an some of the heli boys explain to me how the manifold vacuum works? I remember a mate with a Schweitzer in Canberra used to be able to tell if he could land and take off again depending on how much power was available by checking the inches of mercury, but I don't really understand how all that works .... if you can explain, I would appreciate it

Air pressure measured in In Hg. the engine is rated to give it's maximum rated horsepower when operated with in it's limits.

as the barometric pressure changes or the temperature changes it will also change the 'pressure' measured in In Hg.

Less pressure = less performance . Hot , high = less air pressure( and oxygen) to enter the cylinder on the intake stroke when the air craft is tested the limits are established for sea level performance , thus the red line on the manifold gage,

Laps rate of 2" per a thousand feet... = high altitude 13,000 max for most normally aspirated engines, The air pressure is so low the power is almost nonexistent.

I used to teach my students to do a power check before entering a confined area landing. OGE hovering and pull pitch advance throttle til it poops out. Note the manifold pressure and compare it with the take off manifold pressure. Decide if you want to land! Or if you want your PAX to walk

dont know if its correct but you can correct me... i think its because the engine is derated and its to protect him, to not overtorque the output or overload the engine with colective blade pitch, with similar lycoming in a cesna than a r22 but not derated and with fix blade pitch , the motor is not derated and you dont have a maximum manifold pressure chart to respect, is it correct.. , but i think if you have a constant speed prop, or variable pitch or turbo , even with a not derated engine you have a maximum intake manifold pressure chart to respect to not overtorque or overload and damage the engine, its a better or the best method of monotoring engine output work load, compared to just, rpm, egt, afr, and boost psi in standard race or ground machine, when we look at the same engine n/a and turbo we see how hard the turbo work the engine, with the big manifold pressure addition compared to the n/a low max mp availlable.

Robinson is derated by just drawing that red line at a lower number. If you pull power more at sea level it just wares things out faster.

FORCED INDUCTION!!!!!