lot more air by weight at the same pressure if it is cooler.  That's why RAM puts intercoolers on our converted TSIO-520s in the Chancellor.

 

fair point regarding temperature but in this case I disagree with your assessment because as I understand the scenarios being presented, temperature is irrelevant because all the presented theoretical engines are operating under the same atmospheric conditions and no previous mention of intercoolers has been addressed. As such manifold pressure still represents, to the pilot, the best indicator of air mass flow rate. If we want to get absolutely correct, ignoring partial pressures, under the ideal gas law Pv=nRt, the amount of gas (moles, n) is directly proportional to PV/RT.

This topic is hard to comprehend. There is another thing about supercharging, at low alt when throttle is partially closed, MP will increase while reducing engine rpm, so going from high power setting in to low power setting require throttle readjustment as well. So let say MP goes from 40 to 30 inch then you reduce rpm from 2400 to 2100 during that process MP will rise. Above critical alt things looks opposite.