Simple engine question

[omcaree]

Hey all,

I have what I'm sure will be a simple question about engines, it stems from my thermodynamics coursework but its just as relevant to aviation.

I'll admit that I don't fully understand how manifold pressure and RPM can be used to find engine power output (despite recently completing a VP checkout), thats not really the problem in this case. with the throttle set in a constant position, a reading will be obtained for manifold pressure, if nothing else changes appart from the engine RPM being increased then what should happen to the manifold pressure reading? my logic says it should decrease because the engine is trying to suck more air through the same gap, however from the experiment which i conducted i found that the manifold pressure remained constant from 1800 to 4700 RPM (rover k series engine) which is just making me confused.

Any help would be greatly appreciated

Cheers

Owen

PS. this wont help with my coursework as that concerns thermal efficiency, this was just an observation which confused me!