Full throttle Height
Don't want to fall out with anyone on this. However, Brian Abraham is completely correct in post #5.
However, how FTH is obtained will depend on the method of throttle lever to throttle butterfly (or Corliss valve) connection.
With a directly connected (i.e. no compensating system) throttle system, the pilot will be responsible for ensuring the engine is not overboosted in the climb, as he (or indeed she) will need to gradually move the throttle lever forward to compensate for decreasing ambient pressure with altitude. At the height where the throttle lever is fully forward and the manifold pressure is maintained at the selected MAP, that altitude is the Full Throttle Height for the selected MAP. Any further increase in altitude will result in decreasing MAP and therefore BHP.
For an automatic compensating system, the pilot puts the throttle lever fully forward at a low altitude (engine limitations allowing) or to a limiting MAP (e.g. max continous) and the manifold pressure will be automatically limited to that selected MAP. As full throttle height is reached, the throttle butterfly or Corliss valve will be fully open. Any increrase in altitude above FTH will be noted by a gradual decrease in MAP.
As can be seen, FTH is completely variable dependent on selected MAP. However, quoted manufacturers figures normally relate to Maximum Take Off Power (often with time limit imposed), METO (maximum except take off power) or Maximum Continous, and give figures for low blower speed and high blower speed (or in Rolls Royce parlance, Moderate Speed and Full Speed).
And as an aside, FTH is not always in excess of 10,000 ft. For low-altitude engines, such as the Rolls Royce Griffon 58, FTH low gear @ max rpm of 2750 and 67" MAP was 2000 ft. FTH high gear @ max rpm of 2750 and 81" MAP was 3000 ft.
For eMACaRe's Spit mk IX with a 60 series Rolls Royce Merlin, FTH of 19,000 ft in high gear sounds about right.
In a simple nutshell, for supercharged engines (including turbosupercharged) the altitude reached where the throttle butterfly or Corliss valve can open no more is FTH for the selected MAP.
Hope this helps.
camlobe