manifold air pressure does
not generate any horsepower at all.
it is only an indication of how much power you are pulling and more importantly how much is left.
"less exhaust backpressure" i haven't stopped laughing, what book did you get that out of ???, the book of stupid answers i'll bet.
who teaches you guys this crap. go and get a book on aircraft engines and i bet you won't find any reference to back pressure affecting map.
mongoose, i think you meant at a higher altitude where the pressure is less.
23" of map is 23" of map at any altitude and just tells you that the pressure in the manifold is the same. this has no bearing on the power produced by the motor.