you say 'thus an engine sees the same pressure whether at 0 ft or 30000 ft'
if you mean inlet manifold pressure ....
10 lb boost at sea level was 10 lb forward pressure but 10 lb boost at altitude was more than 10 lb forward pressure
wouldn't this term contribute in the sense of power increase with altitude ?
yes of course the dominant factor is (absolute) inlet manifold pressure, driving massflow
before writing yesterday I checked and got this, written by Bill Gunston
'boost pressure is the difference between ambient atmospheric pressure and the pressure in the supercharger delivery manifold feeding the cylinders'
and 'in German engines 1.7 ata (1.7 times local atmospheric pressure)'
(my take was) ....
if he meant sea level atmospheric pressure why did he say ambient ?
if he meant sea level atmospheric pressure why did he say local atmospheric pressure ? (Berlin or 5000' Munich ? of Thain/Manchester United notoriety)
Gunston's diagram of a British boost controller is not helpful
but today, looking at the Sensitive Aneroid paragraph in this
http://www.enginehistory.org/Piston/Rol ... nABC.shtml
it shows the aneroid pressure was a mass production-line approach to vacuum (elsewhere aneroids are defined as partially evacuated)
and ok the designed system was presumably calibration-checked at nominally sea level
our 2021 car (turbo, if no recovery) might slightly increase power with altitude at fixed MAP
and MAP fixed by rule is difficult to implement, so (as in 1987-8) backup limiters are needed ie fuel quantity