I am a car enthusiast and have absolutely no engineering educational/knoweldge, so keep that in mind. If an internal combustion engine revolutions per minute increase simply by opening the throttle, I assume that this is a function of the pressure differential between the air outside the motor and the low pressure area created within the motor (assume we are discussing a naturally aspirted engine). Assume the parts could withstand the forces (pistons, pins, rods, bearings, springs, etc). What would happen if I just left the throttle open? The RPM's would continue to increase obviously, but at what point would they max? Or would they? I assume at some point other restrictions regarding the intake manifold, cam design & valve events, etc. would take over. But would the pressure differential come close to zero? My assumption is yes based on experience, but I am not for sure what would happen with the RPM's. Just a mind exercise that I haven't worked out in my head.